[sanitizers] [PowerPC] Intercept __tls_get_addr_opt.

[• koriakin]

Summary

On PowerPC, if binutils and glibc are new enough, the linker uses
an optimized code sequence to implement tls_get_addr call stub,
which will end up calling tls_get_addr_opt instead of tls_get_addr.
Thus, we need to intercept it in addition to tls_get_addr.

This symbol is actually an alias of __tls_get_addr - its only purpose
is that its presence in glibc triggers the optimization in linker.
This means we can make our own intercepting symbol an alias as well.

This patch will make the linker attempt optimization even on older
glibc's (since it sees a defined __tls_get_addr_opt symbol in msan)

• however, this is only a very minor performance problem (the linker

generated code will never recognize a filled static TLS descriptor,
always burning a few cycles), not a correctness problem.

This fixes MSan's dtls_test.c, allowing us to finally enable MSan
on PowerPC64.

[aizatsky]

lib/sanitizer_common/sanitizer_common_interceptors.inc

4430

mostly

[• koriakin]

Commited in 273250

lib/sanitizer_common/sanitizer_common_interceptors.inc

INTERCEPTOR(void *, __tls_get_addr, void *arg) {
   COMMON_INTERCEPTOR_GET_TLS_RANGE(&tls_begin, &tls_end);
   DTLS::DTV *dtv = DTLS_on_tls_get_addr(arg, res, tls_begin, tls_end);
   if (dtv) {
     // New DTLS block has been allocated.
     COMMON_INTERCEPTOR_INITIALIZE_RANGE((void *)dtv->beg, dtv->size);
   }
   return res;
 }
+#if SANITIZER_PPC
+// On PowerPC, we also need to intercept __tls_get_addr_opt, which has
+// ostly the same semantics as __tls_get_addr, but its presence enables
+// some optimizations in linker (which are safe to ignore here).
+extern "C" __attribute__((alias("__interceptor___tls_get_addr"),
+                          visibility("default")))
+void *__tls_get_addr_opt(void *arg);
+#endif
 #else // SANITIZER_S390
 // On s390, we have to intercept two functions here:
 // - __tls_get_addr_internal, which is a glibc-internal function that is like
 //   the usual __tls_get_addr, but returns a TP-relative offset instead of
 //   a proper pointer.  It is used by dlsym for TLS symbols.
 // - __tls_get_offset, which is like the above, but also takes a GOT-relative
 //   descriptor offset as an argument instead of a pointer.  GOT address
 //   is passed in r12, so it's necessary to write it in assembly.  This is