[NFC][sanitizer] Update DTLS_on_tls_get_addr failure comment

[vitalybuka]

Also return nullptr on failure. All callers
do nothing for nullptr or {tls_beg, 0},
but with nullptr they continue earlier.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Vitaly Buka (vitalybuka)

Changes

Also return nullptr on failure, as is all caller
handler nullptr and {tls_beg, 0} in the same
manner.

---

Full diff: https://github.com/llvm/llvm-project/pull/109546.diff

1 Files Affected:

• (modified) compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp (+10-2)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
index 6d5fad3e7b4a4b..e08bcb435ca841 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
@@ -158,10 +158,18 @@ DTLS::DTV *DTLS_on_tls_get_addr(void *arg_void, void *res,
     return dtv;
   }
   VReport(2, "__tls_get_addr: Can't guess glibc version\n");
-  // This may happen inside the DTOR of main thread, so just ignore it.
+  // This may happen inside the DTOR a thread, or async signal handlers before
+  // thread initialization, so just ignore it.
+  //
+  // If the unknown block is dynamic TLS, unlikely we will be able to recognize
+  // it in future, mark it as done with '{tls_beg, 0}'.
+  //
+  // If the block is static TLS, possible reason of failed detection is nullptr
+  // in `static_tls_begin`. Regardless of reasons, the future handling of static
+  // TLS is still '{tls_beg, 0}'.
   dtv->beg = tls_beg;
   dtv->size = 0;
-  return dtv;
+  return nullptr;
 }
 
 DTLS *DTLS_Get() { return &dtls; }