mirror of
https://github.com/torvalds/linux.git
synced 2024-12-27 05:11:48 +00:00
9c296b46c6
Cc: Vegard Nossum <vegardno@ifi.uio.no> Cc: Pekka Enberg <penberg@kernel.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
734 lines
30 KiB
ReStructuredText
734 lines
30 KiB
ReStructuredText
Getting started with kmemcheck
|
|
==============================
|
|
|
|
Vegard Nossum <vegardno@ifi.uio.no>
|
|
|
|
|
|
Introduction
|
|
------------
|
|
|
|
kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
|
|
is a dynamic checker that detects and warns about some uses of uninitialized
|
|
memory.
|
|
|
|
Userspace programmers might be familiar with Valgrind's memcheck. The main
|
|
difference between memcheck and kmemcheck is that memcheck works for userspace
|
|
programs only, and kmemcheck works for the kernel only. The implementations
|
|
are of course vastly different. Because of this, kmemcheck is not as accurate
|
|
as memcheck, but it turns out to be good enough in practice to discover real
|
|
programmer errors that the compiler is not able to find through static
|
|
analysis.
|
|
|
|
Enabling kmemcheck on a kernel will probably slow it down to the extent that
|
|
the machine will not be usable for normal workloads such as e.g. an
|
|
interactive desktop. kmemcheck will also cause the kernel to use about twice
|
|
as much memory as normal. For this reason, kmemcheck is strictly a debugging
|
|
feature.
|
|
|
|
|
|
Downloading
|
|
-----------
|
|
|
|
As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel.
|
|
|
|
|
|
Configuring and compiling
|
|
-------------------------
|
|
|
|
kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
|
|
configuration variables must have specific settings in order for the kmemcheck
|
|
menu to even appear in "menuconfig". These are:
|
|
|
|
- ``CONFIG_CC_OPTIMIZE_FOR_SIZE=n``
|
|
This option is located under "General setup" / "Optimize for size".
|
|
|
|
Without this, gcc will use certain optimizations that usually lead to
|
|
false positive warnings from kmemcheck. An example of this is a 16-bit
|
|
field in a struct, where gcc may load 32 bits, then discard the upper
|
|
16 bits. kmemcheck sees only the 32-bit load, and may trigger a
|
|
warning for the upper 16 bits (if they're uninitialized).
|
|
|
|
- ``CONFIG_SLAB=y`` or ``CONFIG_SLUB=y``
|
|
This option is located under "General setup" / "Choose SLAB
|
|
allocator".
|
|
|
|
- ``CONFIG_FUNCTION_TRACER=n``
|
|
This option is located under "Kernel hacking" / "Tracers" / "Kernel
|
|
Function Tracer"
|
|
|
|
When function tracing is compiled in, gcc emits a call to another
|
|
function at the beginning of every function. This means that when the
|
|
page fault handler is called, the ftrace framework will be called
|
|
before kmemcheck has had a chance to handle the fault. If ftrace then
|
|
modifies memory that was tracked by kmemcheck, the result is an
|
|
endless recursive page fault.
|
|
|
|
- ``CONFIG_DEBUG_PAGEALLOC=n``
|
|
This option is located under "Kernel hacking" / "Memory Debugging"
|
|
/ "Debug page memory allocations".
|
|
|
|
In addition, I highly recommend turning on ``CONFIG_DEBUG_INFO=y``. This is also
|
|
located under "Kernel hacking". With this, you will be able to get line number
|
|
information from the kmemcheck warnings, which is extremely valuable in
|
|
debugging a problem. This option is not mandatory, however, because it slows
|
|
down the compilation process and produces a much bigger kernel image.
|
|
|
|
Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory
|
|
Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows
|
|
a description of the kmemcheck configuration variables:
|
|
|
|
- ``CONFIG_KMEMCHECK``
|
|
This must be enabled in order to use kmemcheck at all...
|
|
|
|
- ``CONFIG_KMEMCHECK_``[``DISABLED`` | ``ENABLED`` | ``ONESHOT``]``_BY_DEFAULT``
|
|
This option controls the status of kmemcheck at boot-time. "Enabled"
|
|
will enable kmemcheck right from the start, "disabled" will boot the
|
|
kernel as normal (but with the kmemcheck code compiled in, so it can
|
|
be enabled at run-time after the kernel has booted), and "one-shot" is
|
|
a special mode which will turn kmemcheck off automatically after
|
|
detecting the first use of uninitialized memory.
|
|
|
|
If you are using kmemcheck to actively debug a problem, then you
|
|
probably want to choose "enabled" here.
|
|
|
|
The one-shot mode is mostly useful in automated test setups because it
|
|
can prevent floods of warnings and increase the chances of the machine
|
|
surviving in case something is really wrong. In other cases, the one-
|
|
shot mode could actually be counter-productive because it would turn
|
|
itself off at the very first error -- in the case of a false positive
|
|
too -- and this would come in the way of debugging the specific
|
|
problem you were interested in.
|
|
|
|
If you would like to use your kernel as normal, but with a chance to
|
|
enable kmemcheck in case of some problem, it might be a good idea to
|
|
choose "disabled" here. When kmemcheck is disabled, most of the run-
|
|
time overhead is not incurred, and the kernel will be almost as fast
|
|
as normal.
|
|
|
|
- ``CONFIG_KMEMCHECK_QUEUE_SIZE``
|
|
Select the maximum number of error reports to store in an internal
|
|
(fixed-size) buffer. Since errors can occur virtually anywhere and in
|
|
any context, we need a temporary storage area which is guaranteed not
|
|
to generate any other page faults when accessed. The queue will be
|
|
emptied as soon as a tasklet may be scheduled. If the queue is full,
|
|
new error reports will be lost.
|
|
|
|
The default value of 64 is probably fine. If some code produces more
|
|
than 64 errors within an irqs-off section, then the code is likely to
|
|
produce many, many more, too, and these additional reports seldom give
|
|
any more information (the first report is usually the most valuable
|
|
anyway).
|
|
|
|
This number might have to be adjusted if you are not using serial
|
|
console or similar to capture the kernel log. If you are using the
|
|
"dmesg" command to save the log, then getting a lot of kmemcheck
|
|
warnings might overflow the kernel log itself, and the earlier reports
|
|
will get lost in that way instead. Try setting this to 10 or so on
|
|
such a setup.
|
|
|
|
- ``CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT``
|
|
Select the number of shadow bytes to save along with each entry of the
|
|
error-report queue. These bytes indicate what parts of an allocation
|
|
are initialized, uninitialized, etc. and will be displayed when an
|
|
error is detected to help the debugging of a particular problem.
|
|
|
|
The number entered here is actually the logarithm of the number of
|
|
bytes that will be saved. So if you pick for example 5 here, kmemcheck
|
|
will save 2^5 = 32 bytes.
|
|
|
|
The default value should be fine for debugging most problems. It also
|
|
fits nicely within 80 columns.
|
|
|
|
- ``CONFIG_KMEMCHECK_PARTIAL_OK``
|
|
This option (when enabled) works around certain GCC optimizations that
|
|
produce 32-bit reads from 16-bit variables where the upper 16 bits are
|
|
thrown away afterwards.
|
|
|
|
The default value (enabled) is recommended. This may of course hide
|
|
some real errors, but disabling it would probably produce a lot of
|
|
false positives.
|
|
|
|
- ``CONFIG_KMEMCHECK_BITOPS_OK``
|
|
This option silences warnings that would be generated for bit-field
|
|
accesses where not all the bits are initialized at the same time. This
|
|
may also hide some real bugs.
|
|
|
|
This option is probably obsolete, or it should be replaced with
|
|
the kmemcheck-/bitfield-annotations for the code in question. The
|
|
default value is therefore fine.
|
|
|
|
Now compile the kernel as usual.
|
|
|
|
|
|
How to use
|
|
----------
|
|
|
|
Booting
|
|
~~~~~~~
|
|
|
|
First some information about the command-line options. There is only one
|
|
option specific to kmemcheck, and this is called "kmemcheck". It can be used
|
|
to override the default mode as chosen by the ``CONFIG_KMEMCHECK_*_BY_DEFAULT``
|
|
option. Its possible settings are:
|
|
|
|
- ``kmemcheck=0`` (disabled)
|
|
- ``kmemcheck=1`` (enabled)
|
|
- ``kmemcheck=2`` (one-shot mode)
|
|
|
|
If SLUB debugging has been enabled in the kernel, it may take precedence over
|
|
kmemcheck in such a way that the slab caches which are under SLUB debugging
|
|
will not be tracked by kmemcheck. In order to ensure that this doesn't happen
|
|
(even though it shouldn't by default), use SLUB's boot option ``slub_debug``,
|
|
like this: ``slub_debug=-``
|
|
|
|
In fact, this option may also be used for fine-grained control over SLUB vs.
|
|
kmemcheck. For example, if the command line includes
|
|
``kmemcheck=1 slub_debug=,dentry``, then SLUB debugging will be used only
|
|
for the "dentry" slab cache, and with kmemcheck tracking all the other
|
|
caches. This is advanced usage, however, and is not generally recommended.
|
|
|
|
|
|
Run-time enable/disable
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
When the kernel has booted, it is possible to enable or disable kmemcheck at
|
|
run-time. WARNING: This feature is still experimental and may cause false
|
|
positive warnings to appear. Therefore, try not to use this. If you find that
|
|
it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
|
|
will be happy to take bug reports.
|
|
|
|
Use the file ``/proc/sys/kernel/kmemcheck`` for this purpose, e.g.::
|
|
|
|
$ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
|
|
|
|
The numbers are the same as for the ``kmemcheck=`` command-line option.
|
|
|
|
|
|
Debugging
|
|
~~~~~~~~~
|
|
|
|
A typical report will look something like this::
|
|
|
|
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
|
|
^
|
|
|
|
Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
|
|
RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
|
|
RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
|
|
RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
|
|
RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
|
|
RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
|
|
R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
|
|
R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
|
|
FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
|
|
CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
|
|
CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
|
|
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
|
|
DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
|
|
[<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
|
|
[<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
|
|
[<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
|
|
[<ffffffff8100c7b5>] int_signal+0x12/0x17
|
|
[<ffffffffffffffff>] 0xffffffffffffffff
|
|
|
|
The single most valuable information in this report is the RIP (or EIP on 32-
|
|
bit) value. This will help us pinpoint exactly which instruction that caused
|
|
the warning.
|
|
|
|
If your kernel was compiled with ``CONFIG_DEBUG_INFO=y``, then all we have to do
|
|
is give this address to the addr2line program, like this::
|
|
|
|
$ addr2line -e vmlinux -i ffffffff8104ede8
|
|
arch/x86/include/asm/string_64.h:12
|
|
include/asm-generic/siginfo.h:287
|
|
kernel/signal.c:380
|
|
kernel/signal.c:410
|
|
|
|
The "``-e vmlinux``" tells addr2line which file to look in. **IMPORTANT:**
|
|
This must be the vmlinux of the kernel that produced the warning in the
|
|
first place! If not, the line number information will almost certainly be
|
|
wrong.
|
|
|
|
The "``-i``" tells addr2line to also print the line numbers of inlined
|
|
functions. In this case, the flag was very important, because otherwise,
|
|
it would only have printed the first line, which is just a call to
|
|
``memcpy()``, which could be called from a thousand places in the kernel, and
|
|
is therefore not very useful. These inlined functions would not show up in
|
|
the stack trace above, simply because the kernel doesn't load the extra
|
|
debugging information. This technique can of course be used with ordinary
|
|
kernel oopses as well.
|
|
|
|
In this case, it's the caller of ``memcpy()`` that is interesting, and it can be
|
|
found in ``include/asm-generic/siginfo.h``, line 287::
|
|
|
|
281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
|
|
282 {
|
|
283 if (from->si_code < 0)
|
|
284 memcpy(to, from, sizeof(*to));
|
|
285 else
|
|
286 /* _sigchld is currently the largest know union member */
|
|
287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
|
|
288 }
|
|
|
|
Since this was a read (kmemcheck usually warns about reads only, though it can
|
|
warn about writes to unallocated or freed memory as well), it was probably the
|
|
"from" argument which contained some uninitialized bytes. Following the chain
|
|
of calls, we move upwards to see where "from" was allocated or initialized,
|
|
``kernel/signal.c``, line 380::
|
|
|
|
359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
|
|
360 {
|
|
...
|
|
367 list_for_each_entry(q, &list->list, list) {
|
|
368 if (q->info.si_signo == sig) {
|
|
369 if (first)
|
|
370 goto still_pending;
|
|
371 first = q;
|
|
...
|
|
377 if (first) {
|
|
378 still_pending:
|
|
379 list_del_init(&first->list);
|
|
380 copy_siginfo(info, &first->info);
|
|
381 __sigqueue_free(first);
|
|
...
|
|
392 }
|
|
393 }
|
|
|
|
Here, it is ``&first->info`` that is being passed on to ``copy_siginfo()``. The
|
|
variable ``first`` was found on a list -- passed in as the second argument to
|
|
``collect_signal()``. We continue our journey through the stack, to figure out
|
|
where the item on "list" was allocated or initialized. We move to line 410::
|
|
|
|
395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
|
|
396 siginfo_t *info)
|
|
397 {
|
|
...
|
|
410 collect_signal(sig, pending, info);
|
|
...
|
|
414 }
|
|
|
|
Now we need to follow the ``pending`` pointer, since that is being passed on to
|
|
``collect_signal()`` as ``list``. At this point, we've run out of lines from the
|
|
"addr2line" output. Not to worry, we just paste the next addresses from the
|
|
kmemcheck stack dump, i.e.::
|
|
|
|
[<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
|
|
[<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
|
|
[<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
|
|
[<ffffffff8100c7b5>] int_signal+0x12/0x17
|
|
|
|
$ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
|
|
ffffffff8100b87d ffffffff8100c7b5
|
|
kernel/signal.c:446
|
|
kernel/signal.c:1806
|
|
arch/x86/kernel/signal.c:805
|
|
arch/x86/kernel/signal.c:871
|
|
arch/x86/kernel/entry_64.S:694
|
|
|
|
Remember that since these addresses were found on the stack and not as the
|
|
RIP value, they actually point to the _next_ instruction (they are return
|
|
addresses). This becomes obvious when we look at the code for line 446::
|
|
|
|
422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
|
|
423 {
|
|
...
|
|
431 signr = __dequeue_signal(&tsk->signal->shared_pending,
|
|
432 mask, info);
|
|
433 /*
|
|
434 * itimer signal ?
|
|
435 *
|
|
436 * itimers are process shared and we restart periodic
|
|
437 * itimers in the signal delivery path to prevent DoS
|
|
438 * attacks in the high resolution timer case. This is
|
|
439 * compliant with the old way of self restarting
|
|
440 * itimers, as the SIGALRM is a legacy signal and only
|
|
441 * queued once. Changing the restart behaviour to
|
|
442 * restart the timer in the signal dequeue path is
|
|
443 * reducing the timer noise on heavy loaded !highres
|
|
444 * systems too.
|
|
445 */
|
|
446 if (unlikely(signr == SIGALRM)) {
|
|
...
|
|
489 }
|
|
|
|
So instead of looking at 446, we should be looking at 431, which is the line
|
|
that executes just before 446. Here we see that what we are looking for is
|
|
``&tsk->signal->shared_pending``.
|
|
|
|
Our next task is now to figure out which function that puts items on this
|
|
``shared_pending`` list. A crude, but efficient tool, is ``git grep``::
|
|
|
|
$ git grep -n 'shared_pending' kernel/
|
|
...
|
|
kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
|
|
kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
|
|
...
|
|
|
|
There were more results, but none of them were related to list operations,
|
|
and these were the only assignments. We inspect the line numbers more closely
|
|
and find that this is indeed where items are being added to the list::
|
|
|
|
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);
|
|
...
|
|
890 }
|
|
|
|
and::
|
|
|
|
1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
|
|
1310 {
|
|
....
|
|
1339 pending = group ? &t->signal->shared_pending : &t->pending;
|
|
1340 list_add_tail(&q->list, &pending->list);
|
|
....
|
|
1347 }
|
|
|
|
In the first case, the list element we are looking for, ``q``, is being
|
|
returned from the function ``__sigqueue_alloc()``, which looks like an
|
|
allocation function. Let's take a look at it::
|
|
|
|
187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
|
|
188 int override_rlimit)
|
|
189 {
|
|
190 struct sigqueue *q = NULL;
|
|
191 struct user_struct *user;
|
|
192
|
|
193 /*
|
|
194 * We won't get problems with the target's UID changing under us
|
|
195 * because changing it requires RCU be used, and if t != current, the
|
|
196 * caller must be holding the RCU readlock (by way of a spinlock) and
|
|
197 * we use RCU protection here
|
|
198 */
|
|
199 user = get_uid(__task_cred(t)->user);
|
|
200 atomic_inc(&user->sigpending);
|
|
201 if (override_rlimit ||
|
|
202 atomic_read(&user->sigpending) <=
|
|
203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
|
|
204 q = kmem_cache_alloc(sigqueue_cachep, flags);
|
|
205 if (unlikely(q == NULL)) {
|
|
206 atomic_dec(&user->sigpending);
|
|
207 free_uid(user);
|
|
208 } else {
|
|
209 INIT_LIST_HEAD(&q->list);
|
|
210 q->flags = 0;
|
|
211 q->user = user;
|
|
212 }
|
|
213
|
|
214 return q;
|
|
215 }
|
|
|
|
We see that this function initializes ``q->list``, ``q->flags``, and
|
|
``q->user``. It seems that now is the time to look at the definition of
|
|
``struct sigqueue``, e.g.::
|
|
|
|
14 struct sigqueue {
|
|
15 struct list_head list;
|
|
16 int flags;
|
|
17 siginfo_t info;
|
|
18 struct user_struct *user;
|
|
19 };
|
|
|
|
And, you might remember, it was a ``memcpy()`` on ``&first->info`` that
|
|
caused the warning, so this makes perfect sense. It also seems reasonable
|
|
to assume that it is the caller of ``__sigqueue_alloc()`` that has the
|
|
responsibility of filling out (initializing) this member.
|
|
|
|
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.
|