kmemcheck: add the kmemcheck documentation

Thanks to Sitsofe Wheeler, Randy Dunlap, and Jonathan Corbet for providing
input and feedback on this!

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

1 files changed, 773 insertions, 0 deletions

diff --git a/Documentation/kmemcheck.txt b/Documentation/kmemcheck.txt
new file mode 100644
index 000000000000..363044609dad
--- /dev/null
+++ b/Documentation/kmemcheck.txt
@@ -0,0 +1,773 @@
+GETTING STARTED WITH KMEMCHECK
+==============================
+
+Vegard Nossum <vegardno@ifi.uio.no>
+
+
+Contents
+========
+0. Introduction
+1. Downloading
+2. Configuring and compiling
+3. How to use
+3.1. Booting
+3.2. Run-time enable/disable
+3.3. Debugging
+3.4. Annotating false positives
+4. Reporting errors
+5. Technical description
+
+
+0. 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.
+
+
+1. Downloading
+==============
+
+kmemcheck can only be downloaded using git. If you want to write patches
+against the current code, you should use the kmemcheck development branch of
+the tip tree. It is also possible to use the linux-next tree, which also
+includes the latest version of kmemcheck.
+
+Assuming that you've already cloned the linux-2.6.git repository, all you
+have to do is add the -tip tree as a remote, like this:
+
+        $ git remote add tip git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git
+
+To actually download the tree, fetch the remote:
+
+        $ git fetch tip
+
+And to check out a new local branch with the kmemcheck code:
+
+        $ git checkout -b kmemcheck tip/kmemcheck
+
+General instructions for the -tip tree can be found here:
+http://people.redhat.com/mingo/tip.git/readme.txt
+
+
+2. 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:
+
+  o 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).
+
+  o CONFIG_SLAB=y or CONFIG_SLUB=y
+
+        This option is located under "General setup" / "Choose SLAB
+        allocator".
+
+  o 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.
+
+  o CONFIG_DEBUG_PAGEALLOC=n
+
+        This option is located under "Kernel hacking" / "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" / "kmemcheck:
+trap use of uninitialized memory"). Here follows a description of the
+kmemcheck configuration variables:
+
+  o CONFIG_KMEMCHECK
+
+        This must be enabled in order to use kmemcheck at all...
+
+  o 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.
+
+  o 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.
+
+  o 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.
+
+  o 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.
+
+  o 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.
+
+
+3. How to use
+=============
+
+3.1. 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:
+
+  o kmemcheck=0 (disabled)
+  o kmemcheck=1 (enabled)
+  o 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.
+
+
+3.2. 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.
+
+
+3.3. 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 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.
+
+
+3.4. 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:
+
+  o __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);
+
+  o 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).
+
+
+4. 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!
+
+
+5. 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.