[PATCH 1/2] kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE
christophe.leroy at c-s.fr
Wed Jan 15 06:47:09 PST 2020
Le 15/01/2020 à 15:43, Dmitry Vyukov a écrit :
> On Wed, Jan 15, 2020 at 7:37 AM Daniel Axtens <dja at axtens.net> wrote:
>> 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
>> Cc: Daniel Micay <danielmicay at gmail.com>
>> Cc: Andrey Ryabinin <aryabinin at virtuozzo.com>
>> Cc: Alexander Potapenko <glider at google.com>
>> Cc: Dmitry Vyukov <dvyukov at google.com>
>> Fixes: 0c96350a2d2f ("lib/test_kasan.c: add tests for several string/memory API functions")
>> Signed-off-by: Daniel Axtens <dja at axtens.net>
>> lib/test_kasan.c | 30 +++++++++++++++++++-----------
>> 1 file changed, 19 insertions(+), 11 deletions(-)
>> diff --git a/lib/test_kasan.c b/lib/test_kasan.c
>> index 328d33beae36..58a8cef0d7a2 100644
>> --- a/lib/test_kasan.c
>> +++ b/lib/test_kasan.c
>> @@ -23,6 +23,14 @@
>> #include <asm/page.h>
>> + * We assign some test results to these globals to make sure the tests
>> + * are not eliminated as dead code.
>> + */
>> +int int_result;
>> +void *ptr_result;
> These are globals, but are not static and don't have kasan_ prefix.
> But I guess this does not matter for modules?
> Reviewed-by: Dmitry Vyukov <dvyukov at google.com>
I think if you make them static, GCC will see they aren't used and will
eliminate everything still ?
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