Re: [PATCH] Call REAL(swapcontext) with indirect_return attribute on x86

["Kostya Serebryany via gcc-patches" <gcc-patches at gcc dot gnu dot org>]

What's ENDBR and do we really need to have it in compiler-rt?

As usual, I am opposed to any gcc compiler-rt that bypass upstream.

--kcc

On Wed, Jul 18, 2018 at 8:37 AM H.J. Lu <hongjiu.lu@intel.com> wrote:
>
> asan/asan_interceptors.cc has
>
> ...
>   int res = REAL(swapcontext)(oucp, ucp);
> ...
>
> REAL(swapcontext) is a function pointer to swapcontext in libc.  Since
> swapcontext may return via indirect branch on x86 when shadow stack is
> enabled, we need to call REAL(swapcontext) with indirect_return attribute
> on x86 so that compiler can insert ENDBR after REAL(swapcontext) call.
>
> I opened an LLVM bug:
>
> https://bugs.llvm.org/show_bug.cgi?id=38207
>
> But it won't get fixed before indirect_return attribute is added to
> LLVM.  I'd like to get it fixed in GCC first.
>
> Tested on i386 and x86-64.  OK for trunk after
>
> https://gcc.gnu.org/ml/gcc-patches/2018-07/msg01007.html
>
> is approved?
>
> Thanks.
>
>
> H.J.
> ---
>         PR target/86560
>         * asan/asan_interceptors.cc (swapcontext): Call REAL(swapcontext)
>         with indirect_return attribute on x86.
> ---
>  libsanitizer/asan/asan_interceptors.cc | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/libsanitizer/asan/asan_interceptors.cc b/libsanitizer/asan/asan_interceptors.cc
> index a8f4b72723f..b8dde4f19c5 100644
> --- a/libsanitizer/asan/asan_interceptors.cc
> +++ b/libsanitizer/asan/asan_interceptors.cc
> @@ -267,7 +267,13 @@ INTERCEPTOR(int, swapcontext, struct ucontext_t *oucp,
>    uptr stack, ssize;
>    ReadContextStack(ucp, &stack, &ssize);
>    ClearShadowMemoryForContextStack(stack, ssize);
> +#if defined(__x86_64__) || defined(__i386__)
> +  int (*real_swapcontext) (struct ucontext_t *, struct ucontext_t *)
> +    __attribute__((__indirect_return__)) = REAL(swapcontext);
> +  int res = real_swapcontext(oucp, ucp);
> +#else
>    int res = REAL(swapcontext)(oucp, ucp);
> +#endif
>    // swapcontext technically does not return, but program may swap context to
>    // "oucp" later, that would look as if swapcontext() returned 0.
>    // We need to clear shadow for ucp once again, as it may be in arbitrary
> --
> 2.17.1
>