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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:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
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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.