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1 | GETTING STARTED WITH KMEMCHECK |
2 | ============================== | |
3 | ||
4 | Vegard Nossum <vegardno@ifi.uio.no> | |
5 | ||
6 | ||
7 | Contents | |
8 | ======== | |
9 | 0. Introduction | |
10 | 1. Downloading | |
11 | 2. Configuring and compiling | |
12 | 3. How to use | |
13 | 3.1. Booting | |
14 | 3.2. Run-time enable/disable | |
15 | 3.3. Debugging | |
16 | 3.4. Annotating false positives | |
17 | 4. Reporting errors | |
18 | 5. Technical description | |
19 | ||
20 | ||
21 | 0. Introduction | |
22 | =============== | |
23 | ||
24 | kmemcheck is a debugging feature for the Linux Kernel. More specifically, it | |
25 | is a dynamic checker that detects and warns about some uses of uninitialized | |
26 | memory. | |
27 | ||
28 | Userspace programmers might be familiar with Valgrind's memcheck. The main | |
29 | difference between memcheck and kmemcheck is that memcheck works for userspace | |
30 | programs only, and kmemcheck works for the kernel only. The implementations | |
31 | are of course vastly different. Because of this, kmemcheck is not as accurate | |
32 | as memcheck, but it turns out to be good enough in practice to discover real | |
33 | programmer errors that the compiler is not able to find through static | |
34 | analysis. | |
35 | ||
36 | Enabling kmemcheck on a kernel will probably slow it down to the extent that | |
37 | the machine will not be usable for normal workloads such as e.g. an | |
38 | interactive desktop. kmemcheck will also cause the kernel to use about twice | |
39 | as much memory as normal. For this reason, kmemcheck is strictly a debugging | |
40 | feature. | |
41 | ||
42 | ||
43 | 1. Downloading | |
44 | ============== | |
45 | ||
46 | kmemcheck can only be downloaded using git. If you want to write patches | |
47 | against the current code, you should use the kmemcheck development branch of | |
48 | the tip tree. It is also possible to use the linux-next tree, which also | |
49 | includes the latest version of kmemcheck. | |
50 | ||
51 | Assuming that you've already cloned the linux-2.6.git repository, all you | |
52 | have 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 | ||
56 | To actually download the tree, fetch the remote: | |
57 | ||
58 | $ git fetch tip | |
59 | ||
60 | And to check out a new local branch with the kmemcheck code: | |
61 | ||
62 | $ git checkout -b kmemcheck tip/kmemcheck | |
63 | ||
64 | General instructions for the -tip tree can be found here: | |
65 | http://people.redhat.com/mingo/tip.git/readme.txt | |
66 | ||
67 | ||
68 | 2. Configuring and compiling | |
69 | ============================ | |
70 | ||
71 | kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of | |
72 | configuration variables must have specific settings in order for the kmemcheck | |
73 | menu 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 | ||
107 | In addition, I highly recommend turning on CONFIG_DEBUG_INFO=y. This is also | |
108 | located under "Kernel hacking". With this, you will be able to get line number | |
109 | information from the kmemcheck warnings, which is extremely valuable in | |
110 | debugging a problem. This option is not mandatory, however, because it slows | |
111 | down the compilation process and produces a much bigger kernel image. | |
112 | ||
113 | Now the kmemcheck menu should be visible (under "Kernel hacking" / "kmemcheck: | |
114 | trap use of uninitialized memory"). Here follows a description of the | |
115 | kmemcheck 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 | ||
203 | Now compile the kernel as usual. | |
204 | ||
205 | ||
206 | 3. How to use | |
207 | ============= | |
208 | ||
209 | 3.1. Booting | |
210 | ============ | |
211 | ||
212 | First some information about the command-line options. There is only one | |
213 | option specific to kmemcheck, and this is called "kmemcheck". It can be used | |
214 | to override the default mode as chosen by the CONFIG_KMEMCHECK_*_BY_DEFAULT | |
215 | option. Its possible settings are: | |
216 | ||
217 | o kmemcheck=0 (disabled) | |
218 | o kmemcheck=1 (enabled) | |
219 | o kmemcheck=2 (one-shot mode) | |
220 | ||
221 | If SLUB debugging has been enabled in the kernel, it may take precedence over | |
222 | kmemcheck in such a way that the slab caches which are under SLUB debugging | |
223 | will 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", | |
225 | like this: slub_debug=- | |
226 | ||
227 | In fact, this option may also be used for fine-grained control over SLUB vs. | |
228 | kmemcheck. For example, if the command line includes "kmemcheck=1 | |
229 | slub_debug=,dentry", then SLUB debugging will be used only for the "dentry" | |
230 | slab cache, and with kmemcheck tracking all the other caches. This is advanced | |
231 | usage, however, and is not generally recommended. | |
232 | ||
233 | ||
234 | 3.2. Run-time enable/disable | |
235 | ============================ | |
236 | ||
237 | When the kernel has booted, it is possible to enable or disable kmemcheck at | |
238 | run-time. WARNING: This feature is still experimental and may cause false | |
239 | positive warnings to appear. Therefore, try not to use this. If you find that | |
240 | it doesn't work properly (e.g. you see an unreasonable amount of warnings), I | |
241 | will be happy to take bug reports. | |
242 | ||
243 | Use the file /proc/sys/kernel/kmemcheck for this purpose, e.g.: | |
244 | ||
245 | $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck | |
246 | ||
247 | The numbers are the same as for the kmemcheck= command-line option. | |
248 | ||
249 | ||
250 | 3.3. Debugging | |
251 | ============== | |
252 | ||
253 | A typical report will look something like this: | |
254 | ||
255 | WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024) | |
256 | 80000000000000000000000000000000000000000088ffff0000000000000000 | |
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 | ||
260 | Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A | |
261 | RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190 | |
262 | RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002 | |
263 | RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009 | |
264 | RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84 | |
265 | RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000 | |
266 | R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e | |
267 | R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8 | |
268 | FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000 | |
269 | CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033 | |
270 | CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0 | |
271 | DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 | |
272 | DR3: 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 | ||
279 | The single most valuable information in this report is the RIP (or EIP on 32- | |
280 | bit) value. This will help us pinpoint exactly which instruction that caused | |
281 | the warning. | |
282 | ||
283 | If your kernel was compiled with CONFIG_DEBUG_INFO=y, then all we have to do | |
284 | is 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 | ||
292 | The "-e vmlinux" tells addr2line which file to look in. IMPORTANT: This must | |
293 | be the vmlinux of the kernel that produced the warning in the first place! If | |
294 | not, the line number information will almost certainly be wrong. | |
295 | ||
296 | The "-i" tells addr2line to also print the line numbers of inlined functions. | |
297 | In this case, the flag was very important, because otherwise, it would only | |
298 | have printed the first line, which is just a call to memcpy(), which could be | |
299 | called from a thousand places in the kernel, and is therefore not very useful. | |
300 | These inlined functions would not show up in the stack trace above, simply | |
301 | because the kernel doesn't load the extra debugging information. This | |
302 | technique can of course be used with ordinary kernel oopses as well. | |
303 | ||
304 | In this case, it's the caller of memcpy() that is interesting, and it can be | |
305 | found in include/asm-generic/siginfo.h, line 287: | |
306 | ||
307 | 281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from) | |
308 | 282 { | |
309 | 283 if (from->si_code < 0) | |
310 | 284 memcpy(to, from, sizeof(*to)); | |
311 | 285 else | |
312 | 286 /* _sigchld is currently the largest know union member */ | |
313 | 287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld)); | |
314 | 288 } | |
315 | ||
316 | Since this was a read (kmemcheck usually warns about reads only, though it can | |
317 | warn about writes to unallocated or freed memory as well), it was probably the | |
318 | "from" argument which contained some uninitialized bytes. Following the chain | |
319 | of calls, we move upwards to see where "from" was allocated or initialized, | |
320 | kernel/signal.c, line 380: | |
321 | ||
322 | 359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info) | |
323 | 360 { | |
324 | ... | |
325 | 367 list_for_each_entry(q, &list->list, list) { | |
326 | 368 if (q->info.si_signo == sig) { | |
327 | 369 if (first) | |
328 | 370 goto still_pending; | |
329 | 371 first = q; | |
330 | ... | |
331 | 377 if (first) { | |
332 | 378 still_pending: | |
333 | 379 list_del_init(&first->list); | |
334 | 380 copy_siginfo(info, &first->info); | |
335 | 381 __sigqueue_free(first); | |
336 | ... | |
337 | 392 } | |
338 | 393 } | |
339 | ||
340 | Here, it is &first->info that is being passed on to copy_siginfo(). The | |
341 | variable "first" was found on a list -- passed in as the second argument to | |
342 | collect_signal(). We continue our journey through the stack, to figure out | |
343 | where the item on "list" was allocated or initialized. We move to line 410: | |
344 | ||
345 | 395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask, | |
346 | 396 siginfo_t *info) | |
347 | 397 { | |
348 | ... | |
349 | 410 collect_signal(sig, pending, info); | |
350 | ... | |
351 | 414 } | |
352 | ||
353 | Now we need to follow the "pending" pointer, since that is being passed on to | |
354 | collect_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 | |
356 | kmemcheck 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 | ||
371 | Remember that since these addresses were found on the stack and not as the | |
372 | RIP value, they actually point to the _next_ instruction (they are return | |
373 | addresses). This becomes obvious when we look at the code for line 446: | |
374 | ||
375 | 422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info) | |
376 | 423 { | |
377 | ... | |
378 | 431 signr = __dequeue_signal(&tsk->signal->shared_pending, | |
379 | 432 mask, info); | |
380 | 433 /* | |
381 | 434 * itimer signal ? | |
382 | 435 * | |
383 | 436 * itimers are process shared and we restart periodic | |
384 | 437 * itimers in the signal delivery path to prevent DoS | |
385 | 438 * attacks in the high resolution timer case. This is | |
386 | 439 * compliant with the old way of self restarting | |
387 | 440 * itimers, as the SIGALRM is a legacy signal and only | |
388 | 441 * queued once. Changing the restart behaviour to | |
389 | 442 * restart the timer in the signal dequeue path is | |
390 | 443 * reducing the timer noise on heavy loaded !highres | |
391 | 444 * systems too. | |
392 | 445 */ | |
393 | 446 if (unlikely(signr == SIGALRM)) { | |
394 | ... | |
395 | 489 } | |
396 | ||
397 | So instead of looking at 446, we should be looking at 431, which is the line | |
398 | that executes just before 446. Here we see that what we are looking for is | |
399 | &tsk->signal->shared_pending. | |
400 | ||
401 | Our 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 | ||
410 | There were more results, but none of them were related to list operations, | |
411 | and these were the only assignments. We inspect the line numbers more closely | |
412 | and find that this is indeed where items are being added to the list: | |
413 | ||
414 | 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t, | |
415 | 817 int group) | |
416 | 818 { | |
417 | ... | |
418 | 828 pending = group ? &t->signal->shared_pending : &t->pending; | |
419 | ... | |
420 | 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN && | |
421 | 852 (is_si_special(info) || | |
422 | 853 info->si_code >= 0))); | |
423 | 854 if (q) { | |
424 | 855 list_add_tail(&q->list, &pending->list); | |
425 | ... | |
426 | 890 } | |
427 | ||
428 | and: | |
429 | ||
430 | 1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group) | |
431 | 1310 { | |
432 | .... | |
433 | 1339 pending = group ? &t->signal->shared_pending : &t->pending; | |
434 | 1340 list_add_tail(&q->list, &pending->list); | |
435 | .... | |
436 | 1347 } | |
437 | ||
438 | In the first case, the list element we are looking for, "q", is being returned | |
439 | from the function __sigqueue_alloc(), which looks like an allocation function. | |
440 | Let's take a look at it: | |
441 | ||
442 | 187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags, | |
443 | 188 int override_rlimit) | |
444 | 189 { | |
445 | 190 struct sigqueue *q = NULL; | |
446 | 191 struct user_struct *user; | |
447 | 192 | |
448 | 193 /* | |
449 | 194 * We won't get problems with the target's UID changing under us | |
450 | 195 * because changing it requires RCU be used, and if t != current, the | |
451 | 196 * caller must be holding the RCU readlock (by way of a spinlock) and | |
452 | 197 * we use RCU protection here | |
453 | 198 */ | |
454 | 199 user = get_uid(__task_cred(t)->user); | |
455 | 200 atomic_inc(&user->sigpending); | |
456 | 201 if (override_rlimit || | |
457 | 202 atomic_read(&user->sigpending) <= | |
458 | 203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur) | |
459 | 204 q = kmem_cache_alloc(sigqueue_cachep, flags); | |
460 | 205 if (unlikely(q == NULL)) { | |
461 | 206 atomic_dec(&user->sigpending); | |
462 | 207 free_uid(user); | |
463 | 208 } else { | |
464 | 209 INIT_LIST_HEAD(&q->list); | |
465 | 210 q->flags = 0; | |
466 | 211 q->user = user; | |
467 | 212 } | |
468 | 213 | |
469 | 214 return q; | |
470 | 215 } | |
471 | ||
472 | We see that this function initializes q->list, q->flags, and q->user. It seems | |
473 | that now is the time to look at the definition of "struct sigqueue", e.g.: | |
474 | ||
475 | 14 struct sigqueue { | |
476 | 15 struct list_head list; | |
477 | 16 int flags; | |
478 | 17 siginfo_t info; | |
479 | 18 struct user_struct *user; | |
480 | 19 }; | |
481 | ||
482 | And, you might remember, it was a memcpy() on &first->info that caused the | |
483 | warning, so this makes perfect sense. It also seems reasonable to assume that | |
484 | it is the caller of __sigqueue_alloc() that has the responsibility of filling | |
485 | out (initializing) this member. | |
486 | ||
487 | But just which fields of the struct were uninitialized? Let's look at | |
488 | kmemcheck's report again: | |
489 | ||
490 | WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024) | |
491 | 80000000000000000000000000000000000000000088ffff0000000000000000 | |
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 | ||
495 | These first two lines are the memory dump of the memory object itself, and the | |
496 | shadow 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 | |
498 | object itself! The position of the caret (^) corresponds with the address of | |
499 | the read (ffff88003e4a2024). | |
500 | ||
501 | The 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 | ||
510 | In order to figure out where (relative to the start of the object) the | |
511 | uninitialized memory was located, we have to look at the disassembly. For | |
512 | that, we'll need the RIP address again: | |
513 | ||
514 | RIP: 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 | ||
535 | As expected, it's the "rep movsl" instruction from the memcpy() that causes | |
536 | the warning. We know about REP MOVSL that it uses the register RCX to count | |
537 | the number of remaining iterations. By taking a look at the register dump | |
538 | again (from the kmemcheck report), we can figure out how many bytes were left | |
539 | to copy: | |
540 | ||
541 | RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009 | |
542 | ||
543 | By looking at the disassembly, we also see that %ecx is being loaded with the | |
544 | value $0xc just before (ffffffff8104edd8), so we are very lucky. Keep in mind | |
545 | that this is the number of iterations, not bytes. And since this is a "long" | |
546 | operation, we need to multiply by 4 to get the number of bytes. So this means | |
547 | that the uninitialized value was encountered at 4 * (0xc - 0x9) = 12 bytes | |
548 | from the start of the object. | |
549 | ||
550 | We can now try to figure out which field of the "struct siginfo" that was not | |
551 | initialized. This is the beginning of the struct: | |
552 | ||
553 | 40 typedef struct siginfo { | |
554 | 41 int si_signo; | |
555 | 42 int si_errno; | |
556 | 43 int si_code; | |
557 | 44 | |
558 | 45 union { | |
559 | .. | |
560 | 92 } _sifields; | |
561 | 93 } siginfo_t; | |
562 | ||
563 | On 64-bit, the int is 4 bytes long, so it must the the union member that has | |
564 | not 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 | ||
571 | Actually, it seems that the union member is located at offset 0x10 -- which | |
572 | means that gcc has inserted 4 bytes of padding between the members si_code | |
573 | and _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 | -------|-------|-------|-------| | |
581 | 80000000000000000000000000000000000000000088ffff0000000000000000 | |
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 | ||
584 | This allows us to realize another important fact: si_code contains the value | |
585 | 0x80. Remember that x86 is little endian, so the first 4 bytes "80000000" are | |
586 | really the number 0x00000080. With a bit of research, we find that this is | |
587 | actually the constant SI_KERNEL defined in include/asm-generic/siginfo.h: | |
588 | ||
589 | 144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */ | |
590 | ||
591 | This macro is used in exactly one place in the x86 kernel: In send_signal() | |
592 | in kernel/signal.c: | |
593 | ||
594 | 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t, | |
595 | 817 int group) | |
596 | 818 { | |
597 | ... | |
598 | 828 pending = group ? &t->signal->shared_pending : &t->pending; | |
599 | ... | |
600 | 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN && | |
601 | 852 (is_si_special(info) || | |
602 | 853 info->si_code >= 0))); | |
603 | 854 if (q) { | |
604 | 855 list_add_tail(&q->list, &pending->list); | |
605 | 856 switch ((unsigned long) info) { | |
606 | ... | |
607 | 865 case (unsigned long) SEND_SIG_PRIV: | |
608 | 866 q->info.si_signo = sig; | |
609 | 867 q->info.si_errno = 0; | |
610 | 868 q->info.si_code = SI_KERNEL; | |
611 | 869 q->info.si_pid = 0; | |
612 | 870 q->info.si_uid = 0; | |
613 | 871 break; | |
614 | ... | |
615 | 890 } | |
616 | ||
617 | Not only does this match with the .si_code member, it also matches the place | |
618 | we found earlier when looking for where siginfo_t objects are enqueued on the | |
619 | "shared_pending" list. | |
620 | ||
621 | So to sum up: It seems that it is the padding introduced by the compiler | |
622 | between two struct fields that is uninitialized, and this gets reported when | |
623 | we do a memcpy() on the struct. This means that we have identified a false | |
624 | positive warning. | |
625 | ||
626 | Normally, kmemcheck will not report uninitialized accesses in memcpy() calls | |
627 | when both the source and destination addresses are tracked. (Instead, we copy | |
628 | the shadow bytemap as well). In this case, the destination address clearly | |
629 | was 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 | ||
635 | And we clearly see that the destination siginfo object is located on the | |
636 | stack: | |
637 | ||
638 | 782 static void do_signal(struct pt_regs *regs) | |
639 | 783 { | |
640 | 784 struct k_sigaction ka; | |
641 | 785 siginfo_t info; | |
642 | ... | |
643 | 804 signr = get_signal_to_deliver(&info, &ka, regs, NULL); | |
644 | ... | |
645 | 854 } | |
646 | ||
647 | And this &info is what eventually gets passed to copy_siginfo() as the | |
648 | destination argument. | |
649 | ||
650 | Now, even though we didn't find an actual error here, the example is still a | |
651 | good one, because it shows how one would go about to find out what the report | |
652 | was all about. | |
653 | ||
654 | ||
655 | 3.4. Annotating false positives | |
656 | =============================== | |
657 | ||
658 | There are a few different ways to make annotations in the source code that | |
659 | will keep kmemcheck from checking and reporting certain allocations. Here | |
660 | they 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 | ||
712 | 4. Reporting errors | |
713 | =================== | |
714 | ||
715 | As we have seen, kmemcheck will produce false positive reports. Therefore, it | |
716 | is not very wise to blindly post kmemcheck warnings to mailing lists and | |
717 | maintainers. Instead, I encourage maintainers and developers to find errors | |
718 | in their own code. If you get a warning, you can try to work around it, try | |
719 | to figure out if it's a real error or not, or simply ignore it. Most | |
720 | developers know their own code and will quickly and efficiently determine the | |
721 | root cause of a kmemcheck report. This is therefore also the most efficient | |
722 | way to work with kmemcheck. | |
723 | ||
724 | That said, we (the kmemcheck maintainers) will always be on the lookout for | |
725 | false positives that we can annotate and silence. So whatever you find, | |
726 | please drop us a note privately! Kernel configs and steps to reproduce (if | |
727 | available) are of course a great help too. | |
728 | ||
729 | Happy hacking! | |
730 | ||
731 | ||
732 | 5. Technical description | |
733 | ======================== | |
734 | ||
735 | kmemcheck works by marking memory pages non-present. This means that whenever | |
736 | somebody attempts to access the page, a page fault is generated. The page | |
737 | fault handler notices that the page was in fact only hidden, and so it calls | |
738 | on the kmemcheck code to make further investigations. | |
739 | ||
740 | When the investigations are completed, kmemcheck "shows" the page by marking | |
741 | it present (as it would be under normal circumstances). This way, the | |
742 | interrupted code can continue as usual. | |
743 | ||
744 | But after the instruction has been executed, we should hide the page again, so | |
745 | that we can catch the next access too! Now kmemcheck makes use of a debugging | |
746 | feature of the processor, namely single-stepping. When the processor has | |
747 | finished the one instruction that generated the memory access, a debug | |
748 | exception is raised. From here, we simply hide the page again and continue | |
749 | execution, this time with the single-stepping feature turned off. | |
750 | ||
751 | kmemcheck requires some assistance from the memory allocator in order to work. | |
752 | The 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 | ||
764 | If a slab cache is set up using the SLAB_NOTRACK flag, it will never return | |
765 | memory that can take page faults because of kmemcheck. | |
766 | ||
767 | If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still | |
768 | request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags. | |
769 | This does not prevent the page faults from occurring, however, but marks the | |
770 | object in question as being initialized so that no warnings will ever be | |
771 | produced for this object. | |
772 | ||
773 | Currently, the SLAB and SLUB allocators are supported by kmemcheck. |