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Cc: Vegard Nossum <vegardno@ifi.uio.no> Cc: Pekka Enberg <penberg@kernel.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
733 lines
30 KiB
ReStructuredText
733 lines
30 KiB
ReStructuredText
Getting started with kmemcheck
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==============================
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Vegard Nossum <vegardno@ifi.uio.no>
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Introduction
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------------
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kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
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is a dynamic checker that detects and warns about some uses of uninitialized
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memory.
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Userspace programmers might be familiar with Valgrind's memcheck. The main
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difference between memcheck and kmemcheck is that memcheck works for userspace
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programs only, and kmemcheck works for the kernel only. The implementations
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are of course vastly different. Because of this, kmemcheck is not as accurate
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as memcheck, but it turns out to be good enough in practice to discover real
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programmer errors that the compiler is not able to find through static
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analysis.
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Enabling kmemcheck on a kernel will probably slow it down to the extent that
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the machine will not be usable for normal workloads such as e.g. an
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interactive desktop. kmemcheck will also cause the kernel to use about twice
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as much memory as normal. For this reason, kmemcheck is strictly a debugging
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feature.
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Downloading
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-----------
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As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel.
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Configuring and compiling
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-------------------------
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kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
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configuration variables must have specific settings in order for the kmemcheck
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menu to even appear in "menuconfig". These are:
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- ``CONFIG_CC_OPTIMIZE_FOR_SIZE=n``
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This option is located under "General setup" / "Optimize for size".
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Without this, gcc will use certain optimizations that usually lead to
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false positive warnings from kmemcheck. An example of this is a 16-bit
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field in a struct, where gcc may load 32 bits, then discard the upper
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16 bits. kmemcheck sees only the 32-bit load, and may trigger a
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warning for the upper 16 bits (if they're uninitialized).
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- ``CONFIG_SLAB=y`` or ``CONFIG_SLUB=y``
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This option is located under "General setup" / "Choose SLAB
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allocator".
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- ``CONFIG_FUNCTION_TRACER=n``
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This option is located under "Kernel hacking" / "Tracers" / "Kernel
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Function Tracer"
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When function tracing is compiled in, gcc emits a call to another
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function at the beginning of every function. This means that when the
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page fault handler is called, the ftrace framework will be called
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before kmemcheck has had a chance to handle the fault. If ftrace then
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modifies memory that was tracked by kmemcheck, the result is an
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endless recursive page fault.
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- ``CONFIG_DEBUG_PAGEALLOC=n``
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This option is located under "Kernel hacking" / "Memory Debugging"
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/ "Debug page memory allocations".
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In addition, I highly recommend turning on ``CONFIG_DEBUG_INFO=y``. This is also
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located under "Kernel hacking". With this, you will be able to get line number
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information from the kmemcheck warnings, which is extremely valuable in
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debugging a problem. This option is not mandatory, however, because it slows
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down the compilation process and produces a much bigger kernel image.
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Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory
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Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows
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a description of the kmemcheck configuration variables:
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- ``CONFIG_KMEMCHECK``
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This must be enabled in order to use kmemcheck at all...
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- ``CONFIG_KMEMCHECK_``[``DISABLED`` | ``ENABLED`` | ``ONESHOT``]``_BY_DEFAULT``
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This option controls the status of kmemcheck at boot-time. "Enabled"
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will enable kmemcheck right from the start, "disabled" will boot the
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kernel as normal (but with the kmemcheck code compiled in, so it can
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be enabled at run-time after the kernel has booted), and "one-shot" is
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a special mode which will turn kmemcheck off automatically after
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detecting the first use of uninitialized memory.
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If you are using kmemcheck to actively debug a problem, then you
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probably want to choose "enabled" here.
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The one-shot mode is mostly useful in automated test setups because it
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can prevent floods of warnings and increase the chances of the machine
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surviving in case something is really wrong. In other cases, the one-
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shot mode could actually be counter-productive because it would turn
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itself off at the very first error -- in the case of a false positive
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too -- and this would come in the way of debugging the specific
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problem you were interested in.
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If you would like to use your kernel as normal, but with a chance to
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enable kmemcheck in case of some problem, it might be a good idea to
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choose "disabled" here. When kmemcheck is disabled, most of the run-
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time overhead is not incurred, and the kernel will be almost as fast
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as normal.
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- ``CONFIG_KMEMCHECK_QUEUE_SIZE``
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Select the maximum number of error reports to store in an internal
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(fixed-size) buffer. Since errors can occur virtually anywhere and in
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any context, we need a temporary storage area which is guaranteed not
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to generate any other page faults when accessed. The queue will be
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emptied as soon as a tasklet may be scheduled. If the queue is full,
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new error reports will be lost.
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The default value of 64 is probably fine. If some code produces more
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than 64 errors within an irqs-off section, then the code is likely to
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produce many, many more, too, and these additional reports seldom give
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any more information (the first report is usually the most valuable
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anyway).
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This number might have to be adjusted if you are not using serial
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console or similar to capture the kernel log. If you are using the
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"dmesg" command to save the log, then getting a lot of kmemcheck
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warnings might overflow the kernel log itself, and the earlier reports
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will get lost in that way instead. Try setting this to 10 or so on
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such a setup.
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- ``CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT``
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Select the number of shadow bytes to save along with each entry of the
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error-report queue. These bytes indicate what parts of an allocation
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are initialized, uninitialized, etc. and will be displayed when an
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error is detected to help the debugging of a particular problem.
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The number entered here is actually the logarithm of the number of
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bytes that will be saved. So if you pick for example 5 here, kmemcheck
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will save 2^5 = 32 bytes.
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The default value should be fine for debugging most problems. It also
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fits nicely within 80 columns.
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- ``CONFIG_KMEMCHECK_PARTIAL_OK``
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This option (when enabled) works around certain GCC optimizations that
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produce 32-bit reads from 16-bit variables where the upper 16 bits are
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thrown away afterwards.
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The default value (enabled) is recommended. This may of course hide
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some real errors, but disabling it would probably produce a lot of
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false positives.
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- ``CONFIG_KMEMCHECK_BITOPS_OK``
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This option silences warnings that would be generated for bit-field
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accesses where not all the bits are initialized at the same time. This
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may also hide some real bugs.
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This option is probably obsolete, or it should be replaced with
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the kmemcheck-/bitfield-annotations for the code in question. The
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default value is therefore fine.
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Now compile the kernel as usual.
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How to use
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----------
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Booting
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~~~~~~~
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First some information about the command-line options. There is only one
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option specific to kmemcheck, and this is called "kmemcheck". It can be used
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to override the default mode as chosen by the ``CONFIG_KMEMCHECK_*_BY_DEFAULT``
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option. Its possible settings are:
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- ``kmemcheck=0`` (disabled)
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- ``kmemcheck=1`` (enabled)
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- ``kmemcheck=2`` (one-shot mode)
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If SLUB debugging has been enabled in the kernel, it may take precedence over
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kmemcheck in such a way that the slab caches which are under SLUB debugging
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will not be tracked by kmemcheck. In order to ensure that this doesn't happen
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(even though it shouldn't by default), use SLUB's boot option ``slub_debug``,
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like this: ``slub_debug=-``
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In fact, this option may also be used for fine-grained control over SLUB vs.
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kmemcheck. For example, if the command line includes
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``kmemcheck=1 slub_debug=,dentry``, then SLUB debugging will be used only
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for the "dentry" slab cache, and with kmemcheck tracking all the other
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caches. This is advanced usage, however, and is not generally recommended.
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Run-time enable/disable
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~~~~~~~~~~~~~~~~~~~~~~~
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When the kernel has booted, it is possible to enable or disable kmemcheck at
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run-time. WARNING: This feature is still experimental and may cause false
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positive warnings to appear. Therefore, try not to use this. If you find that
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it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
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will be happy to take bug reports.
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Use the file ``/proc/sys/kernel/kmemcheck`` for this purpose, e.g.::
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$ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
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The numbers are the same as for the ``kmemcheck=`` command-line option.
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Debugging
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~~~~~~~~~
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A typical report will look something like this::
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WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
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80000000000000000000000000000000000000000088ffff0000000000000000
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i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
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^
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Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
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RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
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RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
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RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
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RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
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RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
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R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
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R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
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FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
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CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
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CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
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DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
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DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
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[<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
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[<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
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[<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
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[<ffffffff8100c7b5>] int_signal+0x12/0x17
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[<ffffffffffffffff>] 0xffffffffffffffff
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The single most valuable information in this report is the RIP (or EIP on 32-
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bit) value. This will help us pinpoint exactly which instruction that caused
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the warning.
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If your kernel was compiled with ``CONFIG_DEBUG_INFO=y``, then all we have to do
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is give this address to the addr2line program, like this::
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$ addr2line -e vmlinux -i ffffffff8104ede8
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arch/x86/include/asm/string_64.h:12
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include/asm-generic/siginfo.h:287
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kernel/signal.c:380
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kernel/signal.c:410
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The "``-e vmlinux``" tells addr2line which file to look in. **IMPORTANT:**
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This must be the vmlinux of the kernel that produced the warning in the
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first place! If not, the line number information will almost certainly be
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wrong.
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The "``-i``" tells addr2line to also print the line numbers of inlined
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functions. In this case, the flag was very important, because otherwise,
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it would only have printed the first line, which is just a call to
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``memcpy()``, which could be called from a thousand places in the kernel, and
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is therefore not very useful. These inlined functions would not show up in
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the stack trace above, simply because the kernel doesn't load the extra
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debugging information. This technique can of course be used with ordinary
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kernel oopses as well.
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In this case, it's the caller of ``memcpy()`` that is interesting, and it can be
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found in ``include/asm-generic/siginfo.h``, line 287::
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281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
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282 {
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283 if (from->si_code < 0)
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284 memcpy(to, from, sizeof(*to));
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285 else
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286 /* _sigchld is currently the largest know union member */
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287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
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288 }
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Since this was a read (kmemcheck usually warns about reads only, though it can
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warn about writes to unallocated or freed memory as well), it was probably the
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"from" argument which contained some uninitialized bytes. Following the chain
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of calls, we move upwards to see where "from" was allocated or initialized,
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``kernel/signal.c``, line 380::
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359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
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360 {
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...
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367 list_for_each_entry(q, &list->list, list) {
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368 if (q->info.si_signo == sig) {
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369 if (first)
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370 goto still_pending;
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371 first = q;
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...
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377 if (first) {
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378 still_pending:
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379 list_del_init(&first->list);
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380 copy_siginfo(info, &first->info);
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381 __sigqueue_free(first);
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...
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392 }
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393 }
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Here, it is ``&first->info`` that is being passed on to ``copy_siginfo()``. The
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variable ``first`` was found on a list -- passed in as the second argument to
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``collect_signal()``. We continue our journey through the stack, to figure out
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where the item on "list" was allocated or initialized. We move to line 410::
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395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
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396 siginfo_t *info)
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397 {
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...
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410 collect_signal(sig, pending, info);
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...
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414 }
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Now we need to follow the ``pending`` pointer, since that is being passed on to
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``collect_signal()`` as ``list``. At this point, we've run out of lines from the
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"addr2line" output. Not to worry, we just paste the next addresses from the
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kmemcheck stack dump, i.e.::
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[<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
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[<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
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[<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
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[<ffffffff8100c7b5>] int_signal+0x12/0x17
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$ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
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ffffffff8100b87d ffffffff8100c7b5
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kernel/signal.c:446
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kernel/signal.c:1806
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arch/x86/kernel/signal.c:805
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arch/x86/kernel/signal.c:871
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arch/x86/kernel/entry_64.S:694
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Remember that since these addresses were found on the stack and not as the
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RIP value, they actually point to the _next_ instruction (they are return
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addresses). This becomes obvious when we look at the code for line 446::
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422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
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423 {
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...
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431 signr = __dequeue_signal(&tsk->signal->shared_pending,
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432 mask, info);
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433 /*
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434 * itimer signal ?
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435 *
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436 * itimers are process shared and we restart periodic
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437 * itimers in the signal delivery path to prevent DoS
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438 * attacks in the high resolution timer case. This is
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439 * compliant with the old way of self restarting
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440 * itimers, as the SIGALRM is a legacy signal and only
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441 * queued once. Changing the restart behaviour to
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442 * restart the timer in the signal dequeue path is
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443 * reducing the timer noise on heavy loaded !highres
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444 * systems too.
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445 */
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446 if (unlikely(signr == SIGALRM)) {
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...
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489 }
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So instead of looking at 446, we should be looking at 431, which is the line
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that executes just before 446. Here we see that what we are looking for is
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``&tsk->signal->shared_pending``.
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Our next task is now to figure out which function that puts items on this
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``shared_pending`` list. A crude, but efficient tool, is ``git grep``::
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$ git grep -n 'shared_pending' kernel/
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...
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kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
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kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
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...
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There were more results, but none of them were related to list operations,
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and these were the only assignments. We inspect the line numbers more closely
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and find that this is indeed where items are being added to the list::
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816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
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817 int group)
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818 {
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...
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828 pending = group ? &t->signal->shared_pending : &t->pending;
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...
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851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
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852 (is_si_special(info) ||
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853 info->si_code >= 0)));
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854 if (q) {
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855 list_add_tail(&q->list, &pending->list);
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...
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890 }
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and::
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1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
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1310 {
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....
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1339 pending = group ? &t->signal->shared_pending : &t->pending;
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1340 list_add_tail(&q->list, &pending->list);
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....
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1347 }
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In the first case, the list element we are looking for, ``q``, is being
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returned from the function ``__sigqueue_alloc()``, which looks like an
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allocation function. Let's take a look at it::
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187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
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188 int override_rlimit)
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189 {
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190 struct sigqueue *q = NULL;
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191 struct user_struct *user;
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192
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193 /*
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194 * We won't get problems with the target's UID changing under us
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195 * because changing it requires RCU be used, and if t != current, the
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196 * caller must be holding the RCU readlock (by way of a spinlock) and
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197 * we use RCU protection here
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198 */
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199 user = get_uid(__task_cred(t)->user);
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200 atomic_inc(&user->sigpending);
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201 if (override_rlimit ||
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202 atomic_read(&user->sigpending) <=
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203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
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204 q = kmem_cache_alloc(sigqueue_cachep, flags);
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205 if (unlikely(q == NULL)) {
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206 atomic_dec(&user->sigpending);
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207 free_uid(user);
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208 } else {
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209 INIT_LIST_HEAD(&q->list);
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210 q->flags = 0;
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211 q->user = user;
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212 }
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213
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214 return q;
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215 }
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We see that this function initializes ``q->list``, ``q->flags``, and
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``q->user``. It seems that now is the time to look at the definition of
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``struct sigqueue``, e.g.::
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14 struct sigqueue {
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15 struct list_head list;
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16 int flags;
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17 siginfo_t info;
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18 struct user_struct *user;
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19 };
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And, you might remember, it was a ``memcpy()`` on ``&first->info`` that
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caused the warning, so this makes perfect sense. It also seems reasonable
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to assume that it is the caller of ``__sigqueue_alloc()`` that has the
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responsibility of filling out (initializing) this member.
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|
|
But just which fields of the struct were uninitialized? Let's look at
|
|
kmemcheck's report again::
|
|
|
|
WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
|
|
80000000000000000000000000000000000000000088ffff0000000000000000
|
|
i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
|
|
^
|
|
|
|
These first two lines are the memory dump of the memory object itself, and
|
|
the shadow bytemap, respectively. The memory object itself is in this case
|
|
``&first->info``. Just beware that the start of this dump is NOT the start
|
|
of the object itself! The position of the caret (^) corresponds with the
|
|
address of the read (ffff88003e4a2024).
|
|
|
|
The shadow bytemap dump legend is as follows:
|
|
|
|
- i: initialized
|
|
- u: uninitialized
|
|
- a: unallocated (memory has been allocated by the slab layer, but has not
|
|
yet been handed off to anybody)
|
|
- f: freed (memory has been allocated by the slab layer, but has been freed
|
|
by the previous owner)
|
|
|
|
In order to figure out where (relative to the start of the object) the
|
|
uninitialized memory was located, we have to look at the disassembly. For
|
|
that, we'll need the RIP address again::
|
|
|
|
RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
|
|
|
|
$ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8:
|
|
ffffffff8104edc8: mov %r8,0x8(%r8)
|
|
ffffffff8104edcc: test %r10d,%r10d
|
|
ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168>
|
|
ffffffff8104edd5: mov %rax,%rdx
|
|
ffffffff8104edd8: mov $0xc,%ecx
|
|
ffffffff8104eddd: mov %r13,%rdi
|
|
ffffffff8104ede0: mov $0x30,%eax
|
|
ffffffff8104ede5: mov %rdx,%rsi
|
|
ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi)
|
|
ffffffff8104edea: test $0x2,%al
|
|
ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0>
|
|
ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi)
|
|
ffffffff8104edf0: test $0x1,%al
|
|
ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5>
|
|
ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi)
|
|
ffffffff8104edf5: mov %r8,%rdi
|
|
ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free>
|
|
|
|
As expected, it's the "``rep movsl``" instruction from the ``memcpy()``
|
|
that causes the warning. We know about ``REP MOVSL`` that it uses the register
|
|
``RCX`` to count the number of remaining iterations. By taking a look at the
|
|
register dump again (from the kmemcheck report), we can figure out how many
|
|
bytes were left to copy::
|
|
|
|
RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
|
|
|
|
By looking at the disassembly, we also see that ``%ecx`` is being loaded
|
|
with the value ``$0xc`` just before (ffffffff8104edd8), so we are very
|
|
lucky. Keep in mind that this is the number of iterations, not bytes. And
|
|
since this is a "long" operation, we need to multiply by 4 to get the
|
|
number of bytes. So this means that the uninitialized value was encountered
|
|
at 4 * (0xc - 0x9) = 12 bytes from the start of the object.
|
|
|
|
We can now try to figure out which field of the "``struct siginfo``" that
|
|
was not initialized. This is the beginning of the struct::
|
|
|
|
40 typedef struct siginfo {
|
|
41 int si_signo;
|
|
42 int si_errno;
|
|
43 int si_code;
|
|
44
|
|
45 union {
|
|
..
|
|
92 } _sifields;
|
|
93 } siginfo_t;
|
|
|
|
On 64-bit, the int is 4 bytes long, so it must the union member that has
|
|
not been initialized. We can verify this using gdb::
|
|
|
|
$ gdb vmlinux
|
|
...
|
|
(gdb) p &((struct siginfo *) 0)->_sifields
|
|
$1 = (union {...} *) 0x10
|
|
|
|
Actually, it seems that the union member is located at offset 0x10 -- which
|
|
means that gcc has inserted 4 bytes of padding between the members ``si_code``
|
|
and ``_sifields``. We can now get a fuller picture of the memory dump::
|
|
|
|
_----------------------------=> si_code
|
|
/ _--------------------=> (padding)
|
|
| / _------------=> _sifields(._kill._pid)
|
|
| | / _----=> _sifields(._kill._uid)
|
|
| | | /
|
|
-------|-------|-------|-------|
|
|
80000000000000000000000000000000000000000088ffff0000000000000000
|
|
i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u
|
|
|
|
This allows us to realize another important fact: ``si_code`` contains the
|
|
value 0x80. Remember that x86 is little endian, so the first 4 bytes
|
|
"80000000" are really the number 0x00000080. With a bit of research, we
|
|
find that this is actually the constant ``SI_KERNEL`` defined in
|
|
``include/asm-generic/siginfo.h``::
|
|
|
|
144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */
|
|
|
|
This macro is used in exactly one place in the x86 kernel: In ``send_signal()``
|
|
in ``kernel/signal.c``::
|
|
|
|
816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
|
|
817 int group)
|
|
818 {
|
|
...
|
|
828 pending = group ? &t->signal->shared_pending : &t->pending;
|
|
...
|
|
851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
|
|
852 (is_si_special(info) ||
|
|
853 info->si_code >= 0)));
|
|
854 if (q) {
|
|
855 list_add_tail(&q->list, &pending->list);
|
|
856 switch ((unsigned long) info) {
|
|
...
|
|
865 case (unsigned long) SEND_SIG_PRIV:
|
|
866 q->info.si_signo = sig;
|
|
867 q->info.si_errno = 0;
|
|
868 q->info.si_code = SI_KERNEL;
|
|
869 q->info.si_pid = 0;
|
|
870 q->info.si_uid = 0;
|
|
871 break;
|
|
...
|
|
890 }
|
|
|
|
Not only does this match with the ``.si_code`` member, it also matches the place
|
|
we found earlier when looking for where siginfo_t objects are enqueued on the
|
|
``shared_pending`` list.
|
|
|
|
So to sum up: It seems that it is the padding introduced by the compiler
|
|
between two struct fields that is uninitialized, and this gets reported when
|
|
we do a ``memcpy()`` on the struct. This means that we have identified a false
|
|
positive warning.
|
|
|
|
Normally, kmemcheck will not report uninitialized accesses in ``memcpy()`` calls
|
|
when both the source and destination addresses are tracked. (Instead, we copy
|
|
the shadow bytemap as well). In this case, the destination address clearly
|
|
was not tracked. We can dig a little deeper into the stack trace from above::
|
|
|
|
arch/x86/kernel/signal.c:805
|
|
arch/x86/kernel/signal.c:871
|
|
arch/x86/kernel/entry_64.S:694
|
|
|
|
And we clearly see that the destination siginfo object is located on the
|
|
stack::
|
|
|
|
782 static void do_signal(struct pt_regs *regs)
|
|
783 {
|
|
784 struct k_sigaction ka;
|
|
785 siginfo_t info;
|
|
...
|
|
804 signr = get_signal_to_deliver(&info, &ka, regs, NULL);
|
|
...
|
|
854 }
|
|
|
|
And this ``&info`` is what eventually gets passed to ``copy_siginfo()`` as the
|
|
destination argument.
|
|
|
|
Now, even though we didn't find an actual error here, the example is still a
|
|
good one, because it shows how one would go about to find out what the report
|
|
was all about.
|
|
|
|
|
|
Annotating false positives
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
There are a few different ways to make annotations in the source code that
|
|
will keep kmemcheck from checking and reporting certain allocations. Here
|
|
they are:
|
|
|
|
- ``__GFP_NOTRACK_FALSE_POSITIVE``
|
|
This flag can be passed to ``kmalloc()`` or ``kmem_cache_alloc()``
|
|
(therefore also to other functions that end up calling one of
|
|
these) to indicate that the allocation should not be tracked
|
|
because it would lead to a false positive report. This is a "big
|
|
hammer" way of silencing kmemcheck; after all, even if the false
|
|
positive pertains to particular field in a struct, for example, we
|
|
will now lose the ability to find (real) errors in other parts of
|
|
the same struct.
|
|
|
|
Example::
|
|
|
|
/* No warnings will ever trigger on accessing any part of x */
|
|
x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE);
|
|
|
|
- ``kmemcheck_bitfield_begin(name)``/``kmemcheck_bitfield_end(name)`` and
|
|
``kmemcheck_annotate_bitfield(ptr, name)``
|
|
The first two of these three macros can be used inside struct
|
|
definitions to signal, respectively, the beginning and end of a
|
|
bitfield. Additionally, this will assign the bitfield a name, which
|
|
is given as an argument to the macros.
|
|
|
|
Having used these markers, one can later use
|
|
kmemcheck_annotate_bitfield() at the point of allocation, to indicate
|
|
which parts of the allocation is part of a bitfield.
|
|
|
|
Example::
|
|
|
|
struct foo {
|
|
int x;
|
|
|
|
kmemcheck_bitfield_begin(flags);
|
|
int flag_a:1;
|
|
int flag_b:1;
|
|
kmemcheck_bitfield_end(flags);
|
|
|
|
int y;
|
|
};
|
|
|
|
struct foo *x = kmalloc(sizeof *x);
|
|
|
|
/* No warnings will trigger on accessing the bitfield of x */
|
|
kmemcheck_annotate_bitfield(x, flags);
|
|
|
|
Note that ``kmemcheck_annotate_bitfield()`` can be used even before the
|
|
return value of ``kmalloc()`` is checked -- in other words, passing NULL
|
|
as the first argument is legal (and will do nothing).
|
|
|
|
|
|
Reporting errors
|
|
----------------
|
|
|
|
As we have seen, kmemcheck will produce false positive reports. Therefore, it
|
|
is not very wise to blindly post kmemcheck warnings to mailing lists and
|
|
maintainers. Instead, I encourage maintainers and developers to find errors
|
|
in their own code. If you get a warning, you can try to work around it, try
|
|
to figure out if it's a real error or not, or simply ignore it. Most
|
|
developers know their own code and will quickly and efficiently determine the
|
|
root cause of a kmemcheck report. This is therefore also the most efficient
|
|
way to work with kmemcheck.
|
|
|
|
That said, we (the kmemcheck maintainers) will always be on the lookout for
|
|
false positives that we can annotate and silence. So whatever you find,
|
|
please drop us a note privately! Kernel configs and steps to reproduce (if
|
|
available) are of course a great help too.
|
|
|
|
Happy hacking!
|
|
|
|
|
|
Technical description
|
|
---------------------
|
|
|
|
kmemcheck works by marking memory pages non-present. This means that whenever
|
|
somebody attempts to access the page, a page fault is generated. The page
|
|
fault handler notices that the page was in fact only hidden, and so it calls
|
|
on the kmemcheck code to make further investigations.
|
|
|
|
When the investigations are completed, kmemcheck "shows" the page by marking
|
|
it present (as it would be under normal circumstances). This way, the
|
|
interrupted code can continue as usual.
|
|
|
|
But after the instruction has been executed, we should hide the page again, so
|
|
that we can catch the next access too! Now kmemcheck makes use of a debugging
|
|
feature of the processor, namely single-stepping. When the processor has
|
|
finished the one instruction that generated the memory access, a debug
|
|
exception is raised. From here, we simply hide the page again and continue
|
|
execution, this time with the single-stepping feature turned off.
|
|
|
|
kmemcheck requires some assistance from the memory allocator in order to work.
|
|
The memory allocator needs to
|
|
|
|
1. Tell kmemcheck about newly allocated pages and pages that are about to
|
|
be freed. This allows kmemcheck to set up and tear down the shadow memory
|
|
for the pages in question. The shadow memory stores the status of each
|
|
byte in the allocation proper, e.g. whether it is initialized or
|
|
uninitialized.
|
|
|
|
2. Tell kmemcheck which parts of memory should be marked uninitialized.
|
|
There are actually a few more states, such as "not yet allocated" and
|
|
"recently freed".
|
|
|
|
If a slab cache is set up using the SLAB_NOTRACK flag, it will never return
|
|
memory that can take page faults because of kmemcheck.
|
|
|
|
If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still
|
|
request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags.
|
|
This does not prevent the page faults from occurring, however, but marks the
|
|
object in question as being initialized so that no warnings will ever be
|
|
produced for this object.
|
|
|
|
Currently, the SLAB and SLUB allocators are supported by kmemcheck.
|