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

[Daniel Axtens]

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(-)

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2.20.1