kasan: add kernel address sanitizer infrastructure
Kernel Address sanitizer (KASan) is a dynamic memory error detector. It
provides fast and comprehensive solution for finding use-after-free and
out-of-bounds bugs.
KASAN uses compile-time instrumentation for checking every memory access,
therefore GCC > v4.9.2 required. v4.9.2 almost works, but has issues with
putting symbol aliases into the wrong section, which breaks kasan
instrumentation of globals.
This patch only adds infrastructure for kernel address sanitizer. It's
not available for use yet. The idea and some code was borrowed from [1].
Basic idea:
The main idea of KASAN is to use shadow memory to record whether each byte
of memory is safe to access or not, and use compiler's instrumentation to
check the shadow memory on each memory access.
Address sanitizer uses 1/8 of the memory addressable in kernel for shadow
memory and uses direct mapping with a scale and offset to translate a
memory address to its corresponding shadow address.
Here is function to translate address to corresponding shadow address:
unsigned long kasan_mem_to_shadow(unsigned long addr)
{
return (addr >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET;
}
where KASAN_SHADOW_SCALE_SHIFT = 3.
So for every 8 bytes there is one corresponding byte of shadow memory.
The following encoding used for each shadow byte: 0 means that all 8 bytes
of the corresponding memory region are valid for access; k (1 <= k <= 7)
means that the first k bytes are valid for access, and other (8 - k) bytes
are not; Any negative value indicates that the entire 8-bytes are
inaccessible. Different negative values used to distinguish between
different kinds of inaccessible memory (redzones, freed memory) (see
mm/kasan/kasan.h).
To be able to detect accesses to bad memory we need a special compiler.
Such compiler inserts a specific function calls (__asan_load*(addr),
__asan_store*(addr)) before each memory access of size 1, 2, 4, 8 or 16.
These functions check whether memory region is valid to access or not by
checking corresponding shadow memory. If access is not valid an error
printed.
Historical background of the address sanitizer from Dmitry Vyukov:
"We've developed the set of tools, AddressSanitizer (Asan),
ThreadSanitizer and MemorySanitizer, for user space. We actively use
them for testing inside of Google (continuous testing, fuzzing,
running prod services). To date the tools have found more than 10'000
scary bugs in Chromium, Google internal codebase and various
open-source projects (Firefox, OpenSSL, gcc, clang, ffmpeg, MySQL and
lots of others): [2] [3] [4].
The tools are part of both gcc and clang compilers.
We have not yet done massive testing under the Kernel AddressSanitizer
(it's kind of chicken and egg problem, you need it to be upstream to
start applying it extensively). To date it has found about 50 bugs.
Bugs that we've found in upstream kernel are listed in [5].
We've also found ~20 bugs in out internal version of the kernel. Also
people from Samsung and Oracle have found some.
[...]
As others noted, the main feature of AddressSanitizer is its
performance due to inline compiler instrumentation and simple linear
shadow memory. User-space Asan has ~2x slowdown on computational
programs and ~2x memory consumption increase. Taking into account that
kernel usually consumes only small fraction of CPU and memory when
running real user-space programs, I would expect that kernel Asan will
have ~10-30% slowdown and similar memory consumption increase (when we
finish all tuning).
I agree that Asan can well replace kmemcheck. We have plans to start
working on Kernel MemorySanitizer that finds uses of unitialized
memory. Asan+Msan will provide feature-parity with kmemcheck. As
others noted, Asan will unlikely replace debug slab and pagealloc that
can be enabled at runtime. Asan uses compiler instrumentation, so even
if it is disabled, it still incurs visible overheads.
Asan technology is easily portable to other architectures. Compiler
instrumentation is fully portable. Runtime has some arch-dependent
parts like shadow mapping and atomic operation interception. They are
relatively easy to port."
Comparison with other debugging features:
========================================
KMEMCHECK:
- KASan can do almost everything that kmemcheck can. KASan uses
compile-time instrumentation, which makes it significantly faster than
kmemcheck. The only advantage of kmemcheck over KASan is detection of
uninitialized memory reads.
Some brief performance testing showed that kasan could be
x500-x600 times faster than kmemcheck:
$ netperf -l 30
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost (127.0.0.1) port 0 AF_INET
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec
no debug: 87380 16384 16384 30.00 41624.72
kasan inline: 87380 16384 16384 30.00 12870.54
kasan outline: 87380 16384 16384 30.00 10586.39
kmemcheck: 87380 16384 16384 30.03 20.23
- Also kmemcheck couldn't work on several CPUs. It always sets
number of CPUs to 1. KASan doesn't have such limitation.
DEBUG_PAGEALLOC:
- KASan is slower than DEBUG_PAGEALLOC, but KASan works on sub-page
granularity level, so it able to find more bugs.
SLUB_DEBUG (poisoning, redzones):
- SLUB_DEBUG has lower overhead than KASan.
- SLUB_DEBUG in most cases are not able to detect bad reads,
KASan able to detect both reads and writes.
- In some cases (e.g. redzone overwritten) SLUB_DEBUG detect
bugs only on allocation/freeing of object. KASan catch
bugs right before it will happen, so we always know exact
place of first bad read/write.
[1] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
[2] https://code.google.com/p/address-sanitizer/wiki/FoundBugs
[3] https://code.google.com/p/thread-sanitizer/wiki/FoundBugs
[4] https://code.google.com/p/memory-sanitizer/wiki/FoundBugs
[5] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel#Trophies
Based on work by Andrey Konovalov.
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Acked-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrey Konovalov <adech.fo@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Konstantin Serebryany <kcc@google.com>
Cc: Dmitry Chernenkov <dmitryc@google.com>
Cc: Yuri Gribov <tetra2005@gmail.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[tsoni@codeaurora.org: trivial merge conflicts]
Git-commit: 0b24becc810dc3be6e3f94103a866f214c282394
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Change-Id: If62dffc8bb54d92654f221f5b365ed3f1a07fd3a
Signed-off-by: David Keitel <dkeitel@codeaurora.org>
2015-02-13 22:39:17 +00:00
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Kernel address sanitizer
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================
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0. Overview
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===========
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Kernel Address sanitizer (KASan) is a dynamic memory error detector. It provides
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a fast and comprehensive solution for finding use-after-free and out-of-bounds
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bugs.
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KASan uses compile-time instrumentation for checking every memory access,
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2015-04-24 17:34:19 +00:00
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therefore you will need a certain version of GCC > 4.9.2
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kasan: add kernel address sanitizer infrastructure
Kernel Address sanitizer (KASan) is a dynamic memory error detector. It
provides fast and comprehensive solution for finding use-after-free and
out-of-bounds bugs.
KASAN uses compile-time instrumentation for checking every memory access,
therefore GCC > v4.9.2 required. v4.9.2 almost works, but has issues with
putting symbol aliases into the wrong section, which breaks kasan
instrumentation of globals.
This patch only adds infrastructure for kernel address sanitizer. It's
not available for use yet. The idea and some code was borrowed from [1].
Basic idea:
The main idea of KASAN is to use shadow memory to record whether each byte
of memory is safe to access or not, and use compiler's instrumentation to
check the shadow memory on each memory access.
Address sanitizer uses 1/8 of the memory addressable in kernel for shadow
memory and uses direct mapping with a scale and offset to translate a
memory address to its corresponding shadow address.
Here is function to translate address to corresponding shadow address:
unsigned long kasan_mem_to_shadow(unsigned long addr)
{
return (addr >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET;
}
where KASAN_SHADOW_SCALE_SHIFT = 3.
So for every 8 bytes there is one corresponding byte of shadow memory.
The following encoding used for each shadow byte: 0 means that all 8 bytes
of the corresponding memory region are valid for access; k (1 <= k <= 7)
means that the first k bytes are valid for access, and other (8 - k) bytes
are not; Any negative value indicates that the entire 8-bytes are
inaccessible. Different negative values used to distinguish between
different kinds of inaccessible memory (redzones, freed memory) (see
mm/kasan/kasan.h).
To be able to detect accesses to bad memory we need a special compiler.
Such compiler inserts a specific function calls (__asan_load*(addr),
__asan_store*(addr)) before each memory access of size 1, 2, 4, 8 or 16.
These functions check whether memory region is valid to access or not by
checking corresponding shadow memory. If access is not valid an error
printed.
Historical background of the address sanitizer from Dmitry Vyukov:
"We've developed the set of tools, AddressSanitizer (Asan),
ThreadSanitizer and MemorySanitizer, for user space. We actively use
them for testing inside of Google (continuous testing, fuzzing,
running prod services). To date the tools have found more than 10'000
scary bugs in Chromium, Google internal codebase and various
open-source projects (Firefox, OpenSSL, gcc, clang, ffmpeg, MySQL and
lots of others): [2] [3] [4].
The tools are part of both gcc and clang compilers.
We have not yet done massive testing under the Kernel AddressSanitizer
(it's kind of chicken and egg problem, you need it to be upstream to
start applying it extensively). To date it has found about 50 bugs.
Bugs that we've found in upstream kernel are listed in [5].
We've also found ~20 bugs in out internal version of the kernel. Also
people from Samsung and Oracle have found some.
[...]
As others noted, the main feature of AddressSanitizer is its
performance due to inline compiler instrumentation and simple linear
shadow memory. User-space Asan has ~2x slowdown on computational
programs and ~2x memory consumption increase. Taking into account that
kernel usually consumes only small fraction of CPU and memory when
running real user-space programs, I would expect that kernel Asan will
have ~10-30% slowdown and similar memory consumption increase (when we
finish all tuning).
I agree that Asan can well replace kmemcheck. We have plans to start
working on Kernel MemorySanitizer that finds uses of unitialized
memory. Asan+Msan will provide feature-parity with kmemcheck. As
others noted, Asan will unlikely replace debug slab and pagealloc that
can be enabled at runtime. Asan uses compiler instrumentation, so even
if it is disabled, it still incurs visible overheads.
Asan technology is easily portable to other architectures. Compiler
instrumentation is fully portable. Runtime has some arch-dependent
parts like shadow mapping and atomic operation interception. They are
relatively easy to port."
Comparison with other debugging features:
========================================
KMEMCHECK:
- KASan can do almost everything that kmemcheck can. KASan uses
compile-time instrumentation, which makes it significantly faster than
kmemcheck. The only advantage of kmemcheck over KASan is detection of
uninitialized memory reads.
Some brief performance testing showed that kasan could be
x500-x600 times faster than kmemcheck:
$ netperf -l 30
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to localhost (127.0.0.1) port 0 AF_INET
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec
no debug: 87380 16384 16384 30.00 41624.72
kasan inline: 87380 16384 16384 30.00 12870.54
kasan outline: 87380 16384 16384 30.00 10586.39
kmemcheck: 87380 16384 16384 30.03 20.23
- Also kmemcheck couldn't work on several CPUs. It always sets
number of CPUs to 1. KASan doesn't have such limitation.
DEBUG_PAGEALLOC:
- KASan is slower than DEBUG_PAGEALLOC, but KASan works on sub-page
granularity level, so it able to find more bugs.
SLUB_DEBUG (poisoning, redzones):
- SLUB_DEBUG has lower overhead than KASan.
- SLUB_DEBUG in most cases are not able to detect bad reads,
KASan able to detect both reads and writes.
- In some cases (e.g. redzone overwritten) SLUB_DEBUG detect
bugs only on allocation/freeing of object. KASan catch
bugs right before it will happen, so we always know exact
place of first bad read/write.
[1] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel
[2] https://code.google.com/p/address-sanitizer/wiki/FoundBugs
[3] https://code.google.com/p/thread-sanitizer/wiki/FoundBugs
[4] https://code.google.com/p/memory-sanitizer/wiki/FoundBugs
[5] https://code.google.com/p/address-sanitizer/wiki/AddressSanitizerForKernel#Trophies
Based on work by Andrey Konovalov.
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Acked-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrey Konovalov <adech.fo@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Konstantin Serebryany <kcc@google.com>
Cc: Dmitry Chernenkov <dmitryc@google.com>
Cc: Yuri Gribov <tetra2005@gmail.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[tsoni@codeaurora.org: trivial merge conflicts]
Git-commit: 0b24becc810dc3be6e3f94103a866f214c282394
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
Change-Id: If62dffc8bb54d92654f221f5b365ed3f1a07fd3a
Signed-off-by: David Keitel <dkeitel@codeaurora.org>
2015-02-13 22:39:17 +00:00
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Currently KASan is supported only for x86_64 architecture and requires that the
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kernel be built with the SLUB allocator.
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1. Usage
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=========
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To enable KASAN configure kernel with:
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CONFIG_KASAN = y
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and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline/inline
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is compiler instrumentation types. The former produces smaller binary the
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latter is 1.1 - 2 times faster. Inline instrumentation requires GCC 5.0 or
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latter.
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Currently KASAN works only with the SLUB memory allocator.
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For better bug detection and nicer report, enable CONFIG_STACKTRACE and put
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at least 'slub_debug=U' in the boot cmdline.
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To disable instrumentation for specific files or directories, add a line
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similar to the following to the respective kernel Makefile:
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For a single file (e.g. main.o):
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KASAN_SANITIZE_main.o := n
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For all files in one directory:
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KASAN_SANITIZE := n
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1.1 Error reports
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==========
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A typical out of bounds access report looks like this:
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==================================================================
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BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3
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Write of size 1 by task modprobe/1689
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=============================================================================
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BUG kmalloc-128 (Not tainted): kasan error
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-----------------------------------------------------------------------------
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Disabling lock debugging due to kernel taint
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INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689
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__slab_alloc+0x4b4/0x4f0
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kmem_cache_alloc_trace+0x10b/0x190
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kmalloc_oob_right+0x3d/0x75 [test_kasan]
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init_module+0x9/0x47 [test_kasan]
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do_one_initcall+0x99/0x200
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load_module+0x2cb3/0x3b20
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SyS_finit_module+0x76/0x80
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system_call_fastpath+0x12/0x17
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INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080
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INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720
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Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
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Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
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Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........
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Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
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CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
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ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78
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ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8
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ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558
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Call Trace:
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[<ffffffff81cc68ae>] dump_stack+0x46/0x58
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[<ffffffff811fd848>] print_trailer+0xf8/0x160
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[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
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[<ffffffff811ff0f5>] object_err+0x35/0x40
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[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0
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[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
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[<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40
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[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
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[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
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[<ffffffff8120a995>] __asan_store1+0x75/0xb0
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[<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan]
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[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan]
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[<ffffffff810002d9>] do_one_initcall+0x99/0x200
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[<ffffffff811e4e5c>] ? __vunmap+0xec/0x160
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[<ffffffff81114f63>] load_module+0x2cb3/0x3b20
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[<ffffffff8110fd70>] ? m_show+0x240/0x240
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[<ffffffff81115f06>] SyS_finit_module+0x76/0x80
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[<ffffffff81cd3129>] system_call_fastpath+0x12/0x17
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Memory state around the buggy address:
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ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc
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ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00
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>ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc
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^
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ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb
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ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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==================================================================
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First sections describe slub object where bad access happened.
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See 'SLUB Debug output' section in Documentation/vm/slub.txt for details.
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In the last section the report shows memory state around the accessed address.
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Reading this part requires some more understanding of how KASAN works.
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Each 8 bytes of memory are encoded in one shadow byte as accessible,
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partially accessible, freed or they can be part of a redzone.
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We use the following encoding for each shadow byte: 0 means that all 8 bytes
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of the corresponding memory region are accessible; number N (1 <= N <= 7) means
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that the first N bytes are accessible, and other (8 - N) bytes are not;
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any negative value indicates that the entire 8-byte word is inaccessible.
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We use different negative values to distinguish between different kinds of
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inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
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In the report above the arrows point to the shadow byte 03, which means that
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the accessed address is partially accessible.
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2. Implementation details
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========================
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From a high level, our approach to memory error detection is similar to that
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of kmemcheck: use shadow memory to record whether each byte of memory is safe
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to access, and use compile-time instrumentation to check shadow memory on each
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memory access.
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AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory
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(e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and
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offset to translate a memory address to its corresponding shadow address.
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Here is the function witch translate an address to its corresponding shadow
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address:
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static inline void *kasan_mem_to_shadow(const void *addr)
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{
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return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
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+ KASAN_SHADOW_OFFSET;
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}
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where KASAN_SHADOW_SCALE_SHIFT = 3.
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Compile-time instrumentation used for checking memory accesses. Compiler inserts
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function calls (__asan_load*(addr), __asan_store*(addr)) before each memory
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access of size 1, 2, 4, 8 or 16. These functions check whether memory access is
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valid or not by checking corresponding shadow memory.
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GCC 5.0 has possibility to perform inline instrumentation. Instead of making
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function calls GCC directly inserts the code to check the shadow memory.
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This option significantly enlarges kernel but it gives x1.1-x2 performance
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boost over outline instrumented kernel.
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