From 4052de6cb52e2d902a10b49ffc13bb4085ca092a Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston@google.com>
Date: Wed, 10 Jul 2024 19:34:14 -0700
Subject: [PATCH] [tsan] Fix calculation of shadow end address in
 MemoryAccessRangeT (#98404)

MemoryAccessRangeT overestimates the size of the shadow region by 8x,
occasionally leading to assertion failure:
```
  RawShadow* shadow_mem = MemToShadow(addr);
  ...
  // Check that end of shadow is valid
  if (!IsShadowMem(shadow_mem + size * kShadowCnt - 1)) {
    DCHECK(IsShadowMem(shadow_mem + size * kShadowCnt - 1));
```
It is erroneous for two separate reasons:
- it uses kShadowCnt (== 4) instead of kShadowMultiplier (== 2)
- since shadow_mem is a RawShadow*, pointer arithmetic is multiplied by
sizeof(RawShadow) == 4

This patch fixes the calculation, and also improves the debugging
information.

The assertion error was observed on a buildbot
(https://lab.llvm.org/staging/#/builders/89/builds/656/steps/13/logs/stdio):
```
Bad shadow addr 0x3000000190bc (7fffffffe85f)
ThreadSanitizer: CHECK failed: tsan_rtl_access.cpp:690 "((IsShadowMem(shadow_mem + size * kShadowCnt - 1))) != (0)" (0x0, 0x0) (tid=2202676)
```
Notice that 0x3000000190bc is not the correct shadow for the end address
0x7fffffffe85f.

This error is more commonly observed on high-entropy ASLR systems, since
ASLR may be disabled (if the randomized memory layout is incompatible),
leading to an allocation near the boundaries of the high app memory
region (and therefore a shadow end that may be erroneously calculated to
be past the end of the shadow region). Also note that the assertion is
guarded by SANITIZER_DEBUG.

---------

Co-authored-by: Vitaly Buka <vitalybuka@gmail.com>
---
 compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp | 22 +++++++++++++-------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp b/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
index 8b20984a0100..cf07686d968d 100644
--- a/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_rtl_access.cpp
@@ -672,22 +672,28 @@ void MemoryAccessRangeT(ThreadState* thr, uptr pc, uptr addr, uptr size) {
 
 #if SANITIZER_DEBUG
   if (!IsAppMem(addr)) {
-    Printf("Access to non app mem %zx\n", addr);
+    Printf("Access to non app mem start: %p\n", (void*)addr);
     DCHECK(IsAppMem(addr));
   }
   if (!IsAppMem(addr + size - 1)) {
-    Printf("Access to non app mem %zx\n", addr + size - 1);
+    Printf("Access to non app mem end: %p\n", (void*)(addr + size - 1));
     DCHECK(IsAppMem(addr + size - 1));
   }
   if (!IsShadowMem(shadow_mem)) {
-    Printf("Bad shadow addr %p (%zx)\n", static_cast<void*>(shadow_mem), addr);
+    Printf("Bad shadow start addr: %p (%p)\n", shadow_mem, (void*)addr);
     DCHECK(IsShadowMem(shadow_mem));
   }
-  if (!IsShadowMem(shadow_mem + size * kShadowCnt - 1)) {
-    Printf("Bad shadow addr %p (%zx)\n",
-           static_cast<void*>(shadow_mem + size * kShadowCnt - 1),
-           addr + size - 1);
-    DCHECK(IsShadowMem(shadow_mem + size * kShadowCnt - 1));
+
+  RawShadow* shadow_mem_end = reinterpret_cast<RawShadow*>(
+      reinterpret_cast<uptr>(shadow_mem) + size * kShadowMultiplier - 1);
+  if (!IsShadowMem(shadow_mem_end)) {
+    Printf("Bad shadow end addr: %p (%p)\n", shadow_mem_end,
+           (void*)(addr + size - 1));
+    Printf(
+        "Shadow start addr (ok): %p (%p); size: 0x%zx; kShadowMultiplier: "
+        "%zx\n",
+        shadow_mem, (void*)addr, size, kShadowMultiplier);
+    DCHECK(IsShadowMem(shadow_mem_end));
   }
 #endif