Mark REAL(swapcontext) with indirect_return attribute on x86

[hjl.tools]

Summary

When shadow stack from Intel CET is enabled, the first instruction of all
indirect branch targets must be a special instruction, ENDBR.

lib/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, as in this case,

int res = REAL(swapcontext)(oucp, ucp);

^^^^^^^^^^^^^^^^^^^^^^^^^^^^  This function may be

returned via an indirect branch.

Here compiler must insert ENDBR after call, like

call *bar(%rip)
endbr64

I opened an LLVM bug:

https://bugs.llvm.org/show_bug.cgi?id=38207

to add the indirect_return attribute so that it can be used to inform
compiler to insert ENDBR after REAL(swapcontext) call. We mark
REAL(swapcontext) with the indirect_return attribute if it is available.

[eugenis]

lib/asan/asan_interceptors.cc

278

Please add a __ has_attribute compatibility definition here (same as __ has_feature):
http://llvm-cs.pcc.me.uk/projects/compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h#122

[hjl.tools]

Added a __ has_attribute compatibility definition.

[eugenis]

lib/sanitizer_common/sanitizer_internal_defs.h

127

an extra ")"

[hjl.tools]

Remove the extra ")".

[eugenis]

LGTM

lib/asan/asan_interceptors.cc

Report("WARNING: ASan doesn't fully support makecontext/swapcontext "
            "functions and may produce false positives in some cases!\n");
     reported_warning = true;
   }
   // Clear shadow memory for new context (it may share stack
   // with current context).
   uptr stack, ssize;
   ReadContextStack(ucp, &stack, &ssize);
   ClearShadowMemoryForContextStack(stack, ssize);
+#if __has_attribute(__indirect_return__) && \
+    (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
   // state.
   ClearShadowMemoryForContextStack(stack, ssize);
   return res;
 }
 #endif  // ASAN_INTERCEPT_SWAPCONTEXT

lib/sanitizer_common/sanitizer_internal_defs.h

 #endif
 #endif  // SANITIZER_CAN_USE_PREINIT_ARRAY
 
 // GCC does not understand __has_feature
 #if !defined(__has_feature)
 # define __has_feature(x) 0
 #endif
 
+// Older GCCs do not understand __has_attribute.
+#if !defined(__has_attribute)
+# define __has_attribute(x) 0
+#endif
+
 // For portability reasons we do not include stddef.h, stdint.h or any other
 // system header, but we do need some basic types that are not defined
 // in a portable way by the language itself.
 namespace __sanitizer {
 
 #if defined(_WIN64)
 // 64-bit Windows uses LLP64 data model.
 typedef unsigned long long uptr;  // NOLINT