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1Getting started with kmemcheck
2==============================
3
4Vegard Nossum <vegardno@ifi.uio.no>
5
6
7Introduction
8------------
9
10kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
11is a dynamic checker that detects and warns about some uses of uninitialized
12memory.
13
14Userspace programmers might be familiar with Valgrind's memcheck. The main
15difference between memcheck and kmemcheck is that memcheck works for userspace
16programs only, and kmemcheck works for the kernel only. The implementations
17are of course vastly different. Because of this, kmemcheck is not as accurate
18as memcheck, but it turns out to be good enough in practice to discover real
19programmer errors that the compiler is not able to find through static
20analysis.
21
22Enabling kmemcheck on a kernel will probably slow it down to the extent that
23the machine will not be usable for normal workloads such as e.g. an
24interactive desktop. kmemcheck will also cause the kernel to use about twice
25as much memory as normal. For this reason, kmemcheck is strictly a debugging
26feature.
27
28
29Downloading
30-----------
31
32As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel.
33
34
35Configuring and compiling
36-------------------------
37
38kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
39configuration variables must have specific settings in order for the kmemcheck
40menu to even appear in "menuconfig". These are:
41
42- ``CONFIG_CC_OPTIMIZE_FOR_SIZE=n``
43 This option is located under "General setup" / "Optimize for size".
44
45 Without this, gcc will use certain optimizations that usually lead to
46 false positive warnings from kmemcheck. An example of this is a 16-bit
47 field in a struct, where gcc may load 32 bits, then discard the upper
48 16 bits. kmemcheck sees only the 32-bit load, and may trigger a
49 warning for the upper 16 bits (if they're uninitialized).
50
51- ``CONFIG_SLAB=y`` or ``CONFIG_SLUB=y``
52 This option is located under "General setup" / "Choose SLAB
53 allocator".
54
55- ``CONFIG_FUNCTION_TRACER=n``
56 This option is located under "Kernel hacking" / "Tracers" / "Kernel
57 Function Tracer"
58
59 When function tracing is compiled in, gcc emits a call to another
60 function at the beginning of every function. This means that when the
61 page fault handler is called, the ftrace framework will be called
62 before kmemcheck has had a chance to handle the fault. If ftrace then
63 modifies memory that was tracked by kmemcheck, the result is an
64 endless recursive page fault.
65
66- ``CONFIG_DEBUG_PAGEALLOC=n``
67 This option is located under "Kernel hacking" / "Memory Debugging"
68 / "Debug page memory allocations".
69
70In addition, I highly recommend turning on ``CONFIG_DEBUG_INFO=y``. This is also
71located under "Kernel hacking". With this, you will be able to get line number
72information from the kmemcheck warnings, which is extremely valuable in
73debugging a problem. This option is not mandatory, however, because it slows
74down the compilation process and produces a much bigger kernel image.
75
76Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory
77Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows
78a description of the kmemcheck configuration variables:
79
80- ``CONFIG_KMEMCHECK``
81 This must be enabled in order to use kmemcheck at all...
82
83- ``CONFIG_KMEMCHECK_``[``DISABLED`` | ``ENABLED`` | ``ONESHOT``]``_BY_DEFAULT``
84 This option controls the status of kmemcheck at boot-time. "Enabled"
85 will enable kmemcheck right from the start, "disabled" will boot the
86 kernel as normal (but with the kmemcheck code compiled in, so it can
87 be enabled at run-time after the kernel has booted), and "one-shot" is
88 a special mode which will turn kmemcheck off automatically after
89 detecting the first use of uninitialized memory.
90
91 If you are using kmemcheck to actively debug a problem, then you
92 probably want to choose "enabled" here.
93
94 The one-shot mode is mostly useful in automated test setups because it
95 can prevent floods of warnings and increase the chances of the machine
96 surviving in case something is really wrong. In other cases, the one-
97 shot mode could actually be counter-productive because it would turn
98 itself off at the very first error -- in the case of a false positive
99 too -- and this would come in the way of debugging the specific
100 problem you were interested in.
101
102 If you would like to use your kernel as normal, but with a chance to
103 enable kmemcheck in case of some problem, it might be a good idea to
104 choose "disabled" here. When kmemcheck is disabled, most of the run-
105 time overhead is not incurred, and the kernel will be almost as fast
106 as normal.
107
108- ``CONFIG_KMEMCHECK_QUEUE_SIZE``
109 Select the maximum number of error reports to store in an internal
110 (fixed-size) buffer. Since errors can occur virtually anywhere and in
111 any context, we need a temporary storage area which is guaranteed not
112 to generate any other page faults when accessed. The queue will be
113 emptied as soon as a tasklet may be scheduled. If the queue is full,
114 new error reports will be lost.
115
116 The default value of 64 is probably fine. If some code produces more
117 than 64 errors within an irqs-off section, then the code is likely to
118 produce many, many more, too, and these additional reports seldom give
119 any more information (the first report is usually the most valuable
120 anyway).
121
122 This number might have to be adjusted if you are not using serial
123 console or similar to capture the kernel log. If you are using the
124 "dmesg" command to save the log, then getting a lot of kmemcheck
125 warnings might overflow the kernel log itself, and the earlier reports
126 will get lost in that way instead. Try setting this to 10 or so on
127 such a setup.
128
129- ``CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT``
130 Select the number of shadow bytes to save along with each entry of the
131 error-report queue. These bytes indicate what parts of an allocation
132 are initialized, uninitialized, etc. and will be displayed when an
133 error is detected to help the debugging of a particular problem.
134
135 The number entered here is actually the logarithm of the number of
136 bytes that will be saved. So if you pick for example 5 here, kmemcheck
137 will save 2^5 = 32 bytes.
138
139 The default value should be fine for debugging most problems. It also
140 fits nicely within 80 columns.
141
142- ``CONFIG_KMEMCHECK_PARTIAL_OK``
143 This option (when enabled) works around certain GCC optimizations that
144 produce 32-bit reads from 16-bit variables where the upper 16 bits are
145 thrown away afterwards.
146
147 The default value (enabled) is recommended. This may of course hide
148 some real errors, but disabling it would probably produce a lot of
149 false positives.
150
151- ``CONFIG_KMEMCHECK_BITOPS_OK``
152 This option silences warnings that would be generated for bit-field
153 accesses where not all the bits are initialized at the same time. This
154 may also hide some real bugs.
155
156 This option is probably obsolete, or it should be replaced with
157 the kmemcheck-/bitfield-annotations for the code in question. The
158 default value is therefore fine.
159
160Now compile the kernel as usual.
161
162
163How to use
164----------
165
166Booting
167~~~~~~~
168
169First some information about the command-line options. There is only one
170option specific to kmemcheck, and this is called "kmemcheck". It can be used
171to override the default mode as chosen by the ``CONFIG_KMEMCHECK_*_BY_DEFAULT``
172option. Its possible settings are:
173
174- ``kmemcheck=0`` (disabled)
175- ``kmemcheck=1`` (enabled)
176- ``kmemcheck=2`` (one-shot mode)
177
178If SLUB debugging has been enabled in the kernel, it may take precedence over
179kmemcheck in such a way that the slab caches which are under SLUB debugging
180will not be tracked by kmemcheck. In order to ensure that this doesn't happen
181(even though it shouldn't by default), use SLUB's boot option ``slub_debug``,
182like this: ``slub_debug=-``
183
184In fact, this option may also be used for fine-grained control over SLUB vs.
185kmemcheck. For example, if the command line includes
186``kmemcheck=1 slub_debug=,dentry``, then SLUB debugging will be used only
187for the "dentry" slab cache, and with kmemcheck tracking all the other
188caches. This is advanced usage, however, and is not generally recommended.
189
190
191Run-time enable/disable
192~~~~~~~~~~~~~~~~~~~~~~~
193
194When the kernel has booted, it is possible to enable or disable kmemcheck at
195run-time. WARNING: This feature is still experimental and may cause false
196positive warnings to appear. Therefore, try not to use this. If you find that
197it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
198will be happy to take bug reports.
199
200Use the file ``/proc/sys/kernel/kmemcheck`` for this purpose, e.g.::
201
202 $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
203
204The numbers are the same as for the ``kmemcheck=`` command-line option.
205
206
207Debugging
208~~~~~~~~~
209
210A typical report will look something like this::
211
212 WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
213 80000000000000000000000000000000000000000088ffff0000000000000000
214 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
215 ^
216
217 Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
218 RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
219 RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
220 RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
221 RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
222 RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
223 R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
224 R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
225 FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
226 CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
227 CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
228 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
229 DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
230 [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
231 [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
232 [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
233 [<ffffffff8100c7b5>] int_signal+0x12/0x17
234 [<ffffffffffffffff>] 0xffffffffffffffff
235
236The single most valuable information in this report is the RIP (or EIP on 32-
237bit) value. This will help us pinpoint exactly which instruction that caused
238the warning.
239
240If your kernel was compiled with ``CONFIG_DEBUG_INFO=y``, then all we have to do
241is give this address to the addr2line program, like this::
242
243 $ addr2line -e vmlinux -i ffffffff8104ede8
244 arch/x86/include/asm/string_64.h:12
245 include/asm-generic/siginfo.h:287
246 kernel/signal.c:380
247 kernel/signal.c:410
248
249The "``-e vmlinux``" tells addr2line which file to look in. **IMPORTANT:**
250This must be the vmlinux of the kernel that produced the warning in the
251first place! If not, the line number information will almost certainly be
252wrong.
253
254The "``-i``" tells addr2line to also print the line numbers of inlined
255functions. In this case, the flag was very important, because otherwise,
256it would only have printed the first line, which is just a call to
257``memcpy()``, which could be called from a thousand places in the kernel, and
258is therefore not very useful. These inlined functions would not show up in
259the stack trace above, simply because the kernel doesn't load the extra
260debugging information. This technique can of course be used with ordinary
261kernel oopses as well.
262
263In this case, it's the caller of ``memcpy()`` that is interesting, and it can be
264found in ``include/asm-generic/siginfo.h``, line 287::
265
266 281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
267 282 {
268 283 if (from->si_code < 0)
269 284 memcpy(to, from, sizeof(*to));
270 285 else
271 286 /* _sigchld is currently the largest know union member */
272 287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
273 288 }
274
275Since this was a read (kmemcheck usually warns about reads only, though it can
276warn about writes to unallocated or freed memory as well), it was probably the
277"from" argument which contained some uninitialized bytes. Following the chain
278of calls, we move upwards to see where "from" was allocated or initialized,
279``kernel/signal.c``, line 380::
280
281 359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
282 360 {
283 ...
284 367 list_for_each_entry(q, &list->list, list) {
285 368 if (q->info.si_signo == sig) {
286 369 if (first)
287 370 goto still_pending;
288 371 first = q;
289 ...
290 377 if (first) {
291 378 still_pending:
292 379 list_del_init(&first->list);
293 380 copy_siginfo(info, &first->info);
294 381 __sigqueue_free(first);
295 ...
296 392 }
297 393 }
298
299Here, it is ``&first->info`` that is being passed on to ``copy_siginfo()``. The
300variable ``first`` was found on a list -- passed in as the second argument to
301``collect_signal()``. We continue our journey through the stack, to figure out
302where the item on "list" was allocated or initialized. We move to line 410::
303
304 395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
305 396 siginfo_t *info)
306 397 {
307 ...
308 410 collect_signal(sig, pending, info);
309 ...
310 414 }
311
312Now we need to follow the ``pending`` pointer, since that is being passed on to
313``collect_signal()`` as ``list``. At this point, we've run out of lines from the
314"addr2line" output. Not to worry, we just paste the next addresses from the
315kmemcheck stack dump, i.e.::
316
317 [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
318 [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
319 [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
320 [<ffffffff8100c7b5>] int_signal+0x12/0x17
321
322 $ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
323 ffffffff8100b87d ffffffff8100c7b5
324 kernel/signal.c:446
325 kernel/signal.c:1806
326 arch/x86/kernel/signal.c:805
327 arch/x86/kernel/signal.c:871
328 arch/x86/kernel/entry_64.S:694
329
330Remember that since these addresses were found on the stack and not as the
331RIP value, they actually point to the _next_ instruction (they are return
332addresses). This becomes obvious when we look at the code for line 446::
333
334 422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
335 423 {
336 ...
337 431 signr = __dequeue_signal(&tsk->signal->shared_pending,
338 432 mask, info);
339 433 /*
340 434 * itimer signal ?
341 435 *
342 436 * itimers are process shared and we restart periodic
343 437 * itimers in the signal delivery path to prevent DoS
344 438 * attacks in the high resolution timer case. This is
345 439 * compliant with the old way of self restarting
346 440 * itimers, as the SIGALRM is a legacy signal and only
347 441 * queued once. Changing the restart behaviour to
348 442 * restart the timer in the signal dequeue path is
349 443 * reducing the timer noise on heavy loaded !highres
350 444 * systems too.
351 445 */
352 446 if (unlikely(signr == SIGALRM)) {
353 ...
354 489 }
355
356So instead of looking at 446, we should be looking at 431, which is the line
357that executes just before 446. Here we see that what we are looking for is
358``&tsk->signal->shared_pending``.
359
360Our next task is now to figure out which function that puts items on this
361``shared_pending`` list. A crude, but efficient tool, is ``git grep``::
362
363 $ git grep -n 'shared_pending' kernel/
364 ...
365 kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
366 kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
367 ...
368
369There were more results, but none of them were related to list operations,
370and these were the only assignments. We inspect the line numbers more closely
371and find that this is indeed where items are being added to the list::
372
373 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
374 817 int group)
375 818 {
376 ...
377 828 pending = group ? &t->signal->shared_pending : &t->pending;
378 ...
379 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
380 852 (is_si_special(info) ||
381 853 info->si_code >= 0)));
382 854 if (q) {
383 855 list_add_tail(&q->list, &pending->list);
384 ...
385 890 }
386
387and::
388
389 1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
390 1310 {
391 ....
392 1339 pending = group ? &t->signal->shared_pending : &t->pending;
393 1340 list_add_tail(&q->list, &pending->list);
394 ....
395 1347 }
396
397In the first case, the list element we are looking for, ``q``, is being
398returned from the function ``__sigqueue_alloc()``, which looks like an
399allocation function. Let's take a look at it::
400
401 187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
402 188 int override_rlimit)
403 189 {
404 190 struct sigqueue *q = NULL;
405 191 struct user_struct *user;
406 192
407 193 /*
408 194 * We won't get problems with the target's UID changing under us
409 195 * because changing it requires RCU be used, and if t != current, the
410 196 * caller must be holding the RCU readlock (by way of a spinlock) and
411 197 * we use RCU protection here
412 198 */
413 199 user = get_uid(__task_cred(t)->user);
414 200 atomic_inc(&user->sigpending);
415 201 if (override_rlimit ||
416 202 atomic_read(&user->sigpending) <=
417 203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
418 204 q = kmem_cache_alloc(sigqueue_cachep, flags);
419 205 if (unlikely(q == NULL)) {
420 206 atomic_dec(&user->sigpending);
421 207 free_uid(user);
422 208 } else {
423 209 INIT_LIST_HEAD(&q->list);
424 210 q->flags = 0;
425 211 q->user = user;
426 212 }
427 213
428 214 return q;
429 215 }
430
431We see that this function initializes ``q->list``, ``q->flags``, and
432``q->user``. It seems that now is the time to look at the definition of
433``struct sigqueue``, e.g.::
434
435 14 struct sigqueue {
436 15 struct list_head list;
437 16 int flags;
438 17 siginfo_t info;
439 18 struct user_struct *user;
440 19 };
441
442And, you might remember, it was a ``memcpy()`` on ``&first->info`` that
443caused the warning, so this makes perfect sense. It also seems reasonable
444to assume that it is the caller of ``__sigqueue_alloc()`` that has the
445responsibility of filling out (initializing) this member.
446
447But just which fields of the struct were uninitialized? Let's look at
448kmemcheck's report again::
449
450 WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
451 80000000000000000000000000000000000000000088ffff0000000000000000
452 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
453 ^
454
455These first two lines are the memory dump of the memory object itself, and
456the shadow bytemap, respectively. The memory object itself is in this case
457``&first->info``. Just beware that the start of this dump is NOT the start
458of the object itself! The position of the caret (^) corresponds with the
459address of the read (ffff88003e4a2024).
460
461The shadow bytemap dump legend is as follows:
462
463- i: initialized
464- u: uninitialized
465- a: unallocated (memory has been allocated by the slab layer, but has not
466 yet been handed off to anybody)
467- f: freed (memory has been allocated by the slab layer, but has been freed
468 by the previous owner)
469
470In order to figure out where (relative to the start of the object) the
471uninitialized memory was located, we have to look at the disassembly. For
472that, we'll need the RIP address again::
473
474 RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
475
476 $ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8:
477 ffffffff8104edc8: mov %r8,0x8(%r8)
478 ffffffff8104edcc: test %r10d,%r10d
479 ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168>
480 ffffffff8104edd5: mov %rax,%rdx
481 ffffffff8104edd8: mov $0xc,%ecx
482 ffffffff8104eddd: mov %r13,%rdi
483 ffffffff8104ede0: mov $0x30,%eax
484 ffffffff8104ede5: mov %rdx,%rsi
485 ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi)
486 ffffffff8104edea: test $0x2,%al
487 ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0>
488 ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi)
489 ffffffff8104edf0: test $0x1,%al
490 ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5>
491 ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi)
492 ffffffff8104edf5: mov %r8,%rdi
493 ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free>
494
495As expected, it's the "``rep movsl``" instruction from the ``memcpy()``
496that causes the warning. We know about ``REP MOVSL`` that it uses the register
497``RCX`` to count the number of remaining iterations. By taking a look at the
498register dump again (from the kmemcheck report), we can figure out how many
499bytes were left to copy::
500
501 RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
502
503By looking at the disassembly, we also see that ``%ecx`` is being loaded
504with the value ``$0xc`` just before (ffffffff8104edd8), so we are very
505lucky. Keep in mind that this is the number of iterations, not bytes. And
506since this is a "long" operation, we need to multiply by 4 to get the
507number of bytes. So this means that the uninitialized value was encountered
508at 4 * (0xc - 0x9) = 12 bytes from the start of the object.
509
510We can now try to figure out which field of the "``struct siginfo``" that
511was not initialized. This is the beginning of the struct::
512
513 40 typedef struct siginfo {
514 41 int si_signo;
515 42 int si_errno;
516 43 int si_code;
517 44
518 45 union {
519 ..
520 92 } _sifields;
521 93 } siginfo_t;
522
523On 64-bit, the int is 4 bytes long, so it must the union member that has
524not been initialized. We can verify this using gdb::
525
526 $ gdb vmlinux
527 ...
528 (gdb) p &((struct siginfo *) 0)->_sifields
529 $1 = (union {...} *) 0x10
530
531Actually, it seems that the union member is located at offset 0x10 -- which
532means that gcc has inserted 4 bytes of padding between the members ``si_code``
533and ``_sifields``. We can now get a fuller picture of the memory dump::
534
535 _----------------------------=> si_code
536 / _--------------------=> (padding)
537 | / _------------=> _sifields(._kill._pid)
538 | | / _----=> _sifields(._kill._uid)
539 | | | /
540 -------|-------|-------|-------|
541 80000000000000000000000000000000000000000088ffff0000000000000000
542 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
543
544This allows us to realize another important fact: ``si_code`` contains the
545value 0x80. Remember that x86 is little endian, so the first 4 bytes
546"80000000" are really the number 0x00000080. With a bit of research, we
547find that this is actually the constant ``SI_KERNEL`` defined in
548``include/asm-generic/siginfo.h``::
549
550 144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */
551
552This macro is used in exactly one place in the x86 kernel: In ``send_signal()``
553in ``kernel/signal.c``::
554
555 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
556 817 int group)
557 818 {
558 ...
559 828 pending = group ? &t->signal->shared_pending : &t->pending;
560 ...
561 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
562 852 (is_si_special(info) ||
563 853 info->si_code >= 0)));
564 854 if (q) {
565 855 list_add_tail(&q->list, &pending->list);
566 856 switch ((unsigned long) info) {
567 ...
568 865 case (unsigned long) SEND_SIG_PRIV:
569 866 q->info.si_signo = sig;
570 867 q->info.si_errno = 0;
571 868 q->info.si_code = SI_KERNEL;
572 869 q->info.si_pid = 0;
573 870 q->info.si_uid = 0;
574 871 break;
575 ...
576 890 }
577
578Not only does this match with the ``.si_code`` member, it also matches the place
579we found earlier when looking for where siginfo_t objects are enqueued on the
580``shared_pending`` list.
581
582So to sum up: It seems that it is the padding introduced by the compiler
583between two struct fields that is uninitialized, and this gets reported when
584we do a ``memcpy()`` on the struct. This means that we have identified a false
585positive warning.
586
587Normally, kmemcheck will not report uninitialized accesses in ``memcpy()`` calls
588when both the source and destination addresses are tracked. (Instead, we copy
589the shadow bytemap as well). In this case, the destination address clearly
590was not tracked. We can dig a little deeper into the stack trace from above::
591
592 arch/x86/kernel/signal.c:805
593 arch/x86/kernel/signal.c:871
594 arch/x86/kernel/entry_64.S:694
595
596And we clearly see that the destination siginfo object is located on the
597stack::
598
599 782 static void do_signal(struct pt_regs *regs)
600 783 {
601 784 struct k_sigaction ka;
602 785 siginfo_t info;
603 ...
604 804 signr = get_signal_to_deliver(&info, &ka, regs, NULL);
605 ...
606 854 }
607
608And this ``&info`` is what eventually gets passed to ``copy_siginfo()`` as the
609destination argument.
610
611Now, even though we didn't find an actual error here, the example is still a
612good one, because it shows how one would go about to find out what the report
613was all about.
614
615
616Annotating false positives
617~~~~~~~~~~~~~~~~~~~~~~~~~~
618
619There are a few different ways to make annotations in the source code that
620will keep kmemcheck from checking and reporting certain allocations. Here
621they are:
622
623- ``__GFP_NOTRACK_FALSE_POSITIVE``
624 This flag can be passed to ``kmalloc()`` or ``kmem_cache_alloc()``
625 (therefore also to other functions that end up calling one of
626 these) to indicate that the allocation should not be tracked
627 because it would lead to a false positive report. This is a "big
628 hammer" way of silencing kmemcheck; after all, even if the false
629 positive pertains to particular field in a struct, for example, we
630 will now lose the ability to find (real) errors in other parts of
631 the same struct.
632
633 Example::
634
635 /* No warnings will ever trigger on accessing any part of x */
636 x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE);
637
638- ``kmemcheck_bitfield_begin(name)``/``kmemcheck_bitfield_end(name)`` and
639 ``kmemcheck_annotate_bitfield(ptr, name)``
640 The first two of these three macros can be used inside struct
641 definitions to signal, respectively, the beginning and end of a
642 bitfield. Additionally, this will assign the bitfield a name, which
643 is given as an argument to the macros.
644
645 Having used these markers, one can later use
646 kmemcheck_annotate_bitfield() at the point of allocation, to indicate
647 which parts of the allocation is part of a bitfield.
648
649 Example::
650
651 struct foo {
652 int x;
653
654 kmemcheck_bitfield_begin(flags);
655 int flag_a:1;
656 int flag_b:1;
657 kmemcheck_bitfield_end(flags);
658
659 int y;
660 };
661
662 struct foo *x = kmalloc(sizeof *x);
663
664 /* No warnings will trigger on accessing the bitfield of x */
665 kmemcheck_annotate_bitfield(x, flags);
666
667 Note that ``kmemcheck_annotate_bitfield()`` can be used even before the
668 return value of ``kmalloc()`` is checked -- in other words, passing NULL
669 as the first argument is legal (and will do nothing).
670
671
672Reporting errors
673----------------
674
675As we have seen, kmemcheck will produce false positive reports. Therefore, it
676is not very wise to blindly post kmemcheck warnings to mailing lists and
677maintainers. Instead, I encourage maintainers and developers to find errors
678in their own code. If you get a warning, you can try to work around it, try
679to figure out if it's a real error or not, or simply ignore it. Most
680developers know their own code and will quickly and efficiently determine the
681root cause of a kmemcheck report. This is therefore also the most efficient
682way to work with kmemcheck.
683
684That said, we (the kmemcheck maintainers) will always be on the lookout for
685false positives that we can annotate and silence. So whatever you find,
686please drop us a note privately! Kernel configs and steps to reproduce (if
687available) are of course a great help too.
688
689Happy hacking!
690
691
692Technical description
693---------------------
694
695kmemcheck works by marking memory pages non-present. This means that whenever
696somebody attempts to access the page, a page fault is generated. The page
697fault handler notices that the page was in fact only hidden, and so it calls
698on the kmemcheck code to make further investigations.
699
700When the investigations are completed, kmemcheck "shows" the page by marking
701it present (as it would be under normal circumstances). This way, the
702interrupted code can continue as usual.
703
704But after the instruction has been executed, we should hide the page again, so
705that we can catch the next access too! Now kmemcheck makes use of a debugging
706feature of the processor, namely single-stepping. When the processor has
707finished the one instruction that generated the memory access, a debug
708exception is raised. From here, we simply hide the page again and continue
709execution, this time with the single-stepping feature turned off.
710
711kmemcheck requires some assistance from the memory allocator in order to work.
712The memory allocator needs to
713
714 1. Tell kmemcheck about newly allocated pages and pages that are about to
715 be freed. This allows kmemcheck to set up and tear down the shadow memory
716 for the pages in question. The shadow memory stores the status of each
717 byte in the allocation proper, e.g. whether it is initialized or
718 uninitialized.
719
720 2. Tell kmemcheck which parts of memory should be marked uninitialized.
721 There are actually a few more states, such as "not yet allocated" and
722 "recently freed".
723
724If a slab cache is set up using the SLAB_NOTRACK flag, it will never return
725memory that can take page faults because of kmemcheck.
726
727If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still
728request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags.
729This does not prevent the page faults from occurring, however, but marks the
730object in question as being initialized so that no warnings will ever be
731produced for this object.
732
733Currently, the SLAB and SLUB allocators are supported by kmemcheck.