mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-24 09:50:04 +00:00
![Mauro Carvalho Chehab](/assets/img/avatar_default.png)
The :doc:`foo` tag is auto-generated via automarkup.py. So, use the filename at the sources, instead of :doc:`foo`. Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org> Link: https://lore.kernel.org/r/095b04bff6d49b4097382398bb91102eaa3f0fd3.1623824363.git.mchehab+huawei@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
253 lines
10 KiB
ReStructuredText
253 lines
10 KiB
ReStructuredText
======================
|
|
ioctl based interfaces
|
|
======================
|
|
|
|
ioctl() is the most common way for applications to interface
|
|
with device drivers. It is flexible and easily extended by adding new
|
|
commands and can be passed through character devices, block devices as
|
|
well as sockets and other special file descriptors.
|
|
|
|
However, it is also very easy to get ioctl command definitions wrong,
|
|
and hard to fix them later without breaking existing applications,
|
|
so this documentation tries to help developers get it right.
|
|
|
|
Command number definitions
|
|
==========================
|
|
|
|
The command number, or request number, is the second argument passed to
|
|
the ioctl system call. While this can be any 32-bit number that uniquely
|
|
identifies an action for a particular driver, there are a number of
|
|
conventions around defining them.
|
|
|
|
``include/uapi/asm-generic/ioctl.h`` provides four macros for defining
|
|
ioctl commands that follow modern conventions: ``_IO``, ``_IOR``,
|
|
``_IOW``, and ``_IOWR``. These should be used for all new commands,
|
|
with the correct parameters:
|
|
|
|
_IO/_IOR/_IOW/_IOWR
|
|
The macro name specifies how the argument will be used. It may be a
|
|
pointer to data to be passed into the kernel (_IOW), out of the kernel
|
|
(_IOR), or both (_IOWR). _IO can indicate either commands with no
|
|
argument or those passing an integer value instead of a pointer.
|
|
It is recommended to only use _IO for commands without arguments,
|
|
and use pointers for passing data.
|
|
|
|
type
|
|
An 8-bit number, often a character literal, specific to a subsystem
|
|
or driver, and listed in Documentation/userspace-api/ioctl/ioctl-number.rst
|
|
|
|
nr
|
|
An 8-bit number identifying the specific command, unique for a give
|
|
value of 'type'
|
|
|
|
data_type
|
|
The name of the data type pointed to by the argument, the command number
|
|
encodes the ``sizeof(data_type)`` value in a 13-bit or 14-bit integer,
|
|
leading to a limit of 8191 bytes for the maximum size of the argument.
|
|
Note: do not pass sizeof(data_type) type into _IOR/_IOW/IOWR, as that
|
|
will lead to encoding sizeof(sizeof(data_type)), i.e. sizeof(size_t).
|
|
_IO does not have a data_type parameter.
|
|
|
|
|
|
Interface versions
|
|
==================
|
|
|
|
Some subsystems use version numbers in data structures to overload
|
|
commands with different interpretations of the argument.
|
|
|
|
This is generally a bad idea, since changes to existing commands tend
|
|
to break existing applications.
|
|
|
|
A better approach is to add a new ioctl command with a new number. The
|
|
old command still needs to be implemented in the kernel for compatibility,
|
|
but this can be a wrapper around the new implementation.
|
|
|
|
Return code
|
|
===========
|
|
|
|
ioctl commands can return negative error codes as documented in errno(3);
|
|
these get turned into errno values in user space. On success, the return
|
|
code should be zero. It is also possible but not recommended to return
|
|
a positive 'long' value.
|
|
|
|
When the ioctl callback is called with an unknown command number, the
|
|
handler returns either -ENOTTY or -ENOIOCTLCMD, which also results in
|
|
-ENOTTY being returned from the system call. Some subsystems return
|
|
-ENOSYS or -EINVAL here for historic reasons, but this is wrong.
|
|
|
|
Prior to Linux 5.5, compat_ioctl handlers were required to return
|
|
-ENOIOCTLCMD in order to use the fallback conversion into native
|
|
commands. As all subsystems are now responsible for handling compat
|
|
mode themselves, this is no longer needed, but it may be important to
|
|
consider when backporting bug fixes to older kernels.
|
|
|
|
Timestamps
|
|
==========
|
|
|
|
Traditionally, timestamps and timeout values are passed as ``struct
|
|
timespec`` or ``struct timeval``, but these are problematic because of
|
|
incompatible definitions of these structures in user space after the
|
|
move to 64-bit time_t.
|
|
|
|
The ``struct __kernel_timespec`` type can be used instead to be embedded
|
|
in other data structures when separate second/nanosecond values are
|
|
desired, or passed to user space directly. This is still not ideal though,
|
|
as the structure matches neither the kernel's timespec64 nor the user
|
|
space timespec exactly. The get_timespec64() and put_timespec64() helper
|
|
functions can be used to ensure that the layout remains compatible with
|
|
user space and the padding is treated correctly.
|
|
|
|
As it is cheap to convert seconds to nanoseconds, but the opposite
|
|
requires an expensive 64-bit division, a simple __u64 nanosecond value
|
|
can be simpler and more efficient.
|
|
|
|
Timeout values and timestamps should ideally use CLOCK_MONOTONIC time,
|
|
as returned by ktime_get_ns() or ktime_get_ts64(). Unlike
|
|
CLOCK_REALTIME, this makes the timestamps immune from jumping backwards
|
|
or forwards due to leap second adjustments and clock_settime() calls.
|
|
|
|
ktime_get_real_ns() can be used for CLOCK_REALTIME timestamps that
|
|
need to be persistent across a reboot or between multiple machines.
|
|
|
|
32-bit compat mode
|
|
==================
|
|
|
|
In order to support 32-bit user space running on a 64-bit machine, each
|
|
subsystem or driver that implements an ioctl callback handler must also
|
|
implement the corresponding compat_ioctl handler.
|
|
|
|
As long as all the rules for data structures are followed, this is as
|
|
easy as setting the .compat_ioctl pointer to a helper function such as
|
|
compat_ptr_ioctl() or blkdev_compat_ptr_ioctl().
|
|
|
|
compat_ptr()
|
|
------------
|
|
|
|
On the s390 architecture, 31-bit user space has ambiguous representations
|
|
for data pointers, with the upper bit being ignored. When running such
|
|
a process in compat mode, the compat_ptr() helper must be used to
|
|
clear the upper bit of a compat_uptr_t and turn it into a valid 64-bit
|
|
pointer. On other architectures, this macro only performs a cast to a
|
|
``void __user *`` pointer.
|
|
|
|
In an compat_ioctl() callback, the last argument is an unsigned long,
|
|
which can be interpreted as either a pointer or a scalar depending on
|
|
the command. If it is a scalar, then compat_ptr() must not be used, to
|
|
ensure that the 64-bit kernel behaves the same way as a 32-bit kernel
|
|
for arguments with the upper bit set.
|
|
|
|
The compat_ptr_ioctl() helper can be used in place of a custom
|
|
compat_ioctl file operation for drivers that only take arguments that
|
|
are pointers to compatible data structures.
|
|
|
|
Structure layout
|
|
----------------
|
|
|
|
Compatible data structures have the same layout on all architectures,
|
|
avoiding all problematic members:
|
|
|
|
* ``long`` and ``unsigned long`` are the size of a register, so
|
|
they can be either 32-bit or 64-bit wide and cannot be used in portable
|
|
data structures. Fixed-length replacements are ``__s32``, ``__u32``,
|
|
``__s64`` and ``__u64``.
|
|
|
|
* Pointers have the same problem, in addition to requiring the
|
|
use of compat_ptr(). The best workaround is to use ``__u64``
|
|
in place of pointers, which requires a cast to ``uintptr_t`` in user
|
|
space, and the use of u64_to_user_ptr() in the kernel to convert
|
|
it back into a user pointer.
|
|
|
|
* On the x86-32 (i386) architecture, the alignment of 64-bit variables
|
|
is only 32-bit, but they are naturally aligned on most other
|
|
architectures including x86-64. This means a structure like::
|
|
|
|
struct foo {
|
|
__u32 a;
|
|
__u64 b;
|
|
__u32 c;
|
|
};
|
|
|
|
has four bytes of padding between a and b on x86-64, plus another four
|
|
bytes of padding at the end, but no padding on i386, and it needs a
|
|
compat_ioctl conversion handler to translate between the two formats.
|
|
|
|
To avoid this problem, all structures should have their members
|
|
naturally aligned, or explicit reserved fields added in place of the
|
|
implicit padding. The ``pahole`` tool can be used for checking the
|
|
alignment.
|
|
|
|
* On ARM OABI user space, structures are padded to multiples of 32-bit,
|
|
making some structs incompatible with modern EABI kernels if they
|
|
do not end on a 32-bit boundary.
|
|
|
|
* On the m68k architecture, struct members are not guaranteed to have an
|
|
alignment greater than 16-bit, which is a problem when relying on
|
|
implicit padding.
|
|
|
|
* Bitfields and enums generally work as one would expect them to,
|
|
but some properties of them are implementation-defined, so it is better
|
|
to avoid them completely in ioctl interfaces.
|
|
|
|
* ``char`` members can be either signed or unsigned, depending on
|
|
the architecture, so the __u8 and __s8 types should be used for 8-bit
|
|
integer values, though char arrays are clearer for fixed-length strings.
|
|
|
|
Information leaks
|
|
=================
|
|
|
|
Uninitialized data must not be copied back to user space, as this can
|
|
cause an information leak, which can be used to defeat kernel address
|
|
space layout randomization (KASLR), helping in an attack.
|
|
|
|
For this reason (and for compat support) it is best to avoid any
|
|
implicit padding in data structures. Where there is implicit padding
|
|
in an existing structure, kernel drivers must be careful to fully
|
|
initialize an instance of the structure before copying it to user
|
|
space. This is usually done by calling memset() before assigning to
|
|
individual members.
|
|
|
|
Subsystem abstractions
|
|
======================
|
|
|
|
While some device drivers implement their own ioctl function, most
|
|
subsystems implement the same command for multiple drivers. Ideally the
|
|
subsystem has an .ioctl() handler that copies the arguments from and
|
|
to user space, passing them into subsystem specific callback functions
|
|
through normal kernel pointers.
|
|
|
|
This helps in various ways:
|
|
|
|
* Applications written for one driver are more likely to work for
|
|
another one in the same subsystem if there are no subtle differences
|
|
in the user space ABI.
|
|
|
|
* The complexity of user space access and data structure layout is done
|
|
in one place, reducing the potential for implementation bugs.
|
|
|
|
* It is more likely to be reviewed by experienced developers
|
|
that can spot problems in the interface when the ioctl is shared
|
|
between multiple drivers than when it is only used in a single driver.
|
|
|
|
Alternatives to ioctl
|
|
=====================
|
|
|
|
There are many cases in which ioctl is not the best solution for a
|
|
problem. Alternatives include:
|
|
|
|
* System calls are a better choice for a system-wide feature that
|
|
is not tied to a physical device or constrained by the file system
|
|
permissions of a character device node
|
|
|
|
* netlink is the preferred way of configuring any network related
|
|
objects through sockets.
|
|
|
|
* debugfs is used for ad-hoc interfaces for debugging functionality
|
|
that does not need to be exposed as a stable interface to applications.
|
|
|
|
* sysfs is a good way to expose the state of an in-kernel object
|
|
that is not tied to a file descriptor.
|
|
|
|
* configfs can be used for more complex configuration than sysfs
|
|
|
|
* A custom file system can provide extra flexibility with a simple
|
|
user interface but adds a lot of complexity to the implementation.
|