kmemcheck: rip it out

Fix up makefiles, remove references, and git rm kmemcheck.

Link: http://lkml.kernel.org/r/20171007030159.22241-4-alexander.levin@verizon.com
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vegard Nossum <vegardno@ifi.uio.no>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 0 insertions, 734 deletions

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index a81787cd47d7..e313925fb0fa 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -21,7 +21,6 @@ whole; patches welcome!
    kasan
    ubsan
    kmemleak
-   kmemcheck
    gdb-kernel-debugging
    kgdb
    kselftest
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.