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1GETTING STARTED WITH KMEMCHECK
2==============================
3
4Vegard Nossum <vegardno@ifi.uio.no>
5
6
7Contents
8========
90. Introduction
101. Downloading
112. Configuring and compiling
123. How to use
133.1. Booting
143.2. Run-time enable/disable
153.3. Debugging
163.4. Annotating false positives
174. Reporting errors
185. Technical description
19
20
210. Introduction
22===============
23
24kmemcheck is a debugging feature for the Linux Kernel. More specifically, it
25is a dynamic checker that detects and warns about some uses of uninitialized
26memory.
27
28Userspace programmers might be familiar with Valgrind's memcheck. The main
29difference between memcheck and kmemcheck is that memcheck works for userspace
30programs only, and kmemcheck works for the kernel only. The implementations
31are of course vastly different. Because of this, kmemcheck is not as accurate
32as memcheck, but it turns out to be good enough in practice to discover real
33programmer errors that the compiler is not able to find through static
34analysis.
35
36Enabling kmemcheck on a kernel will probably slow it down to the extent that
37the machine will not be usable for normal workloads such as e.g. an
38interactive desktop. kmemcheck will also cause the kernel to use about twice
39as much memory as normal. For this reason, kmemcheck is strictly a debugging
40feature.
41
42
431. Downloading
44==============
45
46kmemcheck can only be downloaded using git. If you want to write patches
47against the current code, you should use the kmemcheck development branch of
48the tip tree. It is also possible to use the linux-next tree, which also
49includes the latest version of kmemcheck.
50
51Assuming that you've already cloned the linux-2.6.git repository, all you
52have to do is add the -tip tree as a remote, like this:
53
54 $ git remote add tip git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git
55
56To actually download the tree, fetch the remote:
57
58 $ git fetch tip
59
60And to check out a new local branch with the kmemcheck code:
61
62 $ git checkout -b kmemcheck tip/kmemcheck
63
64General instructions for the -tip tree can be found here:
65http://people.redhat.com/mingo/tip.git/readme.txt
66
67
682. Configuring and compiling
69============================
70
71kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of
72configuration variables must have specific settings in order for the kmemcheck
73menu to even appear in "menuconfig". These are:
74
75 o CONFIG_CC_OPTIMIZE_FOR_SIZE=n
76
77 This option is located under "General setup" / "Optimize for size".
78
79 Without this, gcc will use certain optimizations that usually lead to
80 false positive warnings from kmemcheck. An example of this is a 16-bit
81 field in a struct, where gcc may load 32 bits, then discard the upper
82 16 bits. kmemcheck sees only the 32-bit load, and may trigger a
83 warning for the upper 16 bits (if they're uninitialized).
84
85 o CONFIG_SLAB=y or CONFIG_SLUB=y
86
87 This option is located under "General setup" / "Choose SLAB
88 allocator".
89
90 o CONFIG_FUNCTION_TRACER=n
91
92 This option is located under "Kernel hacking" / "Tracers" / "Kernel
93 Function Tracer"
94
95 When function tracing is compiled in, gcc emits a call to another
96 function at the beginning of every function. This means that when the
97 page fault handler is called, the ftrace framework will be called
98 before kmemcheck has had a chance to handle the fault. If ftrace then
99 modifies memory that was tracked by kmemcheck, the result is an
100 endless recursive page fault.
101
102 o CONFIG_DEBUG_PAGEALLOC=n
103
104 This option is located under "Kernel hacking" / "Debug page memory
105 allocations".
106
107In addition, I highly recommend turning on CONFIG_DEBUG_INFO=y. This is also
108located under "Kernel hacking". With this, you will be able to get line number
109information from the kmemcheck warnings, which is extremely valuable in
110debugging a problem. This option is not mandatory, however, because it slows
111down the compilation process and produces a much bigger kernel image.
112
113Now the kmemcheck menu should be visible (under "Kernel hacking" / "kmemcheck:
114trap use of uninitialized memory"). Here follows a description of the
115kmemcheck configuration variables:
116
117 o CONFIG_KMEMCHECK
118
119 This must be enabled in order to use kmemcheck at all...
120
121 o CONFIG_KMEMCHECK_[DISABLED | ENABLED | ONESHOT]_BY_DEFAULT
122
123 This option controls the status of kmemcheck at boot-time. "Enabled"
124 will enable kmemcheck right from the start, "disabled" will boot the
125 kernel as normal (but with the kmemcheck code compiled in, so it can
126 be enabled at run-time after the kernel has booted), and "one-shot" is
127 a special mode which will turn kmemcheck off automatically after
128 detecting the first use of uninitialized memory.
129
130 If you are using kmemcheck to actively debug a problem, then you
131 probably want to choose "enabled" here.
132
133 The one-shot mode is mostly useful in automated test setups because it
134 can prevent floods of warnings and increase the chances of the machine
135 surviving in case something is really wrong. In other cases, the one-
136 shot mode could actually be counter-productive because it would turn
137 itself off at the very first error -- in the case of a false positive
138 too -- and this would come in the way of debugging the specific
139 problem you were interested in.
140
141 If you would like to use your kernel as normal, but with a chance to
142 enable kmemcheck in case of some problem, it might be a good idea to
143 choose "disabled" here. When kmemcheck is disabled, most of the run-
144 time overhead is not incurred, and the kernel will be almost as fast
145 as normal.
146
147 o CONFIG_KMEMCHECK_QUEUE_SIZE
148
149 Select the maximum number of error reports to store in an internal
150 (fixed-size) buffer. Since errors can occur virtually anywhere and in
151 any context, we need a temporary storage area which is guaranteed not
152 to generate any other page faults when accessed. The queue will be
153 emptied as soon as a tasklet may be scheduled. If the queue is full,
154 new error reports will be lost.
155
156 The default value of 64 is probably fine. If some code produces more
157 than 64 errors within an irqs-off section, then the code is likely to
158 produce many, many more, too, and these additional reports seldom give
159 any more information (the first report is usually the most valuable
160 anyway).
161
162 This number might have to be adjusted if you are not using serial
163 console or similar to capture the kernel log. If you are using the
164 "dmesg" command to save the log, then getting a lot of kmemcheck
165 warnings might overflow the kernel log itself, and the earlier reports
166 will get lost in that way instead. Try setting this to 10 or so on
167 such a setup.
168
169 o CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT
170
171 Select the number of shadow bytes to save along with each entry of the
172 error-report queue. These bytes indicate what parts of an allocation
173 are initialized, uninitialized, etc. and will be displayed when an
174 error is detected to help the debugging of a particular problem.
175
176 The number entered here is actually the logarithm of the number of
177 bytes that will be saved. So if you pick for example 5 here, kmemcheck
178 will save 2^5 = 32 bytes.
179
180 The default value should be fine for debugging most problems. It also
181 fits nicely within 80 columns.
182
183 o CONFIG_KMEMCHECK_PARTIAL_OK
184
185 This option (when enabled) works around certain GCC optimizations that
186 produce 32-bit reads from 16-bit variables where the upper 16 bits are
187 thrown away afterwards.
188
189 The default value (enabled) is recommended. This may of course hide
190 some real errors, but disabling it would probably produce a lot of
191 false positives.
192
193 o CONFIG_KMEMCHECK_BITOPS_OK
194
195 This option silences warnings that would be generated for bit-field
196 accesses where not all the bits are initialized at the same time. This
197 may also hide some real bugs.
198
199 This option is probably obsolete, or it should be replaced with
200 the kmemcheck-/bitfield-annotations for the code in question. The
201 default value is therefore fine.
202
203Now compile the kernel as usual.
204
205
2063. How to use
207=============
208
2093.1. Booting
210============
211
212First some information about the command-line options. There is only one
213option specific to kmemcheck, and this is called "kmemcheck". It can be used
214to override the default mode as chosen by the CONFIG_KMEMCHECK_*_BY_DEFAULT
215option. Its possible settings are:
216
217 o kmemcheck=0 (disabled)
218 o kmemcheck=1 (enabled)
219 o kmemcheck=2 (one-shot mode)
220
221If SLUB debugging has been enabled in the kernel, it may take precedence over
222kmemcheck in such a way that the slab caches which are under SLUB debugging
223will not be tracked by kmemcheck. In order to ensure that this doesn't happen
224(even though it shouldn't by default), use SLUB's boot option "slub_debug",
225like this: slub_debug=-
226
227In fact, this option may also be used for fine-grained control over SLUB vs.
228kmemcheck. For example, if the command line includes "kmemcheck=1
229slub_debug=,dentry", then SLUB debugging will be used only for the "dentry"
230slab cache, and with kmemcheck tracking all the other caches. This is advanced
231usage, however, and is not generally recommended.
232
233
2343.2. Run-time enable/disable
235============================
236
237When the kernel has booted, it is possible to enable or disable kmemcheck at
238run-time. WARNING: This feature is still experimental and may cause false
239positive warnings to appear. Therefore, try not to use this. If you find that
240it doesn't work properly (e.g. you see an unreasonable amount of warnings), I
241will be happy to take bug reports.
242
243Use the file /proc/sys/kernel/kmemcheck for this purpose, e.g.:
244
245 $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck
246
247The numbers are the same as for the kmemcheck= command-line option.
248
249
2503.3. Debugging
251==============
252
253A typical report will look something like this:
254
255WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
25680000000000000000000000000000000000000000088ffff0000000000000000
257 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
258 ^
259
260Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A
261RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
262RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002
263RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
264RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84
265RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000
266R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e
267R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8
268FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000
269CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
270CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0
271DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
272DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400
273 [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
274 [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
275 [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
276 [<ffffffff8100c7b5>] int_signal+0x12/0x17
277 [<ffffffffffffffff>] 0xffffffffffffffff
278
279The single most valuable information in this report is the RIP (or EIP on 32-
280bit) value. This will help us pinpoint exactly which instruction that caused
281the warning.
282
283If your kernel was compiled with CONFIG_DEBUG_INFO=y, then all we have to do
284is give this address to the addr2line program, like this:
285
286 $ addr2line -e vmlinux -i ffffffff8104ede8
287 arch/x86/include/asm/string_64.h:12
288 include/asm-generic/siginfo.h:287
289 kernel/signal.c:380
290 kernel/signal.c:410
291
292The "-e vmlinux" tells addr2line which file to look in. IMPORTANT: This must
293be the vmlinux of the kernel that produced the warning in the first place! If
294not, the line number information will almost certainly be wrong.
295
296The "-i" tells addr2line to also print the line numbers of inlined functions.
297In this case, the flag was very important, because otherwise, it would only
298have printed the first line, which is just a call to memcpy(), which could be
299called from a thousand places in the kernel, and is therefore not very useful.
300These inlined functions would not show up in the stack trace above, simply
301because the kernel doesn't load the extra debugging information. This
302technique can of course be used with ordinary kernel oopses as well.
303
304In this case, it's the caller of memcpy() that is interesting, and it can be
305found in include/asm-generic/siginfo.h, line 287:
306
307281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from)
308282 {
309283 if (from->si_code < 0)
310284 memcpy(to, from, sizeof(*to));
311285 else
312286 /* _sigchld is currently the largest know union member */
313287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld));
314288 }
315
316Since this was a read (kmemcheck usually warns about reads only, though it can
317warn about writes to unallocated or freed memory as well), it was probably the
318"from" argument which contained some uninitialized bytes. Following the chain
319of calls, we move upwards to see where "from" was allocated or initialized,
320kernel/signal.c, line 380:
321
322359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info)
323360 {
324...
325367 list_for_each_entry(q, &list->list, list) {
326368 if (q->info.si_signo == sig) {
327369 if (first)
328370 goto still_pending;
329371 first = q;
330...
331377 if (first) {
332378 still_pending:
333379 list_del_init(&first->list);
334380 copy_siginfo(info, &first->info);
335381 __sigqueue_free(first);
336...
337392 }
338393 }
339
340Here, it is &first->info that is being passed on to copy_siginfo(). The
341variable "first" was found on a list -- passed in as the second argument to
342collect_signal(). We continue our journey through the stack, to figure out
343where the item on "list" was allocated or initialized. We move to line 410:
344
345395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
346396 siginfo_t *info)
347397 {
348...
349410 collect_signal(sig, pending, info);
350...
351414 }
352
353Now we need to follow the "pending" pointer, since that is being passed on to
354collect_signal() as "list". At this point, we've run out of lines from the
355"addr2line" output. Not to worry, we just paste the next addresses from the
356kmemcheck stack dump, i.e.:
357
358 [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170
359 [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390
360 [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0
361 [<ffffffff8100c7b5>] int_signal+0x12/0x17
362
363 $ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \
364 ffffffff8100b87d ffffffff8100c7b5
365 kernel/signal.c:446
366 kernel/signal.c:1806
367 arch/x86/kernel/signal.c:805
368 arch/x86/kernel/signal.c:871
369 arch/x86/kernel/entry_64.S:694
370
371Remember that since these addresses were found on the stack and not as the
372RIP value, they actually point to the _next_ instruction (they are return
373addresses). This becomes obvious when we look at the code for line 446:
374
375422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
376423 {
377...
378431 signr = __dequeue_signal(&tsk->signal->shared_pending,
379432 mask, info);
380433 /*
381434 * itimer signal ?
382435 *
383436 * itimers are process shared and we restart periodic
384437 * itimers in the signal delivery path to prevent DoS
385438 * attacks in the high resolution timer case. This is
386439 * compliant with the old way of self restarting
387440 * itimers, as the SIGALRM is a legacy signal and only
388441 * queued once. Changing the restart behaviour to
389442 * restart the timer in the signal dequeue path is
390443 * reducing the timer noise on heavy loaded !highres
391444 * systems too.
392445 */
393446 if (unlikely(signr == SIGALRM)) {
394...
395489 }
396
397So instead of looking at 446, we should be looking at 431, which is the line
398that executes just before 446. Here we see that what we are looking for is
399&tsk->signal->shared_pending.
400
401Our next task is now to figure out which function that puts items on this
402"shared_pending" list. A crude, but efficient tool, is git grep:
403
404 $ git grep -n 'shared_pending' kernel/
405 ...
406 kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending;
407 kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending;
408 ...
409
410There were more results, but none of them were related to list operations,
411and these were the only assignments. We inspect the line numbers more closely
412and find that this is indeed where items are being added to the list:
413
414816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
415817 int group)
416818 {
417...
418828 pending = group ? &t->signal->shared_pending : &t->pending;
419...
420851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
421852 (is_si_special(info) ||
422853 info->si_code >= 0)));
423854 if (q) {
424855 list_add_tail(&q->list, &pending->list);
425...
426890 }
427
428and:
429
4301309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group)
4311310 {
432....
4331339 pending = group ? &t->signal->shared_pending : &t->pending;
4341340 list_add_tail(&q->list, &pending->list);
435....
4361347 }
437
438In the first case, the list element we are looking for, "q", is being returned
439from the function __sigqueue_alloc(), which looks like an allocation function.
440Let's take a look at it:
441
442187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
443188 int override_rlimit)
444189 {
445190 struct sigqueue *q = NULL;
446191 struct user_struct *user;
447192
448193 /*
449194 * We won't get problems with the target's UID changing under us
450195 * because changing it requires RCU be used, and if t != current, the
451196 * caller must be holding the RCU readlock (by way of a spinlock) and
452197 * we use RCU protection here
453198 */
454199 user = get_uid(__task_cred(t)->user);
455200 atomic_inc(&user->sigpending);
456201 if (override_rlimit ||
457202 atomic_read(&user->sigpending) <=
458203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
459204 q = kmem_cache_alloc(sigqueue_cachep, flags);
460205 if (unlikely(q == NULL)) {
461206 atomic_dec(&user->sigpending);
462207 free_uid(user);
463208 } else {
464209 INIT_LIST_HEAD(&q->list);
465210 q->flags = 0;
466211 q->user = user;
467212 }
468213
469214 return q;
470215 }
471
472We see that this function initializes q->list, q->flags, and q->user. It seems
473that now is the time to look at the definition of "struct sigqueue", e.g.:
474
47514 struct sigqueue {
47615 struct list_head list;
47716 int flags;
47817 siginfo_t info;
47918 struct user_struct *user;
48019 };
481
482And, you might remember, it was a memcpy() on &first->info that caused the
483warning, so this makes perfect sense. It also seems reasonable to assume that
484it is the caller of __sigqueue_alloc() that has the responsibility of filling
485out (initializing) this member.
486
487But just which fields of the struct were uninitialized? Let's look at
488kmemcheck's report again:
489
490WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024)
49180000000000000000000000000000000000000000088ffff0000000000000000
492 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
493 ^
494
495These first two lines are the memory dump of the memory object itself, and the
496shadow bytemap, respectively. The memory object itself is in this case
497&first->info. Just beware that the start of this dump is NOT the start of the
498object itself! The position of the caret (^) corresponds with the address of
499the read (ffff88003e4a2024).
500
501The shadow bytemap dump legend is as follows:
502
503 i - initialized
504 u - uninitialized
505 a - unallocated (memory has been allocated by the slab layer, but has not
506 yet been handed off to anybody)
507 f - freed (memory has been allocated by the slab layer, but has been freed
508 by the previous owner)
509
510In order to figure out where (relative to the start of the object) the
511uninitialized memory was located, we have to look at the disassembly. For
512that, we'll need the RIP address again:
513
514RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190
515
516 $ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8:
517 ffffffff8104edc8: mov %r8,0x8(%r8)
518 ffffffff8104edcc: test %r10d,%r10d
519 ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168>
520 ffffffff8104edd5: mov %rax,%rdx
521 ffffffff8104edd8: mov $0xc,%ecx
522 ffffffff8104eddd: mov %r13,%rdi
523 ffffffff8104ede0: mov $0x30,%eax
524 ffffffff8104ede5: mov %rdx,%rsi
525 ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi)
526 ffffffff8104edea: test $0x2,%al
527 ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0>
528 ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi)
529 ffffffff8104edf0: test $0x1,%al
530 ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5>
531 ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi)
532 ffffffff8104edf5: mov %r8,%rdi
533 ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free>
534
535As expected, it's the "rep movsl" instruction from the memcpy() that causes
536the warning. We know about REP MOVSL that it uses the register RCX to count
537the number of remaining iterations. By taking a look at the register dump
538again (from the kmemcheck report), we can figure out how many bytes were left
539to copy:
540
541RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009
542
543By looking at the disassembly, we also see that %ecx is being loaded with the
544value $0xc just before (ffffffff8104edd8), so we are very lucky. Keep in mind
545that this is the number of iterations, not bytes. And since this is a "long"
546operation, we need to multiply by 4 to get the number of bytes. So this means
547that the uninitialized value was encountered at 4 * (0xc - 0x9) = 12 bytes
548from the start of the object.
549
550We can now try to figure out which field of the "struct siginfo" that was not
551initialized. This is the beginning of the struct:
552
55340 typedef struct siginfo {
55441 int si_signo;
55542 int si_errno;
55643 int si_code;
55744
55845 union {
559..
56092 } _sifields;
56193 } siginfo_t;
562
563On 64-bit, the int is 4 bytes long, so it must the the union member that has
564not been initialized. We can verify this using gdb:
565
566 $ gdb vmlinux
567 ...
568 (gdb) p &((struct siginfo *) 0)->_sifields
569 $1 = (union {...} *) 0x10
570
571Actually, it seems that the union member is located at offset 0x10 -- which
572means that gcc has inserted 4 bytes of padding between the members si_code
573and _sifields. We can now get a fuller picture of the memory dump:
574
575 _----------------------------=> si_code
576 / _--------------------=> (padding)
577 | / _------------=> _sifields(._kill._pid)
578 | | / _----=> _sifields(._kill._uid)
579 | | | /
580-------|-------|-------|-------|
58180000000000000000000000000000000000000000088ffff0000000000000000
582 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
583
584This allows us to realize another important fact: si_code contains the value
5850x80. Remember that x86 is little endian, so the first 4 bytes "80000000" are
586really the number 0x00000080. With a bit of research, we find that this is
587actually the constant SI_KERNEL defined in include/asm-generic/siginfo.h:
588
589144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */
590
591This macro is used in exactly one place in the x86 kernel: In send_signal()
592in kernel/signal.c:
593
594816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
595817 int group)
596818 {
597...
598828 pending = group ? &t->signal->shared_pending : &t->pending;
599...
600851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN &&
601852 (is_si_special(info) ||
602853 info->si_code >= 0)));
603854 if (q) {
604855 list_add_tail(&q->list, &pending->list);
605856 switch ((unsigned long) info) {
606...
607865 case (unsigned long) SEND_SIG_PRIV:
608866 q->info.si_signo = sig;
609867 q->info.si_errno = 0;
610868 q->info.si_code = SI_KERNEL;
611869 q->info.si_pid = 0;
612870 q->info.si_uid = 0;
613871 break;
614...
615890 }
616
617Not only does this match with the .si_code member, it also matches the place
618we found earlier when looking for where siginfo_t objects are enqueued on the
619"shared_pending" list.
620
621So to sum up: It seems that it is the padding introduced by the compiler
622between two struct fields that is uninitialized, and this gets reported when
623we do a memcpy() on the struct. This means that we have identified a false
624positive warning.
625
626Normally, kmemcheck will not report uninitialized accesses in memcpy() calls
627when both the source and destination addresses are tracked. (Instead, we copy
628the shadow bytemap as well). In this case, the destination address clearly
629was not tracked. We can dig a little deeper into the stack trace from above:
630
631 arch/x86/kernel/signal.c:805
632 arch/x86/kernel/signal.c:871
633 arch/x86/kernel/entry_64.S:694
634
635And we clearly see that the destination siginfo object is located on the
636stack:
637
638782 static void do_signal(struct pt_regs *regs)
639783 {
640784 struct k_sigaction ka;
641785 siginfo_t info;
642...
643804 signr = get_signal_to_deliver(&info, &ka, regs, NULL);
644...
645854 }
646
647And this &info is what eventually gets passed to copy_siginfo() as the
648destination argument.
649
650Now, even though we didn't find an actual error here, the example is still a
651good one, because it shows how one would go about to find out what the report
652was all about.
653
654
6553.4. Annotating false positives
656===============================
657
658There are a few different ways to make annotations in the source code that
659will keep kmemcheck from checking and reporting certain allocations. Here
660they are:
661
662 o __GFP_NOTRACK_FALSE_POSITIVE
663
664 This flag can be passed to kmalloc() or kmem_cache_alloc() (therefore
665 also to other functions that end up calling one of these) to indicate
666 that the allocation should not be tracked because it would lead to
667 a false positive report. This is a "big hammer" way of silencing
668 kmemcheck; after all, even if the false positive pertains to
669 particular field in a struct, for example, we will now lose the
670 ability to find (real) errors in other parts of the same struct.
671
672 Example:
673
674 /* No warnings will ever trigger on accessing any part of x */
675 x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE);
676
677 o kmemcheck_bitfield_begin(name)/kmemcheck_bitfield_end(name) and
678 kmemcheck_annotate_bitfield(ptr, name)
679
680 The first two of these three macros can be used inside struct
681 definitions to signal, respectively, the beginning and end of a
682 bitfield. Additionally, this will assign the bitfield a name, which
683 is given as an argument to the macros.
684
685 Having used these markers, one can later use
686 kmemcheck_annotate_bitfield() at the point of allocation, to indicate
687 which parts of the allocation is part of a bitfield.
688
689 Example:
690
691 struct foo {
692 int x;
693
694 kmemcheck_bitfield_begin(flags);
695 int flag_a:1;
696 int flag_b:1;
697 kmemcheck_bitfield_end(flags);
698
699 int y;
700 };
701
702 struct foo *x = kmalloc(sizeof *x);
703
704 /* No warnings will trigger on accessing the bitfield of x */
705 kmemcheck_annotate_bitfield(x, flags);
706
707 Note that kmemcheck_annotate_bitfield() can be used even before the
708 return value of kmalloc() is checked -- in other words, passing NULL
709 as the first argument is legal (and will do nothing).
710
711
7124. Reporting errors
713===================
714
715As we have seen, kmemcheck will produce false positive reports. Therefore, it
716is not very wise to blindly post kmemcheck warnings to mailing lists and
717maintainers. Instead, I encourage maintainers and developers to find errors
718in their own code. If you get a warning, you can try to work around it, try
719to figure out if it's a real error or not, or simply ignore it. Most
720developers know their own code and will quickly and efficiently determine the
721root cause of a kmemcheck report. This is therefore also the most efficient
722way to work with kmemcheck.
723
724That said, we (the kmemcheck maintainers) will always be on the lookout for
725false positives that we can annotate and silence. So whatever you find,
726please drop us a note privately! Kernel configs and steps to reproduce (if
727available) are of course a great help too.
728
729Happy hacking!
730
731
7325. Technical description
733========================
734
735kmemcheck works by marking memory pages non-present. This means that whenever
736somebody attempts to access the page, a page fault is generated. The page
737fault handler notices that the page was in fact only hidden, and so it calls
738on the kmemcheck code to make further investigations.
739
740When the investigations are completed, kmemcheck "shows" the page by marking
741it present (as it would be under normal circumstances). This way, the
742interrupted code can continue as usual.
743
744But after the instruction has been executed, we should hide the page again, so
745that we can catch the next access too! Now kmemcheck makes use of a debugging
746feature of the processor, namely single-stepping. When the processor has
747finished the one instruction that generated the memory access, a debug
748exception is raised. From here, we simply hide the page again and continue
749execution, this time with the single-stepping feature turned off.
750
751kmemcheck requires some assistance from the memory allocator in order to work.
752The memory allocator needs to
753
754 1. Tell kmemcheck about newly allocated pages and pages that are about to
755 be freed. This allows kmemcheck to set up and tear down the shadow memory
756 for the pages in question. The shadow memory stores the status of each
757 byte in the allocation proper, e.g. whether it is initialized or
758 uninitialized.
759
760 2. Tell kmemcheck which parts of memory should be marked uninitialized.
761 There are actually a few more states, such as "not yet allocated" and
762 "recently freed".
763
764If a slab cache is set up using the SLAB_NOTRACK flag, it will never return
765memory that can take page faults because of kmemcheck.
766
767If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still
768request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags.
769This does not prevent the page faults from occurring, however, but marks the
770object in question as being initialized so that no warnings will ever be
771produced for this object.
772
773Currently, the SLAB and SLUB allocators are supported by kmemcheck.