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


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


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