Make sure that the std::optional<> result is checked before being accessed

[romainthomas]

In some cases (c.f. the attached binary), getAsSectionOffset() is failing and thus, the inline de-referencing
is wrong.

D_test.bin.zip

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[llvmbot]

@llvm/pr-subscribers-debuginfo

Author: Romain Thomas (romainthomas)

Changes

In some cases (c.f. the attached binary), getAsSectionOffset() is failing and thus, the inline de-referencing
is wrong.

D_test.bin.zip

---

Full diff: https://github.com/llvm/llvm-project/pull/116479.diff

1 Files Affected:

• (modified) llvm/lib/DebugInfo/DWARF/DWARFDie.cpp (+6-2)

diff --git a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
index c1cd587877de0c..a14e4451d5dba8 100644
--- a/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFDie.cpp
@@ -394,9 +394,13 @@ Expected<DWARFAddressRangesVector> DWARFDie::getAddressRanges() const {
 
   std::optional<DWARFFormValue> Value = find(DW_AT_ranges);
   if (Value) {
+    std::optional<uint64_t> SecOff = Value->getAsSectionOffset();
+    if (!SecOff) {
+      return DWARFAddressRangesVector();
+    }
     if (Value->getForm() == DW_FORM_rnglistx)
-      return U->findRnglistFromIndex(*Value->getAsSectionOffset());
-    return U->findRnglistFromOffset(*Value->getAsSectionOffset());
+      return U->findRnglistFromIndex(*SecOff);
+    return U->findRnglistFromOffset(*SecOff);
   }
   return DWARFAddressRangesVector();
 }

[dwblaikie]

Please omit the {} from single-line if statements like this.

Might be worth making this code a bit more consistent in the way it handles non-present optionals, since currentnly the two cases right next to each other are handled differently (one with an if, the other with an if!+early return). I guess the early return might be preferable, consistent with LLVM's style preference to reduce indentation:

optional Value = ...
if (!Value)
  return;
optional SecOff = ...
if (!SecOff)
  return;
...

Also - can you include a test case that demonstrates/validates this bug/fix?

[romainthomas]

I fixed the coding in the commit 394adb0.

How would you like to have the test case? Should I include the binary attached to the description of this issue in the LLVM repo?

[dwblaikie]

We try to avoid checking in binary files, especially unbounded ones/ones without source code/asm code we could use to regenerate them.

Could you run the fixed dwarfdump on your test case, dump it out, then try to create a test case in assembly by hand that is minimal and exercises the interesting codepath (ie: crashes without this patch applied)?