[GWP-ASan] Only pack frames that are stored.

Summary:
Backtrace() returns the number of frames that are *available*, rather
than the number of frames stored. When we pack, we supply the number of
frames we have stored. The number of available frames can exceed the
number of stored frames, leading to stack OOB read.

Fix up this problem.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: #sanitizers, morehouse, cferris, pcc

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D76722

compiler-rt/lib/gwp_asan/common.cpp

@@ -59,6 +59,11 @@ void AllocationMetadata::CallSiteInfo::RecordBacktrace(
   uintptr_t UncompressedBuffer[kMaxTraceLengthToCollect];
   size_t BacktraceLength =
       Backtrace(UncompressedBuffer, kMaxTraceLengthToCollect);
+  // Backtrace() returns the number of available frames, which may be greater
+  // than the number of frames in the buffer. In this case, we need to only pack
+  // the number of frames that are in the buffer.
+  if (BacktraceLength > kMaxTraceLengthToCollect)
+    BacktraceLength = kMaxTraceLengthToCollect;
   TraceSize =
       compression::pack(UncompressedBuffer, BacktraceLength, CompressedTrace,
                         AllocationMetadata::kStackFrameStorageBytes);

compiler-rt/lib/gwp_asan/tests/backtrace.cpp

@@ -8,20 +8,21 @@
 
 #include <string>
 
+#include "gwp_asan/crash_handler.h"
 #include "gwp_asan/tests/harness.h"
 
 TEST_F(BacktraceGuardedPoolAllocator, DoubleFree) {
   void *Ptr = GPA.allocate(1);
   GPA.deallocate(Ptr);
 
   std::string DeathRegex = "Double Free.*";
-  DeathRegex.append("backtrace\\.cpp:25.*");
+  DeathRegex.append("backtrace\\.cpp:26.*");
 
   DeathRegex.append("was deallocated.*");
-  DeathRegex.append("backtrace\\.cpp:15.*");
+  DeathRegex.append("backtrace\\.cpp:16.*");
 
   DeathRegex.append("was allocated.*");
-  DeathRegex.append("backtrace\\.cpp:14.*");
+  DeathRegex.append("backtrace\\.cpp:15.*");
   ASSERT_DEATH(GPA.deallocate(Ptr), DeathRegex);
 }
 
@@ -30,12 +31,36 @@ TEST_F(BacktraceGuardedPoolAllocator, UseAfterFree) {
   GPA.deallocate(Ptr);
 
   std::string DeathRegex = "Use After Free.*";
-  DeathRegex.append("backtrace\\.cpp:40.*");
+  DeathRegex.append("backtrace\\.cpp:41.*");
 
   DeathRegex.append("was deallocated.*");
-  DeathRegex.append("backtrace\\.cpp:30.*");
+  DeathRegex.append("backtrace\\.cpp:31.*");
 
   DeathRegex.append("was allocated.*");
-  DeathRegex.append("backtrace\\.cpp:29.*");
+  DeathRegex.append("backtrace\\.cpp:30.*");
   ASSERT_DEATH({ *Ptr = 7; }, DeathRegex);
 }
+
+TEST(Backtrace, Short) {
+  gwp_asan::AllocationMetadata Meta;
+  Meta.AllocationTrace.RecordBacktrace(
+      [](uintptr_t *TraceBuffer, size_t /* Size */) -> size_t {
+        TraceBuffer[0] = 123u;
+        TraceBuffer[1] = 321u;
+        return 2u;
+      });
+  uintptr_t TraceOutput[2] = {};
+  EXPECT_EQ(2u, __gwp_asan_get_allocation_trace(&Meta, TraceOutput, 2));
+  EXPECT_EQ(TraceOutput[0], 123u);
+  EXPECT_EQ(TraceOutput[1], 321u);
+}
+
+TEST(Backtrace, ExceedsStorableLength) {
+  gwp_asan::AllocationMetadata Meta;
+  Meta.AllocationTrace.RecordBacktrace(
+      [](uintptr_t * /* TraceBuffer */, size_t /* Size */) -> size_t {
+        return SIZE_MAX; // Wow, that's big!
+      });
+  uintptr_t TraceOutput;
+  EXPECT_EQ(1u, __gwp_asan_get_allocation_trace(&Meta, &TraceOutput, 1));
+}