[compiler-rt] [tsan] Fix GetTls for aarch64

[zatrazz]

Summary

This patch fix the function GetTls for aarch64, where it assumes it
follows the x86_64 way where the TLS initial address is at the end
of TLS. Instead aarch64 set the TLS address as the thread pointer.

[dvyukov]

Not that I understand why this is correct, but if you need a rubber stamp...
LGTM

[zatrazz]

To give you more context on GLIBC code for aarch64:

sysdeps/aarch64/nptl/tls.h:
49 /* The TP points to the start of the thread blocks. */
50 # define TLS_DTV_AT_TP 1
51 # define TLS_TCB_AT_TP 0

93 # define TLS_INIT_TP(tcbp) \
94 ({ asm volatile ("msr tpidr_el0, %0" : : "r" (tcbp)); NULL; })

And on:

csu/libc-tls.c:

/* Initialize the thread pointer.  */

#if TLS_TCB_AT_TP

INSTALL_DTV ((char *) tlsblock + tcb_offset, _dl_static_dtv);

const char *lossage = TLS_INIT_TP ((char *) tlsblock + tcb_offset);

#elif TLS_DTV_AT_TP

INSTALL_DTV (tlsblock, _dl_static_dtv);
const char *lossage = TLS_INIT_TP (tlsblock);

So different than x86_64 (which defines TLS_TCB_AT_TP), aarch64 set the thread register pointers as the TLS allocated block.

lib/sanitizer_common/sanitizer_linux_libcdep.cc

 uptr ThreadSelf() {
   return (uptr)ThreadSelfSegbase()[2];
 }
 #endif  // SANITIZER_FREEBSD
 
 #if !SANITIZER_GO
 static void GetTls(uptr *addr, uptr *size) {
 #if SANITIZER_LINUX && !SANITIZER_ANDROID
-# if defined(__x86_64__) || defined(__i386__) || defined(__aarch64__)
+# if defined(__x86_64__) || defined(__i386__)
   *addr = ThreadSelf();
   *size = GetTlsSize();
   *addr -= *size;
   *addr += ThreadDescriptorSize();
-# elif defined(__mips__)
+# elif defined(__mips__) || defined(__aarch64__)
   *addr = ThreadSelf();
   *size = GetTlsSize();
 # else
   *addr = 0;
   *size = 0;
 # endif
 #elif SANITIZER_FREEBSD
   void** segbase = ThreadSelfSegbase();