[flang-commits] [flang] [flang][unittests] Fix buffer underrun in LengthWithoutTrailingSpaces (PR #84382)

[Krzysztof Parzyszek via flang-commits]

https://github.com/kparzysz created https://github.com/llvm/llvm-project/pull/84382

Account for the descriptor containing a zero-length string. Also, avoid iterating backwards too far.

This was detected by address sanitizer.

>From fecccb8e27c808d9b7812fe86b50667543875fc8 Mon Sep 17 00:00:00 2001
From: Krzysztof Parzyszek <Krzysztof.Parzyszek at amd.com>
Date: Thu, 7 Mar 2024 15:34:49 -0600
Subject: [PATCH] [flang][unittests] Fix buffer underrun in
 LengthWithoutTrailingSpaces

Account for the descriptor containing a zero-length string. Also, avoid
iterating backwards too far.

This was detected by address sanitizer.
---
 flang/runtime/command.cpp | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/flang/runtime/command.cpp b/flang/runtime/command.cpp
index 7c44890545bd3f..141518f324bbcc 100644
--- a/flang/runtime/command.cpp
+++ b/flang/runtime/command.cpp
@@ -196,11 +196,14 @@ std::int32_t RTNAME(GetCommand)(const Descriptor *value,
 }
 
 static std::size_t LengthWithoutTrailingSpaces(const Descriptor &d) {
-  std::size_t s{d.ElementBytes() - 1};
-  while (*d.OffsetElement(s) == ' ') {
+  std::size_t s{d.ElementBytes()}; // This can be 0.
+  if (s == 0) {
+    return 0;
+  }
+  while (s != 0 && *d.OffsetElement(s - 1) == ' ') {
     --s;
   }
-  return s + 1;
+  return s;
 }
 
 std::int32_t RTNAME(GetEnvVariable)(const Descriptor &name,