Re: [PATCH 1/2] kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE

[Dmitry Vyukov]

On Wed, Jan 15, 2020 at 3:47 PM Christophe Leroy
<christophe.leroy@xxxxxx> wrote:
>
> Le 15/01/2020 Ã 15:43, Dmitry Vyukov a Ãcrit :
> > On Wed, Jan 15, 2020 at 7:37 AM Daniel Axtens <dja@xxxxxxxxxx> 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 âkasan_memchrâ:
> >> lib/test_kasan.c:606:2: warning: statement with no effect [-Wunused-value]
> >>    memchr(ptr, '1', size + 1);
> >>    ^~~~~~~~~~~~~~~~~~~~~~~~~~
> >> lib/test_kasan.c: In function âkasan_memcmpâ:
> >> lib/test_kasan.c:622:2: warning: statement with no effect [-Wunused-value]
> >>    memcmp(ptr, arr, size+1);
> >>    ^~~~~~~~~~~~~~~~~~~~~~~~
> >> lib/test_kasan.c: In function âkasan_stringsâ:
> >> 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.
> >>
> >> Cc: Daniel Micay <danielmicay@xxxxxxxxx>
> >> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> >> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> >> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> >> Fixes: 0c96350a2d2f ("lib/test_kasan.c: add tests for several string/memory API functions")
> >> Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
> >> ---
> >>   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?
> > Otherwise:
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> >
>
> I think if you make them static, GCC will see they aren't used and will
> eliminate everything still ?

static volatile? :)