[llvm] [yaml2obj][MachO] Fix crash from integer underflow with invalid cmdsize (PR #165924)

[James Henderson via llvm-commits]

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@@ -285,7 +285,15 @@ void MachOWriter::writeLoadCommands(raw_ostream &OS) {
 
     // Fill remaining bytes with 0. This will only get hit in partially
     // specified test cases.
-    auto BytesRemaining = LC.Data.load_command_data.cmdsize - BytesWritten;
+    // Prevent integer underflow if BytesWritten exceeds cmdsize.
+    if (BytesWritten > LC.Data.load_command_data.cmdsize) {
+      errs() << "warning: load command " << LC.Data.load_command_data.cmd
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jh7370 wrote:

Prefer `WithColor::warning`, same as e.g. https://github.com/rjmansfield/llvm-project/blob/1857805719a0c32263f4356c3902c63d32db15c8/llvm/lib/ObjectYAML/ELFEmitter.cpp#L1555

Is it possible to map `LC.Data.load_command_data.cmd` back to its named value (e.g. `LC_SEGMENT_64`) for a clearer warning? Alternatively, might it make more sense to use the index of the entry? Indeed, on first read of the test, I thought that was what was being used! I don't know Mach-O well enough, but might there be more than one of the same load command type? If so, the index will disambiguate which one is the problem.

https://github.com/llvm/llvm-project/pull/165924