[sanitizer] Adjust size for begin/start mismatch (#109079)

Follow up to 51d913a.

Author: vitalybuka

compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp

@@ -15,6 +15,7 @@
 #include "sanitizer_allocator_interface.h"
 #include "sanitizer_atomic.h"
 #include "sanitizer_common/sanitizer_common.h"
+#include "sanitizer_common/sanitizer_internal_defs.h"
 #include "sanitizer_flags.h"
 #include "sanitizer_platform_interceptors.h"
 
@@ -116,10 +117,14 @@ SANITIZER_INTERFACE_WEAK_DEF(uptr, __sanitizer_get_dtls_size,
   const void *start = __sanitizer_get_allocated_begin(tls_begin);
   if (!start)
     return 0;
+  CHECK_LE(start, tls_begin);
   uptr tls_size = __sanitizer_get_allocated_size(start);
   VReport(2, "__tls_get_addr: glibc DTLS suspected; tls={%p,0x%zx}\n",
           tls_begin, tls_size);
-  return tls_size;
+  uptr offset =
+      (reinterpret_cast<uptr>(tls_begin) - reinterpret_cast<uptr>(start));
+  CHECK_LE(offset, tls_size);
+  return tls_size - offset;
 }
 
 DTLS::DTV *DTLS_on_tls_get_addr(void *arg_void, void *res,