Re: [PATCH v5 1/4] kasan: Emit different calls for instrumentable memintrinsics

[Marco Elver]

On Thu, 13 Apr 2023 at 16:14, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> Hi Marco,
>
> On Fri, Feb 24, 2023 at 09:59:39AM +0100, Marco Elver wrote:
> > Clang 15 provides an option to prefix memcpy/memset/memmove calls with
> > __asan_/__hwasan_ in instrumented functions: https://reviews.llvm.org/D122724
> >
> > GCC will add support in future:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108777
> >
> > Use it to regain KASAN instrumentation of memcpy/memset/memmove on
> > architectures that require noinstr to be really free from instrumented
> > mem*() functions (all GENERIC_ENTRY architectures).
> >
> > Fixes: 69d4c0d32186 ("entry, kasan, x86: Disallow overriding mem*() functions")
> > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> > Tested-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> > Tested-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> [...]
> > diff --git a/scripts/Makefile.kasan b/scripts/Makefile.kasan
> > index b9e94c5e7097..fa9f836f8039 100644
> > --- a/scripts/Makefile.kasan
> > +++ b/scripts/Makefile.kasan
> > @@ -38,6 +38,11 @@ endif
> >
> >  CFLAGS_KASAN += $(call cc-param,asan-stack=$(stack_enable))
> >
> > +# Instrument memcpy/memset/memmove calls by using instrumented __asan_mem*()
> > +# instead. With compilers that don't support this option, compiler-inserted
> > +# memintrinsics won't be checked by KASAN on GENERIC_ENTRY architectures.
> > +CFLAGS_KASAN += $(call cc-param,asan-kernel-mem-intrinsic-prefix=1)
> > +
> >  endif # CONFIG_KASAN_GENERIC
> >
> >  ifdef CONFIG_KASAN_SW_TAGS
> > @@ -54,6 +59,9 @@ CFLAGS_KASAN := -fsanitize=kernel-hwaddress \
> >               $(call cc-param,hwasan-inline-all-checks=0) \
> >               $(instrumentation_flags)
> >
> > +# Instrument memcpy/memset/memmove calls by using instrumented __hwasan_mem*().
> > +CFLAGS_KASAN += $(call cc-param,hwasan-kernel-mem-intrinsic-prefix=1)
>
> This patch breaks the arm64 kernel builds with KASAN_SW_TAGS enabled and
> clang prior to version 15. Those prior clang versions don't like the
> '-mllvm -hwasan-kernel-mem-intrinsic-prefix=1' option, end up printing
> the help text instead of generating the object.
>
> Do we need some combination of cc-option and cc-param? Or at least
> disable this instrumentation if earlier clang versions are used.
>
> It's already in mainline as commit
> 51287dcb00cc715c27bf6a6b4dbd431621c5b65a.

Arnd posted a patch, but the reason why a workaround is needed is
quite unfortunate:
https://lore.kernel.org/all/CANpmjNMwYosrvqh4ogDO8rgn+SeDHM2b-shD21wTypm_6MMe=g@xxxxxxxxxxxxxx/

Clang apparently interprets unknown options that start with "-h..",
i.e. "-mllvm -h..." as a request to print help text, which has exit
code 0. So this is only a problem for hwasan options.