diff options
Diffstat (limited to 'Documentation/dev-tools/kmemcheck.rst')
| -rw-r--r-- | Documentation/dev-tools/kmemcheck.rst | 733 |
1 files changed, 0 insertions, 733 deletions
diff --git a/Documentation/dev-tools/kmemcheck.rst b/Documentation/dev-tools/kmemcheck.rst deleted file mode 100644 index 7f3d1985de74..000000000000 --- a/Documentation/dev-tools/kmemcheck.rst +++ /dev/null @@ -1,733 +0,0 @@ -Getting started with kmemcheck -============================== - -Vegard Nossum <vegardno@ifi.uio.no> - - -Introduction ------------- - -kmemcheck is a debugging feature for the Linux Kernel. More specifically, it -is a dynamic checker that detects and warns about some uses of uninitialized -memory. - -Userspace programmers might be familiar with Valgrind's memcheck. The main -difference between memcheck and kmemcheck is that memcheck works for userspace -programs only, and kmemcheck works for the kernel only. The implementations -are of course vastly different. Because of this, kmemcheck is not as accurate -as memcheck, but it turns out to be good enough in practice to discover real -programmer errors that the compiler is not able to find through static -analysis. - -Enabling kmemcheck on a kernel will probably slow it down to the extent that -the machine will not be usable for normal workloads such as e.g. an -interactive desktop. kmemcheck will also cause the kernel to use about twice -as much memory as normal. For this reason, kmemcheck is strictly a debugging -feature. - - -Downloading ------------ - -As of version 2.6.31-rc1, kmemcheck is included in the mainline kernel. - - -Configuring and compiling -------------------------- - -kmemcheck only works for the x86 (both 32- and 64-bit) platform. A number of -configuration variables must have specific settings in order for the kmemcheck -menu to even appear in "menuconfig". These are: - -- ``CONFIG_CC_OPTIMIZE_FOR_SIZE=n`` - This option is located under "General setup" / "Optimize for size". - - Without this, gcc will use certain optimizations that usually lead to - false positive warnings from kmemcheck. An example of this is a 16-bit - field in a struct, where gcc may load 32 bits, then discard the upper - 16 bits. kmemcheck sees only the 32-bit load, and may trigger a - warning for the upper 16 bits (if they're uninitialized). - -- ``CONFIG_SLAB=y`` or ``CONFIG_SLUB=y`` - This option is located under "General setup" / "Choose SLAB - allocator". - -- ``CONFIG_FUNCTION_TRACER=n`` - This option is located under "Kernel hacking" / "Tracers" / "Kernel - Function Tracer" - - When function tracing is compiled in, gcc emits a call to another - function at the beginning of every function. This means that when the - page fault handler is called, the ftrace framework will be called - before kmemcheck has had a chance to handle the fault. If ftrace then - modifies memory that was tracked by kmemcheck, the result is an - endless recursive page fault. - -- ``CONFIG_DEBUG_PAGEALLOC=n`` - This option is located under "Kernel hacking" / "Memory Debugging" - / "Debug page memory allocations". - -In addition, I highly recommend turning on ``CONFIG_DEBUG_INFO=y``. This is also -located under "Kernel hacking". With this, you will be able to get line number -information from the kmemcheck warnings, which is extremely valuable in -debugging a problem. This option is not mandatory, however, because it slows -down the compilation process and produces a much bigger kernel image. - -Now the kmemcheck menu should be visible (under "Kernel hacking" / "Memory -Debugging" / "kmemcheck: trap use of uninitialized memory"). Here follows -a description of the kmemcheck configuration variables: - -- ``CONFIG_KMEMCHECK`` - This must be enabled in order to use kmemcheck at all... - -- ``CONFIG_KMEMCHECK_``[``DISABLED`` | ``ENABLED`` | ``ONESHOT``]``_BY_DEFAULT`` - This option controls the status of kmemcheck at boot-time. "Enabled" - will enable kmemcheck right from the start, "disabled" will boot the - kernel as normal (but with the kmemcheck code compiled in, so it can - be enabled at run-time after the kernel has booted), and "one-shot" is - a special mode which will turn kmemcheck off automatically after - detecting the first use of uninitialized memory. - - If you are using kmemcheck to actively debug a problem, then you - probably want to choose "enabled" here. - - The one-shot mode is mostly useful in automated test setups because it - can prevent floods of warnings and increase the chances of the machine - surviving in case something is really wrong. In other cases, the one- - shot mode could actually be counter-productive because it would turn - itself off at the very first error -- in the case of a false positive - too -- and this would come in the way of debugging the specific - problem you were interested in. - - If you would like to use your kernel as normal, but with a chance to - enable kmemcheck in case of some problem, it might be a good idea to - choose "disabled" here. When kmemcheck is disabled, most of the run- - time overhead is not incurred, and the kernel will be almost as fast - as normal. - -- ``CONFIG_KMEMCHECK_QUEUE_SIZE`` - Select the maximum number of error reports to store in an internal - (fixed-size) buffer. Since errors can occur virtually anywhere and in - any context, we need a temporary storage area which is guaranteed not - to generate any other page faults when accessed. The queue will be - emptied as soon as a tasklet may be scheduled. If the queue is full, - new error reports will be lost. - - The default value of 64 is probably fine. If some code produces more - than 64 errors within an irqs-off section, then the code is likely to - produce many, many more, too, and these additional reports seldom give - any more information (the first report is usually the most valuable - anyway). - - This number might have to be adjusted if you are not using serial - console or similar to capture the kernel log. If you are using the - "dmesg" command to save the log, then getting a lot of kmemcheck - warnings might overflow the kernel log itself, and the earlier reports - will get lost in that way instead. Try setting this to 10 or so on - such a setup. - -- ``CONFIG_KMEMCHECK_SHADOW_COPY_SHIFT`` - Select the number of shadow bytes to save along with each entry of the - error-report queue. These bytes indicate what parts of an allocation - are initialized, uninitialized, etc. and will be displayed when an - error is detected to help the debugging of a particular problem. - - The number entered here is actually the logarithm of the number of - bytes that will be saved. So if you pick for example 5 here, kmemcheck - will save 2^5 = 32 bytes. - - The default value should be fine for debugging most problems. It also - fits nicely within 80 columns. - -- ``CONFIG_KMEMCHECK_PARTIAL_OK`` - This option (when enabled) works around certain GCC optimizations that - produce 32-bit reads from 16-bit variables where the upper 16 bits are - thrown away afterwards. - - The default value (enabled) is recommended. This may of course hide - some real errors, but disabling it would probably produce a lot of - false positives. - -- ``CONFIG_KMEMCHECK_BITOPS_OK`` - This option silences warnings that would be generated for bit-field - accesses where not all the bits are initialized at the same time. This - may also hide some real bugs. - - This option is probably obsolete, or it should be replaced with - the kmemcheck-/bitfield-annotations for the code in question. The - default value is therefore fine. - -Now compile the kernel as usual. - - -How to use ----------- - -Booting -~~~~~~~ - -First some information about the command-line options. There is only one -option specific to kmemcheck, and this is called "kmemcheck". It can be used -to override the default mode as chosen by the ``CONFIG_KMEMCHECK_*_BY_DEFAULT`` -option. Its possible settings are: - -- ``kmemcheck=0`` (disabled) -- ``kmemcheck=1`` (enabled) -- ``kmemcheck=2`` (one-shot mode) - -If SLUB debugging has been enabled in the kernel, it may take precedence over -kmemcheck in such a way that the slab caches which are under SLUB debugging -will not be tracked by kmemcheck. In order to ensure that this doesn't happen -(even though it shouldn't by default), use SLUB's boot option ``slub_debug``, -like this: ``slub_debug=-`` - -In fact, this option may also be used for fine-grained control over SLUB vs. -kmemcheck. For example, if the command line includes -``kmemcheck=1 slub_debug=,dentry``, then SLUB debugging will be used only -for the "dentry" slab cache, and with kmemcheck tracking all the other -caches. This is advanced usage, however, and is not generally recommended. - - -Run-time enable/disable -~~~~~~~~~~~~~~~~~~~~~~~ - -When the kernel has booted, it is possible to enable or disable kmemcheck at -run-time. WARNING: This feature is still experimental and may cause false -positive warnings to appear. Therefore, try not to use this. If you find that -it doesn't work properly (e.g. you see an unreasonable amount of warnings), I -will be happy to take bug reports. - -Use the file ``/proc/sys/kernel/kmemcheck`` for this purpose, e.g.:: - - $ echo 0 > /proc/sys/kernel/kmemcheck # disables kmemcheck - -The numbers are the same as for the ``kmemcheck=`` command-line option. - - -Debugging -~~~~~~~~~ - -A typical report will look something like this:: - - WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024) - 80000000000000000000000000000000000000000088ffff0000000000000000 - i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u - ^ - - Pid: 1856, comm: ntpdate Not tainted 2.6.29-rc5 #264 945P-A - RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190 - RSP: 0018:ffff88003cdf7d98 EFLAGS: 00210002 - RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009 - RDX: ffff88003e5d6018 RSI: ffff88003e5d6024 RDI: ffff88003cdf7e84 - RBP: ffff88003cdf7db8 R08: ffff88003e5d6000 R09: 0000000000000000 - R10: 0000000000000080 R11: 0000000000000000 R12: 000000000000000e - R13: ffff88003cdf7e78 R14: ffff88003d530710 R15: ffff88003d5a98c8 - FS: 0000000000000000(0000) GS:ffff880001982000(0063) knlGS:00000 - CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033 - CR2: ffff88003f806ea0 CR3: 000000003c036000 CR4: 00000000000006a0 - DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 - DR3: 0000000000000000 DR6: 00000000ffff4ff0 DR7: 0000000000000400 - [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170 - [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390 - [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0 - [<ffffffff8100c7b5>] int_signal+0x12/0x17 - [<ffffffffffffffff>] 0xffffffffffffffff - -The single most valuable information in this report is the RIP (or EIP on 32- -bit) value. This will help us pinpoint exactly which instruction that caused -the warning. - -If your kernel was compiled with ``CONFIG_DEBUG_INFO=y``, then all we have to do -is give this address to the addr2line program, like this:: - - $ addr2line -e vmlinux -i ffffffff8104ede8 - arch/x86/include/asm/string_64.h:12 - include/asm-generic/siginfo.h:287 - kernel/signal.c:380 - kernel/signal.c:410 - -The "``-e vmlinux``" tells addr2line which file to look in. **IMPORTANT:** -This must be the vmlinux of the kernel that produced the warning in the -first place! If not, the line number information will almost certainly be -wrong. - -The "``-i``" tells addr2line to also print the line numbers of inlined -functions. In this case, the flag was very important, because otherwise, -it would only have printed the first line, which is just a call to -``memcpy()``, which could be called from a thousand places in the kernel, and -is therefore not very useful. These inlined functions would not show up in -the stack trace above, simply because the kernel doesn't load the extra -debugging information. This technique can of course be used with ordinary -kernel oopses as well. - -In this case, it's the caller of ``memcpy()`` that is interesting, and it can be -found in ``include/asm-generic/siginfo.h``, line 287:: - - 281 static inline void copy_siginfo(struct siginfo *to, struct siginfo *from) - 282 { - 283 if (from->si_code < 0) - 284 memcpy(to, from, sizeof(*to)); - 285 else - 286 /* _sigchld is currently the largest know union member */ - 287 memcpy(to, from, __ARCH_SI_PREAMBLE_SIZE + sizeof(from->_sifields._sigchld)); - 288 } - -Since this was a read (kmemcheck usually warns about reads only, though it can -warn about writes to unallocated or freed memory as well), it was probably the -"from" argument which contained some uninitialized bytes. Following the chain -of calls, we move upwards to see where "from" was allocated or initialized, -``kernel/signal.c``, line 380:: - - 359 static void collect_signal(int sig, struct sigpending *list, siginfo_t *info) - 360 { - ... - 367 list_for_each_entry(q, &list->list, list) { - 368 if (q->info.si_signo == sig) { - 369 if (first) - 370 goto still_pending; - 371 first = q; - ... - 377 if (first) { - 378 still_pending: - 379 list_del_init(&first->list); - 380 copy_siginfo(info, &first->info); - 381 __sigqueue_free(first); - ... - 392 } - 393 } - -Here, it is ``&first->info`` that is being passed on to ``copy_siginfo()``. The -variable ``first`` was found on a list -- passed in as the second argument to -``collect_signal()``. We continue our journey through the stack, to figure out -where the item on "list" was allocated or initialized. We move to line 410:: - - 395 static int __dequeue_signal(struct sigpending *pending, sigset_t *mask, - 396 siginfo_t *info) - 397 { - ... - 410 collect_signal(sig, pending, info); - ... - 414 } - -Now we need to follow the ``pending`` pointer, since that is being passed on to -``collect_signal()`` as ``list``. At this point, we've run out of lines from the -"addr2line" output. Not to worry, we just paste the next addresses from the -kmemcheck stack dump, i.e.:: - - [<ffffffff8104f04e>] dequeue_signal+0x8e/0x170 - [<ffffffff81050bd8>] get_signal_to_deliver+0x98/0x390 - [<ffffffff8100b87d>] do_notify_resume+0xad/0x7d0 - [<ffffffff8100c7b5>] int_signal+0x12/0x17 - - $ addr2line -e vmlinux -i ffffffff8104f04e ffffffff81050bd8 \ - ffffffff8100b87d ffffffff8100c7b5 - kernel/signal.c:446 - kernel/signal.c:1806 - arch/x86/kernel/signal.c:805 - arch/x86/kernel/signal.c:871 - arch/x86/kernel/entry_64.S:694 - -Remember that since these addresses were found on the stack and not as the -RIP value, they actually point to the _next_ instruction (they are return -addresses). This becomes obvious when we look at the code for line 446:: - - 422 int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info) - 423 { - ... - 431 signr = __dequeue_signal(&tsk->signal->shared_pending, - 432 mask, info); - 433 /* - 434 * itimer signal ? - 435 * - 436 * itimers are process shared and we restart periodic - 437 * itimers in the signal delivery path to prevent DoS - 438 * attacks in the high resolution timer case. This is - 439 * compliant with the old way of self restarting - 440 * itimers, as the SIGALRM is a legacy signal and only - 441 * queued once. Changing the restart behaviour to - 442 * restart the timer in the signal dequeue path is - 443 * reducing the timer noise on heavy loaded !highres - 444 * systems too. - 445 */ - 446 if (unlikely(signr == SIGALRM)) { - ... - 489 } - -So instead of looking at 446, we should be looking at 431, which is the line -that executes just before 446. Here we see that what we are looking for is -``&tsk->signal->shared_pending``. - -Our next task is now to figure out which function that puts items on this -``shared_pending`` list. A crude, but efficient tool, is ``git grep``:: - - $ git grep -n 'shared_pending' kernel/ - ... - kernel/signal.c:828: pending = group ? &t->signal->shared_pending : &t->pending; - kernel/signal.c:1339: pending = group ? &t->signal->shared_pending : &t->pending; - ... - -There were more results, but none of them were related to list operations, -and these were the only assignments. We inspect the line numbers more closely -and find that this is indeed where items are being added to the list:: - - 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t, - 817 int group) - 818 { - ... - 828 pending = group ? &t->signal->shared_pending : &t->pending; - ... - 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN && - 852 (is_si_special(info) || - 853 info->si_code >= 0))); - 854 if (q) { - 855 list_add_tail(&q->list, &pending->list); - ... - 890 } - -and:: - - 1309 int send_sigqueue(struct sigqueue *q, struct task_struct *t, int group) - 1310 { - .... - 1339 pending = group ? &t->signal->shared_pending : &t->pending; - 1340 list_add_tail(&q->list, &pending->list); - .... - 1347 } - -In the first case, the list element we are looking for, ``q``, is being -returned from the function ``__sigqueue_alloc()``, which looks like an -allocation function. Let's take a look at it:: - - 187 static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags, - 188 int override_rlimit) - 189 { - 190 struct sigqueue *q = NULL; - 191 struct user_struct *user; - 192 - 193 /* - 194 * We won't get problems with the target's UID changing under us - 195 * because changing it requires RCU be used, and if t != current, the - 196 * caller must be holding the RCU readlock (by way of a spinlock) and - 197 * we use RCU protection here - 198 */ - 199 user = get_uid(__task_cred(t)->user); - 200 atomic_inc(&user->sigpending); - 201 if (override_rlimit || - 202 atomic_read(&user->sigpending) <= - 203 t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur) - 204 q = kmem_cache_alloc(sigqueue_cachep, flags); - 205 if (unlikely(q == NULL)) { - 206 atomic_dec(&user->sigpending); - 207 free_uid(user); - 208 } else { - 209 INIT_LIST_HEAD(&q->list); - 210 q->flags = 0; - 211 q->user = user; - 212 } - 213 - 214 return q; - 215 } - -We see that this function initializes ``q->list``, ``q->flags``, and -``q->user``. It seems that now is the time to look at the definition of -``struct sigqueue``, e.g.:: - - 14 struct sigqueue { - 15 struct list_head list; - 16 int flags; - 17 siginfo_t info; - 18 struct user_struct *user; - 19 }; - -And, you might remember, it was a ``memcpy()`` on ``&first->info`` that -caused the warning, so this makes perfect sense. It also seems reasonable -to assume that it is the caller of ``__sigqueue_alloc()`` that has the -responsibility of filling out (initializing) this member. - -But just which fields of the struct were uninitialized? Let's look at -kmemcheck's report again:: - - WARNING: kmemcheck: Caught 32-bit read from uninitialized memory (ffff88003e4a2024) - 80000000000000000000000000000000000000000088ffff0000000000000000 - i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u - ^ - -These first two lines are the memory dump of the memory object itself, and -the shadow bytemap, respectively. The memory object itself is in this case -``&first->info``. Just beware that the start of this dump is NOT the start -of the object itself! The position of the caret (^) corresponds with the -address of the read (ffff88003e4a2024). - -The shadow bytemap dump legend is as follows: - -- i: initialized -- u: uninitialized -- a: unallocated (memory has been allocated by the slab layer, but has not - yet been handed off to anybody) -- f: freed (memory has been allocated by the slab layer, but has been freed - by the previous owner) - -In order to figure out where (relative to the start of the object) the -uninitialized memory was located, we have to look at the disassembly. For -that, we'll need the RIP address again:: - - RIP: 0010:[<ffffffff8104ede8>] [<ffffffff8104ede8>] __dequeue_signal+0xc8/0x190 - - $ objdump -d --no-show-raw-insn vmlinux | grep -C 8 ffffffff8104ede8: - ffffffff8104edc8: mov %r8,0x8(%r8) - ffffffff8104edcc: test %r10d,%r10d - ffffffff8104edcf: js ffffffff8104ee88 <__dequeue_signal+0x168> - ffffffff8104edd5: mov %rax,%rdx - ffffffff8104edd8: mov $0xc,%ecx - ffffffff8104eddd: mov %r13,%rdi - ffffffff8104ede0: mov $0x30,%eax - ffffffff8104ede5: mov %rdx,%rsi - ffffffff8104ede8: rep movsl %ds:(%rsi),%es:(%rdi) - ffffffff8104edea: test $0x2,%al - ffffffff8104edec: je ffffffff8104edf0 <__dequeue_signal+0xd0> - ffffffff8104edee: movsw %ds:(%rsi),%es:(%rdi) - ffffffff8104edf0: test $0x1,%al - ffffffff8104edf2: je ffffffff8104edf5 <__dequeue_signal+0xd5> - ffffffff8104edf4: movsb %ds:(%rsi),%es:(%rdi) - ffffffff8104edf5: mov %r8,%rdi - ffffffff8104edf8: callq ffffffff8104de60 <__sigqueue_free> - -As expected, it's the "``rep movsl``" instruction from the ``memcpy()`` -that causes the warning. We know about ``REP MOVSL`` that it uses the register -``RCX`` to count the number of remaining iterations. By taking a look at the -register dump again (from the kmemcheck report), we can figure out how many -bytes were left to copy:: - - RAX: 0000000000000030 RBX: ffff88003d4ea968 RCX: 0000000000000009 - -By looking at the disassembly, we also see that ``%ecx`` is being loaded -with the value ``$0xc`` just before (ffffffff8104edd8), so we are very -lucky. Keep in mind that this is the number of iterations, not bytes. And -since this is a "long" operation, we need to multiply by 4 to get the -number of bytes. So this means that the uninitialized value was encountered -at 4 * (0xc - 0x9) = 12 bytes from the start of the object. - -We can now try to figure out which field of the "``struct siginfo``" that -was not initialized. This is the beginning of the struct:: - - 40 typedef struct siginfo { - 41 int si_signo; - 42 int si_errno; - 43 int si_code; - 44 - 45 union { - .. - 92 } _sifields; - 93 } siginfo_t; - -On 64-bit, the int is 4 bytes long, so it must the union member that has -not been initialized. We can verify this using gdb:: - - $ gdb vmlinux - ... - (gdb) p &((struct siginfo *) 0)->_sifields - $1 = (union {...} *) 0x10 - -Actually, it seems that the union member is located at offset 0x10 -- which -means that gcc has inserted 4 bytes of padding between the members ``si_code`` -and ``_sifields``. We can now get a fuller picture of the memory dump:: - - _----------------------------=> si_code - / _--------------------=> (padding) - | / _------------=> _sifields(._kill._pid) - | | / _----=> _sifields(._kill._uid) - | | | / - -------|-------|-------|-------| - 80000000000000000000000000000000000000000088ffff0000000000000000 - i i i i u u u u i i i i i i i i u u u u u u u u u u u u u u u u - -This allows us to realize another important fact: ``si_code`` contains the -value 0x80. Remember that x86 is little endian, so the first 4 bytes -"80000000" are really the number 0x00000080. With a bit of research, we -find that this is actually the constant ``SI_KERNEL`` defined in -``include/asm-generic/siginfo.h``:: - - 144 #define SI_KERNEL 0x80 /* sent by the kernel from somewhere */ - -This macro is used in exactly one place in the x86 kernel: In ``send_signal()`` -in ``kernel/signal.c``:: - - 816 static int send_signal(int sig, struct siginfo *info, struct task_struct *t, - 817 int group) - 818 { - ... - 828 pending = group ? &t->signal->shared_pending : &t->pending; - ... - 851 q = __sigqueue_alloc(t, GFP_ATOMIC, (sig < SIGRTMIN && - 852 (is_si_special(info) || - 853 info->si_code >= 0))); - 854 if (q) { - 855 list_add_tail(&q->list, &pending->list); - 856 switch ((unsigned long) info) { - ... - 865 case (unsigned long) SEND_SIG_PRIV: - 866 q->info.si_signo = sig; - 867 q->info.si_errno = 0; - 868 q->info.si_code = SI_KERNEL; - 869 q->info.si_pid = 0; - 870 q->info.si_uid = 0; - 871 break; - ... - 890 } - -Not only does this match with the ``.si_code`` member, it also matches the place -we found earlier when looking for where siginfo_t objects are enqueued on the -``shared_pending`` list. - -So to sum up: It seems that it is the padding introduced by the compiler -between two struct fields that is uninitialized, and this gets reported when -we do a ``memcpy()`` on the struct. This means that we have identified a false -positive warning. - -Normally, kmemcheck will not report uninitialized accesses in ``memcpy()`` calls -when both the source and destination addresses are tracked. (Instead, we copy -the shadow bytemap as well). In this case, the destination address clearly -was not tracked. We can dig a little deeper into the stack trace from above:: - - arch/x86/kernel/signal.c:805 - arch/x86/kernel/signal.c:871 - arch/x86/kernel/entry_64.S:694 - -And we clearly see that the destination siginfo object is located on the -stack:: - - 782 static void do_signal(struct pt_regs *regs) - 783 { - 784 struct k_sigaction ka; - 785 siginfo_t info; - ... - 804 signr = get_signal_to_deliver(&info, &ka, regs, NULL); - ... - 854 } - -And this ``&info`` is what eventually gets passed to ``copy_siginfo()`` as the -destination argument. - -Now, even though we didn't find an actual error here, the example is still a -good one, because it shows how one would go about to find out what the report -was all about. - - -Annotating false positives -~~~~~~~~~~~~~~~~~~~~~~~~~~ - -There are a few different ways to make annotations in the source code that -will keep kmemcheck from checking and reporting certain allocations. Here -they are: - -- ``__GFP_NOTRACK_FALSE_POSITIVE`` - This flag can be passed to ``kmalloc()`` or ``kmem_cache_alloc()`` - (therefore also to other functions that end up calling one of - these) to indicate that the allocation should not be tracked - because it would lead to a false positive report. This is a "big - hammer" way of silencing kmemcheck; after all, even if the false - positive pertains to particular field in a struct, for example, we - will now lose the ability to find (real) errors in other parts of - the same struct. - - Example:: - - /* No warnings will ever trigger on accessing any part of x */ - x = kmalloc(sizeof *x, GFP_KERNEL | __GFP_NOTRACK_FALSE_POSITIVE); - -- ``kmemcheck_bitfield_begin(name)``/``kmemcheck_bitfield_end(name)`` and - ``kmemcheck_annotate_bitfield(ptr, name)`` - The first two of these three macros can be used inside struct - definitions to signal, respectively, the beginning and end of a - bitfield. Additionally, this will assign the bitfield a name, which - is given as an argument to the macros. - - Having used these markers, one can later use - kmemcheck_annotate_bitfield() at the point of allocation, to indicate - which parts of the allocation is part of a bitfield. - - Example:: - - struct foo { - int x; - - kmemcheck_bitfield_begin(flags); - int flag_a:1; - int flag_b:1; - kmemcheck_bitfield_end(flags); - - int y; - }; - - struct foo *x = kmalloc(sizeof *x); - - /* No warnings will trigger on accessing the bitfield of x */ - kmemcheck_annotate_bitfield(x, flags); - - Note that ``kmemcheck_annotate_bitfield()`` can be used even before the - return value of ``kmalloc()`` is checked -- in other words, passing NULL - as the first argument is legal (and will do nothing). - - -Reporting errors ----------------- - -As we have seen, kmemcheck will produce false positive reports. Therefore, it -is not very wise to blindly post kmemcheck warnings to mailing lists and -maintainers. Instead, I encourage maintainers and developers to find errors -in their own code. If you get a warning, you can try to work around it, try -to figure out if it's a real error or not, or simply ignore it. Most -developers know their own code and will quickly and efficiently determine the -root cause of a kmemcheck report. This is therefore also the most efficient -way to work with kmemcheck. - -That said, we (the kmemcheck maintainers) will always be on the lookout for -false positives that we can annotate and silence. So whatever you find, -please drop us a note privately! Kernel configs and steps to reproduce (if -available) are of course a great help too. - -Happy hacking! - - -Technical description ---------------------- - -kmemcheck works by marking memory pages non-present. This means that whenever -somebody attempts to access the page, a page fault is generated. The page -fault handler notices that the page was in fact only hidden, and so it calls -on the kmemcheck code to make further investigations. - -When the investigations are completed, kmemcheck "shows" the page by marking -it present (as it would be under normal circumstances). This way, the -interrupted code can continue as usual. - -But after the instruction has been executed, we should hide the page again, so -that we can catch the next access too! Now kmemcheck makes use of a debugging -feature of the processor, namely single-stepping. When the processor has -finished the one instruction that generated the memory access, a debug -exception is raised. From here, we simply hide the page again and continue -execution, this time with the single-stepping feature turned off. - -kmemcheck requires some assistance from the memory allocator in order to work. -The memory allocator needs to - - 1. Tell kmemcheck about newly allocated pages and pages that are about to - be freed. This allows kmemcheck to set up and tear down the shadow memory - for the pages in question. The shadow memory stores the status of each - byte in the allocation proper, e.g. whether it is initialized or - uninitialized. - - 2. Tell kmemcheck which parts of memory should be marked uninitialized. - There are actually a few more states, such as "not yet allocated" and - "recently freed". - -If a slab cache is set up using the SLAB_NOTRACK flag, it will never return -memory that can take page faults because of kmemcheck. - -If a slab cache is NOT set up using the SLAB_NOTRACK flag, callers can still -request memory with the __GFP_NOTRACK or __GFP_NOTRACK_FALSE_POSITIVE flags. -This does not prevent the page faults from occurring, however, but marks the -object in question as being initialized so that no warnings will ever be -produced for this object. - -Currently, the SLAB and SLUB allocators are supported by kmemcheck. |