Re: [PATCH v3 0/3] Fix some incompatibilites between KASAN and FORTIFY_SOURCE

[David Gow]

On Thu, Apr 23, 2020 at 11:45 PM Daniel Axtens <dja@xxxxxxxxxx> wrote:
>
> 3 KASAN self-tests fail on a kernel with both KASAN and FORTIFY_SOURCE:
> memchr, memcmp and strlen. I have observed this on x86 and powerpc.
>
> 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.
>
> This breaks things in 2 ways:
>
>  - the three function calls are technically dead code, and can be
>    eliminated. When __builtin_ versions are used, the compiler can detect
>    this.
>
>  - Using __builtins may bypass KASAN checks if the compiler decides to
>    inline it's own implementation as sequence of instructions, rather than
>    emit a function call that goes out to a KASAN-instrumented
>    implementation.
>
> The patches address each reason in turn. Finally, test_memcmp used a
> stack array without explicit initialisation, which can sometimes break
> too, so fix that up.
>
> v3: resend with Reviewed-bys, hopefully for inclusion in 5.8.
>
> v2: - some cleanups, don't mess with arch code as I missed some wrinkles.
>     - add stack array init (patch 3)
>
> Daniel Axtens (3):
>   kasan: stop tests being eliminated as dead code with FORTIFY_SOURCE
>   string.h: fix incompatibility between FORTIFY_SOURCE and KASAN
>   kasan: initialise array in kasan_memcmp test
>
>  include/linux/string.h | 60 +++++++++++++++++++++++++++++++++---------
>  lib/test_kasan.c       | 32 +++++++++++++---------
>  2 files changed, 68 insertions(+), 24 deletions(-)
>
> --
> 2.20.1
>
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Thanks, Daniel!

For the series:
Tested-by: David Gow <davidgow@xxxxxxxxxx>

(Though I will mirror Dmitry's comment[1] on patch 3 -- I also have a
memset() already present in my branch...)

I'd been digging into what turns out to be this issue, which we were
seeing sporadically[2] with the KUnit port of these tests. v7 of the
KUnit port[3] includes your changes, and fixes the issues.

Cheers,
-- David

[1]: https://lkml.org/lkml/2020/4/23/838
[2]: https://lkml.org/lkml/2020/4/18/570
[3]: https://lkml.org/lkml/2020/4/24/80