[XRay][compiler-rt] Fix oob memory access in FDR BufferQueue iterator

[rickyz]

Before this change, the FDR BufferQueue iterator could access oob memory
due to checks of the form !Buffers[Offset].Used && Offset != Max. This
allows access to Buffers[Max], which is past the end of the Buffers
array. This can lead to crashes when that memory is not mapped. Fix this
by testing Offset != Max first.

[llvmbot]

@llvm/pr-subscribers-xray

Author: Ricky Zhou (rickyz)

Changes

Before this change, the FDR BufferQueue iterator could access memory out
of due to checks of the form !Buffers[Offset].Used && Offset != Max,
which can potentially access Buffers[Max], which is past the end of
the Buffers. Fix this by testing Offset != Max first.

---

Full diff: https://github.com/llvm/llvm-project/pull/90940.diff

1 Files Affected:

• (modified) compiler-rt/lib/xray/xray_buffer_queue.h (+2-2)

diff --git a/compiler-rt/lib/xray/xray_buffer_queue.h b/compiler-rt/lib/xray/xray_buffer_queue.h
index e1739d050f3d0d..8d33f73576b5e2 100644
--- a/compiler-rt/lib/xray/xray_buffer_queue.h
+++ b/compiler-rt/lib/xray/xray_buffer_queue.h
@@ -87,7 +87,7 @@ class BufferQueue {
       DCHECK_NE(Offset, Max);
       do {
         ++Offset;
-      } while (!Buffers[Offset].Used && Offset != Max);
+      } while (Offset != Max && !Buffers[Offset].Used);
       return *this;
     }
 
@@ -107,7 +107,7 @@ class BufferQueue {
           Max(M) {
       // We want to advance to the first Offset where the 'Used' property is
       // true, or to the end of the list/queue.
-      while (!Buffers[Offset].Used && Offset != Max) {
+      while (Offset != Max && !Buffers[Offset].Used) {
         ++Offset;
       }
     }