[compiler-rt] [sanitizer] Adjust size for begin/start mismatch (PR #109079)

[Vitaly Buka via llvm-commits]

https://github.com/vitalybuka created https://github.com/llvm/llvm-project/pull/109079

Follow up to 51d913af827567e6a0999609e7e624a422781870.


>From d65663b308c99e27ef984cf021b76f107bc337ad Mon Sep 17 00:00:00 2001
From: Vitaly Buka <vitalybuka at google.com>
Date: Tue, 17 Sep 2024 20:34:17 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.4
---
 .../lib/sanitizer_common/sanitizer_tls_get_addr.cpp        | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

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 0db8547268f4e2..6f3b6af3c58474 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_tls_get_addr.cpp
+++ b/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,