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around. Highlights include: - Conversion of a bunch of security documentation into RST - The conversion of the remaining DocBook templates by The Amazing Mauro Machine. We can now drop the entire DocBook build chain. - The usual collection of fixes and minor updates. -----BEGIN PGP SIGNATURE----- iQIcBAABAgAGBQJZWkGAAAoJEI3ONVYwIuV6rf0P/0B3JTiVPKS/WUx53+jzbAi4 1BN7dmmuMxE1bWpgdEq+ac4aKxm07iAojuntuMj0qz/ZB1WARcmvEqqzI5i4wfq9 5MrLduLkyuWfr4MOPseKJ2VK83p8nkMOiO7jmnBsilu7fE4nF+5YY9j4cVaArfMy cCQvAGjQzvej2eiWMGUSLHn4QFKh00aD7cwKyBVsJ08b27C9xL0J2LQyCDZ4yDgf 37/MH3puEd3HX/4qAwLonIxT3xrIrrbDturqLU7OSKcWTtGZNrYyTFbwR3RQtqWd H8YZVg2Uyhzg9MYhkbQ2E5dEjUP4mkegcp6/JTINH++OOPpTbdTJgirTx7VTkSf1 +kL8t7+Ayxd0FH3+77GJ5RMj8LUK6rj5cZfU5nClFQKWXP9UL3IelQ3Nl+SpdM8v ZAbR2KjKgH9KS6+cbIhgFYlvY+JgPkOVruwbIAc7wXVM3ibk1sWoBOFEujcbueWh yDpQv3l1UX0CKr3jnevJoW26LtEbGFtC7gSKZ+3btyeSBpWFGlii42KNycEGwUW0 ezlwryDVHzyTUiKllNmkdK4v73mvPsZHEjgmme4afKAIiUilmcUF4XcqD86hISFT t+UJLA/zEU+0sJe26o2nK6GNJzmo4oCtVyxfhRe26Ojs1n80xlYgnZRfuIYdd31Z nwLBnwDCHAOyX91WXp9G =cVjZ -----END PGP SIGNATURE----- Merge tag 'docs-4.13' of git://git.lwn.net/linux Pull documentation updates from Jonathan Corbet: "There has been a fair amount of activity in the docs tree this time around. Highlights include: - Conversion of a bunch of security documentation into RST - The conversion of the remaining DocBook templates by The Amazing Mauro Machine. We can now drop the entire DocBook build chain. - The usual collection of fixes and minor updates" * tag 'docs-4.13' of git://git.lwn.net/linux: (90 commits) scripts/kernel-doc: handle DECLARE_HASHTABLE Documentation: atomic_ops.txt is core-api/atomic_ops.rst Docs: clean up some DocBook loose ends Make the main documentation title less Geocities Docs: Use kernel-figure in vidioc-g-selection.rst Docs: fix table problems in ras.rst Docs: Fix breakage with Sphinx 1.5 and upper Docs: Include the Latex "ifthen" package doc/kokr/howto: Only send regression fixes after -rc1 docs-rst: fix broken links to dynamic-debug-howto in kernel-parameters doc: Document suitability of IBM Verse for kernel development Doc: fix a markup error in coding-style.rst docs: driver-api: i2c: remove some outdated information Documentation: DMA API: fix a typo in a function name Docs: Insert missing space to separate link from text doc/ko_KR/memory-barriers: Update control-dependencies example Documentation, kbuild: fix typo "minimun" -> "minimum" docs: Fix some formatting issues in request-key.rst doc: ReSTify keys-trusted-encrypted.txt doc: ReSTify keys-request-key.txt ...
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ReStructuredText
============================================
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Unreliable Guide To Hacking The Linux Kernel
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============================================
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:Author: Rusty Russell
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Introduction
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============
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Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
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Kernel Hacking. This document describes the common routines and general
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requirements for kernel code: its goal is to serve as a primer for Linux
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kernel development for experienced C programmers. I avoid implementation
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details: that's what the code is for, and I ignore whole tracts of
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useful routines.
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Before you read this, please understand that I never wanted to write
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this document, being grossly under-qualified, but I always wanted to
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read it, and this was the only way. I hope it will grow into a
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compendium of best practice, common starting points and random
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information.
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The Players
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===========
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At any time each of the CPUs in a system can be:
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- not associated with any process, serving a hardware interrupt;
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- not associated with any process, serving a softirq or tasklet;
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- running in kernel space, associated with a process (user context);
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- running a process in user space.
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There is an ordering between these. The bottom two can preempt each
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other, but above that is a strict hierarchy: each can only be preempted
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by the ones above it. For example, while a softirq is running on a CPU,
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no other softirq will preempt it, but a hardware interrupt can. However,
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any other CPUs in the system execute independently.
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We'll see a number of ways that the user context can block interrupts,
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to become truly non-preemptable.
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User Context
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------------
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User context is when you are coming in from a system call or other trap:
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like userspace, you can be preempted by more important tasks and by
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interrupts. You can sleep, by calling :c:func:`schedule()`.
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.. note::
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You are always in user context on module load and unload, and on
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operations on the block device layer.
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In user context, the ``current`` pointer (indicating the task we are
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currently executing) is valid, and :c:func:`in_interrupt()`
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(``include/linux/preempt.h``) is false.
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.. warning::
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Beware that if you have preemption or softirqs disabled (see below),
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:c:func:`in_interrupt()` will return a false positive.
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Hardware Interrupts (Hard IRQs)
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-------------------------------
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Timer ticks, network cards and keyboard are examples of real hardware
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which produce interrupts at any time. The kernel runs interrupt
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handlers, which services the hardware. The kernel guarantees that this
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handler is never re-entered: if the same interrupt arrives, it is queued
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(or dropped). Because it disables interrupts, this handler has to be
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fast: frequently it simply acknowledges the interrupt, marks a 'software
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interrupt' for execution and exits.
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You can tell you are in a hardware interrupt, because
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:c:func:`in_irq()` returns true.
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.. warning::
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Beware that this will return a false positive if interrupts are
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disabled (see below).
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Software Interrupt Context: Softirqs and Tasklets
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-------------------------------------------------
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Whenever a system call is about to return to userspace, or a hardware
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interrupt handler exits, any 'software interrupts' which are marked
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pending (usually by hardware interrupts) are run (``kernel/softirq.c``).
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Much of the real interrupt handling work is done here. Early in the
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transition to SMP, there were only 'bottom halves' (BHs), which didn't
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take advantage of multiple CPUs. Shortly after we switched from wind-up
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computers made of match-sticks and snot, we abandoned this limitation
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and switched to 'softirqs'.
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``include/linux/interrupt.h`` lists the different softirqs. A very
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important softirq is the timer softirq (``include/linux/timer.h``): you
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can register to have it call functions for you in a given length of
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time.
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Softirqs are often a pain to deal with, since the same softirq will run
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simultaneously on more than one CPU. For this reason, tasklets
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(``include/linux/interrupt.h``) are more often used: they are
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dynamically-registrable (meaning you can have as many as you want), and
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they also guarantee that any tasklet will only run on one CPU at any
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time, although different tasklets can run simultaneously.
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.. warning::
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The name 'tasklet' is misleading: they have nothing to do with
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'tasks', and probably more to do with some bad vodka Alexey
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Kuznetsov had at the time.
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You can tell you are in a softirq (or tasklet) using the
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:c:func:`in_softirq()` macro (``include/linux/preempt.h``).
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.. warning::
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Beware that this will return a false positive if a
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:ref:`botton half lock <local_bh_disable>` is held.
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Some Basic Rules
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================
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No memory protection
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If you corrupt memory, whether in user context or interrupt context,
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the whole machine will crash. Are you sure you can't do what you
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want in userspace?
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No floating point or MMX
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The FPU context is not saved; even in user context the FPU state
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probably won't correspond with the current process: you would mess
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with some user process' FPU state. If you really want to do this,
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you would have to explicitly save/restore the full FPU state (and
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avoid context switches). It is generally a bad idea; use fixed point
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arithmetic first.
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A rigid stack limit
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Depending on configuration options the kernel stack is about 3K to
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6K for most 32-bit architectures: it's about 14K on most 64-bit
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archs, and often shared with interrupts so you can't use it all.
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Avoid deep recursion and huge local arrays on the stack (allocate
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them dynamically instead).
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The Linux kernel is portable
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Let's keep it that way. Your code should be 64-bit clean, and
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endian-independent. You should also minimize CPU specific stuff,
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e.g. inline assembly should be cleanly encapsulated and minimized to
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ease porting. Generally it should be restricted to the
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architecture-dependent part of the kernel tree.
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ioctls: Not writing a new system call
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=====================================
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A system call generally looks like this::
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asmlinkage long sys_mycall(int arg)
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{
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return 0;
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}
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First, in most cases you don't want to create a new system call. You
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create a character device and implement an appropriate ioctl for it.
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This is much more flexible than system calls, doesn't have to be entered
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in every architecture's ``include/asm/unistd.h`` and
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``arch/kernel/entry.S`` file, and is much more likely to be accepted by
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Linus.
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If all your routine does is read or write some parameter, consider
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implementing a :c:func:`sysfs()` interface instead.
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Inside the ioctl you're in user context to a process. When a error
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occurs you return a negated errno (see
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``include/uapi/asm-generic/errno-base.h``,
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``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``),
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otherwise you return 0.
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After you slept you should check if a signal occurred: the Unix/Linux
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way of handling signals is to temporarily exit the system call with the
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``-ERESTARTSYS`` error. The system call entry code will switch back to
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user context, process the signal handler and then your system call will
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be restarted (unless the user disabled that). So you should be prepared
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to process the restart, e.g. if you're in the middle of manipulating
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some data structure.
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::
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if (signal_pending(current))
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return -ERESTARTSYS;
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If you're doing longer computations: first think userspace. If you
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**really** want to do it in kernel you should regularly check if you need
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to give up the CPU (remember there is cooperative multitasking per CPU).
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Idiom::
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cond_resched(); /* Will sleep */
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A short note on interface design: the UNIX system call motto is "Provide
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mechanism not policy".
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Recipes for Deadlock
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====================
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You cannot call any routines which may sleep, unless:
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- You are in user context.
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- You do not own any spinlocks.
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- You have interrupts enabled (actually, Andi Kleen says that the
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scheduling code will enable them for you, but that's probably not
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what you wanted).
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Note that some functions may sleep implicitly: common ones are the user
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space access functions (\*_user) and memory allocation functions
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without ``GFP_ATOMIC``.
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You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on,
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and it will warn you if you break these rules. If you **do** break the
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rules, you will eventually lock up your box.
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Really.
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Common Routines
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===============
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:c:func:`printk()`
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------------------
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Defined in ``include/linux/printk.h``
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:c:func:`printk()` feeds kernel messages to the console, dmesg, and
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the syslog daemon. It is useful for debugging and reporting errors, and
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can be used inside interrupt context, but use with caution: a machine
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which has its console flooded with printk messages is unusable. It uses
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a format string mostly compatible with ANSI C printf, and C string
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concatenation to give it a first "priority" argument::
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printk(KERN_INFO "i = %u\n", i);
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See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are
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interpreted by syslog as the level. Special case: for printing an IP
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address use::
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__be32 ipaddress;
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printk(KERN_INFO "my ip: %pI4\n", &ipaddress);
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:c:func:`printk()` internally uses a 1K buffer and does not catch
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overruns. Make sure that will be enough.
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.. note::
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You will know when you are a real kernel hacker when you start
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typoing printf as printk in your user programs :)
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.. note::
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Another sidenote: the original Unix Version 6 sources had a comment
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on top of its printf function: "Printf should not be used for
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chit-chat". You should follow that advice.
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:c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()`
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---------------------------------------------------------------------------------------------------
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Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h``
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**[SLEEPS]**
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:c:func:`put_user()` and :c:func:`get_user()` are used to get
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and put single values (such as an int, char, or long) from and to
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userspace. A pointer into userspace should never be simply dereferenced:
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data should be copied using these routines. Both return ``-EFAULT`` or
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0.
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:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are
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more general: they copy an arbitrary amount of data to and from
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userspace.
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.. warning::
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Unlike :c:func:`put_user()` and :c:func:`get_user()`, they
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return the amount of uncopied data (ie. 0 still means success).
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[Yes, this moronic interface makes me cringe. The flamewar comes up
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every year or so. --RR.]
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The functions may sleep implicitly. This should never be called outside
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user context (it makes no sense), with interrupts disabled, or a
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spinlock held.
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:c:func:`kmalloc()`/:c:func:`kfree()`
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-------------------------------------
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Defined in ``include/linux/slab.h``
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**[MAY SLEEP: SEE BELOW]**
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These routines are used to dynamically request pointer-aligned chunks of
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memory, like malloc and free do in userspace, but
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:c:func:`kmalloc()` takes an extra flag word. Important values:
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``GFP_KERNEL``
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May sleep and swap to free memory. Only allowed in user context, but
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is the most reliable way to allocate memory.
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``GFP_ATOMIC``
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Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called
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from interrupt context. You should **really** have a good
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out-of-memory error-handling strategy.
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``GFP_DMA``
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Allocate ISA DMA lower than 16MB. If you don't know what that is you
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don't need it. Very unreliable.
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If you see a sleeping function called from invalid context warning
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message, then maybe you called a sleeping allocation function from
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interrupt context without ``GFP_ATOMIC``. You should really fix that.
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Run, don't walk.
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If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or
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``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()`
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(``include/linux/gfp.h``). It takes an order argument (0 for page sized,
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1 for double page, 2 for four pages etc.) and the same memory priority
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flag word as above.
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If you are allocating more than a page worth of bytes you can use
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:c:func:`vmalloc()`. It'll allocate virtual memory in the kernel
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map. This block is not contiguous in physical memory, but the MMU makes
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it look like it is for you (so it'll only look contiguous to the CPUs,
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not to external device drivers). If you really need large physically
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contiguous memory for some weird device, you have a problem: it is
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poorly supported in Linux because after some time memory fragmentation
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in a running kernel makes it hard. The best way is to allocate the block
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early in the boot process via the :c:func:`alloc_bootmem()`
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routine.
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Before inventing your own cache of often-used objects consider using a
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slab cache in ``include/linux/slab.h``
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:c:func:`current()`
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-------------------
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Defined in ``include/asm/current.h``
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This global variable (really a macro) contains a pointer to the current
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task structure, so is only valid in user context. For example, when a
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process makes a system call, this will point to the task structure of
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the calling process. It is **not NULL** in interrupt context.
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:c:func:`mdelay()`/:c:func:`udelay()`
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-------------------------------------
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Defined in ``include/asm/delay.h`` / ``include/linux/delay.h``
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The :c:func:`udelay()` and :c:func:`ndelay()` functions can be
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used for small pauses. Do not use large values with them as you risk
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overflow - the helper function :c:func:`mdelay()` is useful here, or
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consider :c:func:`msleep()`.
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:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()`
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-----------------------------------------------------------------------------------------------
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Defined in ``include/asm/byteorder.h``
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The :c:func:`cpu_to_be32()` family (where the "32" can be replaced
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by 64 or 16, and the "be" can be replaced by "le") are the general way
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to do endian conversions in the kernel: they return the converted value.
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All variations supply the reverse as well:
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:c:func:`be32_to_cpu()`, etc.
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There are two major variations of these functions: the pointer
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variation, such as :c:func:`cpu_to_be32p()`, which take a pointer
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to the given type, and return the converted value. The other variation
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is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which
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convert value referred to by the pointer, and return void.
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:c:func:`local_irq_save()`/:c:func:`local_irq_restore()`
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--------------------------------------------------------
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Defined in ``include/linux/irqflags.h``
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These routines disable hard interrupts on the local CPU, and restore
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them. They are reentrant; saving the previous state in their one
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``unsigned long flags`` argument. If you know that interrupts are
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enabled, you can simply use :c:func:`local_irq_disable()` and
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:c:func:`local_irq_enable()`.
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.. _local_bh_disable:
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:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()`
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--------------------------------------------------------
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Defined in ``include/linux/bottom_half.h``
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These routines disable soft interrupts on the local CPU, and restore
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them. They are reentrant; if soft interrupts were disabled before, they
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will still be disabled after this pair of functions has been called.
|
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They prevent softirqs and tasklets from running on the current CPU.
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:c:func:`smp_processor_id()`
|
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----------------------------
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Defined in ``include/linux/smp.h``
|
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:c:func:`get_cpu()` disables preemption (so you won't suddenly get
|
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moved to another CPU) and returns the current processor number, between
|
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0 and ``NR_CPUS``. Note that the CPU numbers are not necessarily
|
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continuous. You return it again with :c:func:`put_cpu()` when you
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are done.
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If you know you cannot be preempted by another task (ie. you are in
|
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interrupt context, or have preemption disabled) you can use
|
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smp_processor_id().
|
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|
|
``__init``/``__exit``/``__initdata``
|
|
------------------------------------
|
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|
Defined in ``include/linux/init.h``
|
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|
|
After boot, the kernel frees up a special section; functions marked with
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``__init`` and data structures marked with ``__initdata`` are dropped
|
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after boot is complete: similarly modules discard this memory after
|
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initialization. ``__exit`` is used to declare a function which is only
|
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required on exit: the function will be dropped if this file is not
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compiled as a module. See the header file for use. Note that it makes no
|
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sense for a function marked with ``__init`` to be exported to modules
|
|
with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this
|
|
will break.
|
|
|
|
:c:func:`__initcall()`/:c:func:`module_init()`
|
|
----------------------------------------------
|
|
|
|
Defined in ``include/linux/init.h`` / ``include/linux/module.h``
|
|
|
|
Many parts of the kernel are well served as a module
|
|
(dynamically-loadable parts of the kernel). Using the
|
|
:c:func:`module_init()` and :c:func:`module_exit()` macros it
|
|
is easy to write code without #ifdefs which can operate both as a module
|
|
or built into the kernel.
|
|
|
|
The :c:func:`module_init()` macro defines which function is to be
|
|
called at module insertion time (if the file is compiled as a module),
|
|
or at boot time: if the file is not compiled as a module the
|
|
:c:func:`module_init()` macro becomes equivalent to
|
|
:c:func:`__initcall()`, which through linker magic ensures that
|
|
the function is called on boot.
|
|
|
|
The function can return a negative error number to cause module loading
|
|
to fail (unfortunately, this has no effect if the module is compiled
|
|
into the kernel). This function is called in user context with
|
|
interrupts enabled, so it can sleep.
|
|
|
|
:c:func:`module_exit()`
|
|
-----------------------
|
|
|
|
|
|
Defined in ``include/linux/module.h``
|
|
|
|
This macro defines the function to be called at module removal time (or
|
|
never, in the case of the file compiled into the kernel). It will only
|
|
be called if the module usage count has reached zero. This function can
|
|
also sleep, but cannot fail: everything must be cleaned up by the time
|
|
it returns.
|
|
|
|
Note that this macro is optional: if it is not present, your module will
|
|
not be removable (except for 'rmmod -f').
|
|
|
|
:c:func:`try_module_get()`/:c:func:`module_put()`
|
|
-------------------------------------------------
|
|
|
|
Defined in ``include/linux/module.h``
|
|
|
|
These manipulate the module usage count, to protect against removal (a
|
|
module also can't be removed if another module uses one of its exported
|
|
symbols: see below). Before calling into module code, you should call
|
|
:c:func:`try_module_get()` on that module: if it fails, then the
|
|
module is being removed and you should act as if it wasn't there.
|
|
Otherwise, you can safely enter the module, and call
|
|
:c:func:`module_put()` when you're finished.
|
|
|
|
Most registerable structures have an owner field, such as in the
|
|
:c:type:`struct file_operations <file_operations>` structure.
|
|
Set this field to the macro ``THIS_MODULE``.
|
|
|
|
Wait Queues ``include/linux/wait.h``
|
|
====================================
|
|
|
|
**[SLEEPS]**
|
|
|
|
A wait queue is used to wait for someone to wake you up when a certain
|
|
condition is true. They must be used carefully to ensure there is no
|
|
race condition. You declare a :c:type:`wait_queue_head_t`, and then processes
|
|
which want to wait for that condition declare a :c:type:`wait_queue_entry_t`
|
|
referring to themselves, and place that in the queue.
|
|
|
|
Declaring
|
|
---------
|
|
|
|
You declare a ``wait_queue_head_t`` using the
|
|
:c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the
|
|
:c:func:`init_waitqueue_head()` routine in your initialization
|
|
code.
|
|
|
|
Queuing
|
|
-------
|
|
|
|
Placing yourself in the waitqueue is fairly complex, because you must
|
|
put yourself in the queue before checking the condition. There is a
|
|
macro to do this: :c:func:`wait_event_interruptible()`
|
|
(``include/linux/wait.h``) The first argument is the wait queue head, and
|
|
the second is an expression which is evaluated; the macro returns 0 when
|
|
this expression is true, or ``-ERESTARTSYS`` if a signal is received. The
|
|
:c:func:`wait_event()` version ignores signals.
|
|
|
|
Waking Up Queued Tasks
|
|
----------------------
|
|
|
|
Call :c:func:`wake_up()` (``include/linux/wait.h``);, which will wake
|
|
up every process in the queue. The exception is if one has
|
|
``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will
|
|
not be woken. There are other variants of this basic function available
|
|
in the same header.
|
|
|
|
Atomic Operations
|
|
=================
|
|
|
|
Certain operations are guaranteed atomic on all platforms. The first
|
|
class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``);
|
|
this contains a signed integer (at least 32 bits long), and you must use
|
|
these functions to manipulate or read :c:type:`atomic_t` variables.
|
|
:c:func:`atomic_read()` and :c:func:`atomic_set()` get and set
|
|
the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`,
|
|
:c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and
|
|
:c:func:`atomic_dec_and_test()` (returns true if it was
|
|
decremented to zero).
|
|
|
|
Yes. It returns true (i.e. != 0) if the atomic variable is zero.
|
|
|
|
Note that these functions are slower than normal arithmetic, and so
|
|
should not be used unnecessarily.
|
|
|
|
The second class of atomic operations is atomic bit operations on an
|
|
``unsigned long``, defined in ``include/linux/bitops.h``. These
|
|
operations generally take a pointer to the bit pattern, and a bit
|
|
number: 0 is the least significant bit. :c:func:`set_bit()`,
|
|
:c:func:`clear_bit()` and :c:func:`change_bit()` set, clear,
|
|
and flip the given bit. :c:func:`test_and_set_bit()`,
|
|
:c:func:`test_and_clear_bit()` and
|
|
:c:func:`test_and_change_bit()` do the same thing, except return
|
|
true if the bit was previously set; these are particularly useful for
|
|
atomically setting flags.
|
|
|
|
It is possible to call these operations with bit indices greater than
|
|
``BITS_PER_LONG``. The resulting behavior is strange on big-endian
|
|
platforms though so it is a good idea not to do this.
|
|
|
|
Symbols
|
|
=======
|
|
|
|
Within the kernel proper, the normal linking rules apply (ie. unless a
|
|
symbol is declared to be file scope with the ``static`` keyword, it can
|
|
be used anywhere in the kernel). However, for modules, a special
|
|
exported symbol table is kept which limits the entry points to the
|
|
kernel proper. Modules can also export symbols.
|
|
|
|
:c:func:`EXPORT_SYMBOL()`
|
|
-------------------------
|
|
|
|
Defined in ``include/linux/export.h``
|
|
|
|
This is the classic method of exporting a symbol: dynamically loaded
|
|
modules will be able to use the symbol as normal.
|
|
|
|
:c:func:`EXPORT_SYMBOL_GPL()`
|
|
-----------------------------
|
|
|
|
Defined in ``include/linux/export.h``
|
|
|
|
Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols
|
|
exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by
|
|
modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL
|
|
compatible license. It implies that the function is considered an
|
|
internal implementation issue, and not really an interface. Some
|
|
maintainers and developers may however require EXPORT_SYMBOL_GPL()
|
|
when adding any new APIs or functionality.
|
|
|
|
Routines and Conventions
|
|
========================
|
|
|
|
Double-linked lists ``include/linux/list.h``
|
|
--------------------------------------------
|
|
|
|
There used to be three sets of linked-list routines in the kernel
|
|
headers, but this one is the winner. If you don't have some particular
|
|
pressing need for a single list, it's a good choice.
|
|
|
|
In particular, :c:func:`list_for_each_entry()` is useful.
|
|
|
|
Return Conventions
|
|
------------------
|
|
|
|
For code called in user context, it's very common to defy C convention,
|
|
and return 0 for success, and a negative error number (eg. ``-EFAULT``) for
|
|
failure. This can be unintuitive at first, but it's fairly widespread in
|
|
the kernel.
|
|
|
|
Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a
|
|
negative error number into a pointer, and :c:func:`IS_ERR()` and
|
|
:c:func:`PTR_ERR()` to get it back out again: avoids a separate
|
|
pointer parameter for the error number. Icky, but in a good way.
|
|
|
|
Breaking Compilation
|
|
--------------------
|
|
|
|
Linus and the other developers sometimes change function or structure
|
|
names in development kernels; this is not done just to keep everyone on
|
|
their toes: it reflects a fundamental change (eg. can no longer be
|
|
called with interrupts on, or does extra checks, or doesn't do checks
|
|
which were caught before). Usually this is accompanied by a fairly
|
|
complete note to the linux-kernel mailing list; search the archive.
|
|
Simply doing a global replace on the file usually makes things **worse**.
|
|
|
|
Initializing structure members
|
|
------------------------------
|
|
|
|
The preferred method of initializing structures is to use designated
|
|
initialisers, as defined by ISO C99, eg::
|
|
|
|
static struct block_device_operations opt_fops = {
|
|
.open = opt_open,
|
|
.release = opt_release,
|
|
.ioctl = opt_ioctl,
|
|
.check_media_change = opt_media_change,
|
|
};
|
|
|
|
|
|
This makes it easy to grep for, and makes it clear which structure
|
|
fields are set. You should do this because it looks cool.
|
|
|
|
GNU Extensions
|
|
--------------
|
|
|
|
GNU Extensions are explicitly allowed in the Linux kernel. Note that
|
|
some of the more complex ones are not very well supported, due to lack
|
|
of general use, but the following are considered standard (see the GCC
|
|
info page section "C Extensions" for more details - Yes, really the info
|
|
page, the man page is only a short summary of the stuff in info).
|
|
|
|
- Inline functions
|
|
|
|
- Statement expressions (ie. the ({ and }) constructs).
|
|
|
|
- Declaring attributes of a function / variable / type
|
|
(__attribute__)
|
|
|
|
- typeof
|
|
|
|
- Zero length arrays
|
|
|
|
- Macro varargs
|
|
|
|
- Arithmetic on void pointers
|
|
|
|
- Non-Constant initializers
|
|
|
|
- Assembler Instructions (not outside arch/ and include/asm/)
|
|
|
|
- Function names as strings (__func__).
|
|
|
|
- __builtin_constant_p()
|
|
|
|
Be wary when using long long in the kernel, the code gcc generates for
|
|
it is horrible and worse: division and multiplication does not work on
|
|
i386 because the GCC runtime functions for it are missing from the
|
|
kernel environment.
|
|
|
|
C++
|
|
---
|
|
|
|
Using C++ in the kernel is usually a bad idea, because the kernel does
|
|
not provide the necessary runtime environment and the include files are
|
|
not tested for it. It is still possible, but not recommended. If you
|
|
really want to do this, forget about exceptions at least.
|
|
|
|
NUMif
|
|
-----
|
|
|
|
It is generally considered cleaner to use macros in header files (or at
|
|
the top of .c files) to abstract away functions rather than using \`#if'
|
|
pre-processor statements throughout the source code.
|
|
|
|
Putting Your Stuff in the Kernel
|
|
================================
|
|
|
|
In order to get your stuff into shape for official inclusion, or even to
|
|
make a neat patch, there's administrative work to be done:
|
|
|
|
- Figure out whose pond you've been pissing in. Look at the top of the
|
|
source files, inside the ``MAINTAINERS`` file, and last of all in the
|
|
``CREDITS`` file. You should coordinate with this person to make sure
|
|
you're not duplicating effort, or trying something that's already
|
|
been rejected.
|
|
|
|
Make sure you put your name and EMail address at the top of any files
|
|
you create or mangle significantly. This is the first place people
|
|
will look when they find a bug, or when **they** want to make a change.
|
|
|
|
- Usually you want a configuration option for your kernel hack. Edit
|
|
``Kconfig`` in the appropriate directory. The Config language is
|
|
simple to use by cut and paste, and there's complete documentation in
|
|
``Documentation/kbuild/kconfig-language.txt``.
|
|
|
|
In your description of the option, make sure you address both the
|
|
expert user and the user who knows nothing about your feature.
|
|
Mention incompatibilities and issues here. **Definitely** end your
|
|
description with “if in doubt, say N” (or, occasionally, \`Y'); this
|
|
is for people who have no idea what you are talking about.
|
|
|
|
- Edit the ``Makefile``: the CONFIG variables are exported here so you
|
|
can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax
|
|
is documented in ``Documentation/kbuild/makefiles.txt``.
|
|
|
|
- Put yourself in ``CREDITS`` if you've done something noteworthy,
|
|
usually beyond a single file (your name should be at the top of the
|
|
source files anyway). ``MAINTAINERS`` means you want to be consulted
|
|
when changes are made to a subsystem, and hear about bugs; it implies
|
|
a more-than-passing commitment to some part of the code.
|
|
|
|
- Finally, don't forget to read
|
|
``Documentation/process/submitting-patches.rst`` and possibly
|
|
``Documentation/process/submitting-drivers.rst``.
|
|
|
|
Kernel Cantrips
|
|
===============
|
|
|
|
Some favorites from browsing the source. Feel free to add to this list.
|
|
|
|
``arch/x86/include/asm/delay.h``::
|
|
|
|
#define ndelay(n) (__builtin_constant_p(n) ? \
|
|
((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
|
|
__ndelay(n))
|
|
|
|
|
|
``include/linux/fs.h``::
|
|
|
|
/*
|
|
* Kernel pointers have redundant information, so we can use a
|
|
* scheme where we can return either an error code or a dentry
|
|
* pointer with the same return value.
|
|
*
|
|
* This should be a per-architecture thing, to allow different
|
|
* error and pointer decisions.
|
|
*/
|
|
#define ERR_PTR(err) ((void *)((long)(err)))
|
|
#define PTR_ERR(ptr) ((long)(ptr))
|
|
#define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000))
|
|
|
|
``arch/x86/include/asm/uaccess_32.h:``::
|
|
|
|
#define copy_to_user(to,from,n) \
|
|
(__builtin_constant_p(n) ? \
|
|
__constant_copy_to_user((to),(from),(n)) : \
|
|
__generic_copy_to_user((to),(from),(n)))
|
|
|
|
|
|
``arch/sparc/kernel/head.S:``::
|
|
|
|
/*
|
|
* Sun people can't spell worth damn. "compatability" indeed.
|
|
* At least we *know* we can't spell, and use a spell-checker.
|
|
*/
|
|
|
|
/* Uh, actually Linus it is I who cannot spell. Too much murky
|
|
* Sparc assembly will do this to ya.
|
|
*/
|
|
C_LABEL(cputypvar):
|
|
.asciz "compatibility"
|
|
|
|
/* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
|
|
.align 4
|
|
C_LABEL(cputypvar_sun4m):
|
|
.asciz "compatible"
|
|
|
|
|
|
``arch/sparc/lib/checksum.S:``::
|
|
|
|
/* Sun, you just can't beat me, you just can't. Stop trying,
|
|
* give up. I'm serious, I am going to kick the living shit
|
|
* out of you, game over, lights out.
|
|
*/
|
|
|
|
|
|
Thanks
|
|
======
|
|
|
|
Thanks to Andi Kleen for the idea, answering my questions, fixing my
|
|
mistakes, filling content, etc. Philipp Rumpf for more spelling and
|
|
clarity fixes, and some excellent non-obvious points. Werner Almesberger
|
|
for giving me a great summary of :c:func:`disable_irq()`, and Jes
|
|
Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain
|
|
for checking and adding to the Configure section. Telsa Gwynne for
|
|
teaching me DocBook.
|