[HWASan] Run LAM tests with -hwasan-generate-tags-with-calls.

[morehouse]

Summary

The default callback instrumentation in x86 LAM mode uses ASLR bits
to randomly choose a tag, and thus has a 1/64 chance of choosing a
stack tag of 0, causing stack tests to fail intermittently. By using
__hwasan_generate_tag to pick tags, we guarantee non-zero tags and
eliminate the test flakiness.

aarch64 doesn't seem to have this problem using thread-local addresses
to pick tags, so perhaps we can remove this workaround once we implement
a similar mechanism for LAM.

[eugenis]

LGTM

compiler-rt/lib/hwasan/hwasan.cpp

 }
 
 void __hwasan_print_memory_usage() {
   InternalScopedString s;
   HwasanFormatMemoryUsage(s);
   Printf("%s\n", s.data());
 }
 
-static const u8 kFallbackTag = 0xBB;
+static const u8 kFallbackTag = 0xBB & kTagMask;
 
 u8 __hwasan_generate_tag() {
   Thread *t = GetCurrentThread();
   if (!t) return kFallbackTag;
   return t->GenerateRandomTag();
 }
 
 #if !SANITIZER_SUPPORTS_WEAK_HOOKS

compiler-rt/test/hwasan/lit.cfg.py

if config.target_arch == 'x86_64' and config.enable_aliases == '1':
   clang_hwasan_common_cflags += ["-fsanitize-hwaddress-experimental-aliasing"]
 if config.enable_aliases != '1':
   config.available_features.add('pointer-tagging')
 if config.target_arch == 'x86_64':
   # This does basically the same thing as tagged-globals on aarch64. Because
   # the x86_64 implementation is for testing purposes only there is no
   # equivalent target feature implemented on x86_64.
   clang_hwasan_common_cflags += ["-mcmodel=large"]
+
+  # The callback instrumentation used on x86_64 has a 1/64 chance of choosing a
+  # stack tag of 0.  This causes stack tests to become flaky, so we force tags
+  # to be generated via calls to __hwasan_generate_tag, which never returns 0.
+  # TODO: See if we can remove this once we use the outlined instrumentation.
+  clang_hwasan_common_cflags += ["-mllvm", "-hwasan-generate-tags-with-calls=1"]
 clang_hwasan_cflags = clang_hwasan_common_cflags + ["-mllvm", "-hwasan-globals",
                                                    "-mllvm", "-hwasan-use-short-granules",
                                                    "-mllvm", "-hwasan-instrument-landing-pads=0",
                                                    "-mllvm", "-hwasan-instrument-personality-functions"]
 clang_hwasan_oldrt_cflags = clang_hwasan_common_cflags + ["-mllvm", "-hwasan-use-short-granules=0",
                                                           "-mllvm", "-hwasan-instrument-landing-pads=1",
                                                           "-mllvm", "-hwasan-instrument-personality-functions=0"]