Switch Android from TLS_SLOT_TSAN(8) to TLS_SLOT_SANITIZER(6)

[rprichard]

Summary

The TLS_SLOT_TSAN slot is available starting in N, but its location (8)
is incompatible with the proposed solution for implementing ELF TLS on
Android (i.e. bump ARM/AArch64 alignment to reserve an 8-word TCB).

Instead, starting in Q, Bionic replaced TLS_SLOT_DLERROR(6) with
TLS_SLOT_SANITIZER(6). Switch compiler-rt to the new slot.

[rprichard]

For context:

• https://reviews.llvm.org/D53906
• https://android-review.googlesource.com/c/platform/bionic/+/847554
• https://android-review.googlesource.com/c/platform/bionic/+/825741/3#message-cce1a13518d9c314080be3decb05d784aa13a0cc

[eugenis]

LGTM, thanks.

[eugenis]

A follow-up to this change:
https://reviews.llvm.org/D55587

[eugenis]

Hmm, this is not enough, you also need to update instrumentation in HWAddressSanitizer::getHwasanThreadSlotPtr

[rprichard]

I switched HWAddressSanitizer::getHwasanThreadSlotPtr over:
https://reviews.llvm.org/D55592

I searched for other references of thread_pointer and only found the (TLS_SLOT_SAFESTACK == 9) reference that presumably isn't in use. The CL adding TLS_SLOT_SAFESTACK to Bionic is still in progress (https://android-review.googlesource.com/c/platform/bionic/+/170988). If it's revived, it'll need to use a different slot value (probably negative on AArch64).

[eugenis]

LGTM

[eugenis]

And of course, this broke the android O bot:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-android/builds/17949

I'll try swapping the device with a current aosp master build.

[rprichard]

Thanks for looking at the buildbot failure. Let me know if I need to revert my two commits.

[eugenis]

I've reverted both commits.
The aosp build I've tried is broken in a different way.
I'll reapply the changes once the bot can handle them.

lib/sanitizer_common/sanitizer_linux.h

# define __get_tls() \
     ({ void** __v; __asm__("movl %%gs:0, %0" : "=r"(__v)); __v; })
 #elif defined(__x86_64__)
 # define __get_tls() \
     ({ void** __v; __asm__("mov %%fs:0, %0" : "=r"(__v)); __v; })
 #else
 #error "Unsupported architecture."
 #endif
 
-// The Android Bionic team has allocated a TLS slot for TSan starting with N,
-// given that Android currently doesn't support ELF TLS. It is used to store
-// Sanitizers thread specific data.
-static const int TLS_SLOT_TSAN = 8;
+// The Android Bionic team has allocated a TLS slot for sanitizers starting
+// with Q, given that Android currently doesn't support ELF TLS. It is used to
+// store sanitizer thread specific data.
+static const int TLS_SLOT_SANITIZER = 6;
 
 ALWAYS_INLINE uptr *get_android_tls_ptr() {
-  return reinterpret_cast<uptr *>(&__get_tls()[TLS_SLOT_TSAN]);
+  return reinterpret_cast<uptr *>(&__get_tls()[TLS_SLOT_SANITIZER]);
 }
 
 #endif  // SANITIZER_ANDROID
 
 }  // namespace __sanitizer
 
 #endif
 #endif  // SANITIZER_LINUX_H