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Daniel Axtens d6c2b4d246 kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE 
[ Upstream commit adb72ae191 ]

Patch series "Fix some incompatibilites between KASAN and FORTIFY_SOURCE", v4.

3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
memchr, memcmp and strlen.

When FORTIFY_SOURCE is on, a number of functions are replaced with
fortified versions, which attempt to check the sizes of the operands.
However, these functions often directly invoke __builtin_foo() once they
have performed the fortify check.  The compiler can detect that the
results of these functions are not used, and knows that they have no other
side effects, and so can eliminate them as dead code.

Why are only memchr, memcmp and strlen affected?
================================================

Of string and string-like functions, kasan_test tests:

 * strchr  ->  not affected, no fortified version
 * strrchr ->  likewise
 * strcmp  ->  likewise
 * strncmp ->  likewise

 * strnlen ->  not affected, the fortify source implementation calls the
               underlying strnlen implementation which is instrumented, not
               a builtin

 * strlen  ->  affected, the fortify souce implementation calls a __builtin
               version which the compiler can determine is dead.

 * memchr  ->  likewise
 * memcmp  ->  likewise

 * memset ->   not affected, the compiler knows that memset writes to its
	       first argument and therefore is not dead.

Why does this not affect the functions normally?
================================================

In string.h, these functions are not marked as __pure, so the compiler
cannot know that they do not have side effects.  If relevant functions are
marked as __pure in string.h, we see the following warnings and the
functions are elided:

lib/test_kasan.c: In function `kasan_memchr':
lib/test_kasan.c:606:2: warning: statement with no effect [-Wunused-value]
  memchr(ptr, '1', size + 1);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_memcmp':
lib/test_kasan.c:622:2: warning: statement with no effect [-Wunused-value]
  memcmp(ptr, arr, size+1);
  ^~~~~~~~~~~~~~~~~~~~~~~~
lib/test_kasan.c: In function `kasan_strings':
lib/test_kasan.c:645:2: warning: statement with no effect [-Wunused-value]
  strchr(ptr, '1');
  ^~~~~~~~~~~~~~~~
...

This annotation would make sense to add and could be added at any point,
so the behaviour of test_kasan.c should change.

The fix
=======

Make all the functions that are pure write their results to a global,
which makes them live.  The strlen and memchr tests now pass.

The memcmp test still fails to trigger, which is addressed in the next
patch.

[dja@axtens.net: drop patch 3]
  Link: http://lkml.kernel.org/r/20200424145521.8203-2-dja@axtens.net
Fixes: 0c96350a2d ("lib/test_kasan.c: add tests for several string/memory API functions")
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: David Gow <davidgow@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Link: http://lkml.kernel.org/r/20200423154503.5103-1-dja@axtens.net
Link: http://lkml.kernel.org/r/20200423154503.5103-2-dja@axtens.net
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
 		2020-06-22 09:31:12 +02:00
Documentation dt-bindings: display: mediatek: control dpi pins mode to avoid leakage 2020-06-22 09:30:58 +02:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch MIPS: Fix IRQ tracing when call handle_fpe() and handle_msa_fpe() 2020-06-22 09:31:09 +02:00
block iocost: don't let vrate run wild while there's no saturation signal 2020-06-22 09:31:06 +02:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto crypto: drbg - fix error return code in drbg_alloc_state() 2020-06-17 16:40:31 +02:00
drivers platform/x86: asus_wmi: Reserve more space for struct bias_args 2020-06-22 09:31:11 +02:00
fs btrfs: qgroup: mark qgroup inconsistent if we're inherting snapshot to a new qgroup 2020-06-22 09:31:09 +02:00
include bpf: Fix running sk_skb program types with ktls 2020-06-22 09:31:12 +02:00
init x86: Fix early boot crash on gcc-10, third try 2020-05-20 08:20:34 +02:00
ipc ipc/util.c: sysvipc_find_ipc() incorrectly updates position index 2020-05-20 08:20:16 +02:00
kernel bpf: Fix map permissions check 2020-06-22 09:31:11 +02:00
lib kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE 2020-06-22 09:31:12 +02:00
mm mm/slub: fix a memory leak in sysfs_slab_add() 2020-06-17 16:40:36 +02:00
net bpf: Fix running sk_skb program types with ktls 2020-06-22 09:31:12 +02:00
samples samples: bpf: Fix build error 2020-06-03 08:21:15 +02:00
scripts bpf: Support llvm-objcopy for vmlinux BTF 2020-06-17 16:40:20 +02:00
security selinux: fix error return code in policydb_read() 2020-06-22 09:31:01 +02:00
sound ALSA: usb-audio: Add vendor, product and profile name for HP Thunderbolt Dock 2020-06-17 16:40:28 +02:00
tools selftests/bpf, flow_dissector: Close TAP device FD after the test 2020-06-22 09:31:12 +02:00
usr initramfs: restore default compression behavior 2020-04-08 09:08:38 +02:00
virt KVM: arm64: Save the host's PtrAuth keys in non-preemptible context 2020-06-17 16:40:38 +02:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Modules updates for v5.4 2019-09-22 10:34:46 -07:00
.mailmap ARM: SoC fixes 2019-11-10 13:41:59 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer 2019-10-10 08:12:51 -07:00
Kbuild kbuild: do not descend to ./Kbuild when cleaning 2019-08-21 21:03:58 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS MAINTAINERS: Update drm/i915 bug filing URL 2020-02-28 17:22:19 +01:00
Makefile Linux 5.4.47 2020-06-17 16:40:38 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.