[flang] [flang-rt] Subscript overrun could occur in namelists during a READ command. by kwyatt-ext · Pull Request #176959 · llvm/llvm-project

kwyatt-ext · 2026-01-20T16:20:15Z

NOTE: This is a new pull request, as the prior didn't have labels properly applied.

If a bad subscript is provided in a namelisted record, the HandleSubscripts() routine can read off into infinity. This patch ensures that a read will not go beyond the rank of the expected variable.

The failure will then be captured in the return status (IOSTAT) of the READ.

The small test demonstrates the failure before and after the fix.

github-actions · 2026-01-20T16:20:35Z

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I realized that this is an executable test so it needs to go into llvm-test-suite.

kwyatt-ext · 2026-03-03T17:16:28Z

I was looking over my open PRs. I realized this one has an executable test associated with it. I will remove that test and put it within the llvm-test-suite. A link to that PR to follow.

Otherwise, I'd like to "Ping" this PR, as it's been open for a while.

kwyatt-ext · 2026-03-03T18:00:50Z

Here are the test artifacts for this change:

check-flang:

Testing Time: 141.34s

Total Discovered Tests: 4239
  Unsupported      :   81 (1.91%)
  Passed           : 4147 (97.83%)
  Expectedly Failed:   11 (0.26%)


FLANG Container Test completed 3 minutes (201 s).

Total Time: 3 minutes (201 s)
Completed : Tue Mar  3 11:20:53 AM CST 2026

check-flang-rt:

Testing Time: 1.59s

Total Discovered Tests: 279
  Passed: 279 (100.00%)


FLANG Container Test completed 0 minutes (58 s).

Total Time: 0 minutes (58 s)
Completed : Tue Mar  3 11:23:24 AM CST 2026

llvm-test-suite:

Testing Time: 218.56s

Total Discovered Tests: 6931
  Passed: 6931 (100.00%)


LLVM Test Suite Container Test completed in 3 minutes (223 s).

Total Time: 3 minutes (223 s)
Completed : Tue Mar  3 11:00:50 AM CST 2026

kwyatt-ext · 2026-03-03T18:20:15Z

PR for the new executable test:
llvm/llvm-test-suite#361

tarunprabhu · 2026-03-05T11:02:35Z

Is there an associated issue that demonstrates the buffer overflow?

kwyatt-ext · 2026-03-05T14:52:09Z

There was not an opened issue. This was my first fix and I never opened one. It was identified when we ran an internal cray test against flang. If I need to (or you'd just prefer), I will open one. The PR will end up being a description of a core/hanging and the test associated with this PR: llvm/llvm-test-suite#361.

Please let me know how I should proceed and I'll be happy to. Thank you for looking.

tarunprabhu · 2026-03-05T14:56:41Z

There was not an opened issue. This was my first fix and I never opened one. It was identified when we ran an internal cray test against flang. If I need to (or you'd just prefer), I will open one. The PR will end up being a description of a core/hanging and the test associated with this PR: llvm/llvm-test-suite#361.

Please let me know how I should proceed and I'll be happy to. Thank you for looking.

You don't strictly, but the title of the PR said "could occur", so I was wondering if there was a test that actually triggered it that others could use to reproduce. Might be worth opening an issue.

kwyatt-ext · 2026-03-05T21:01:50Z

I created an issue and added linking comments to this and the llvm-test-suite update:
#184884

kwyatt-ext · 2026-03-17T21:18:12Z

Ping.

I'm not sure who the right reviewer would be. I do see that @klausler touched this code a number of years ago.

Thank you for your time.

klausler · 2026-03-17T21:42:54Z

-  for (; ch && *ch != ')'; ++j) {
+
+  // Read subscripts, but don't exceed rank to prevent buffer overrun.
+  for (int Rank = source.rank(); ch && *ch != ')' && j <= Rank; ++j) {


The coding style for the modern C++ bits of flang-new always uses braced initialization and lower-case variable names.

Thank you. I corrected the coding standard issues.

If you are satisfied with the changes, would you please also merge them for me? I don't have write permissions yet.

Ok, but please apply for them soon!

github-actions · 2026-03-18T21:26:34Z

@kwyatt-ext Congratulations on having your first Pull Request (PR) merged into the LLVM Project!

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kwyatt-ext · 2026-03-19T13:26:50Z

@klausler Thank you. Will do.

Subscript overrun could occur in namelists during a READ command.

a3eab98

llvmbot added the flang-rt label Jan 20, 2026

kwyatt-ext added 2 commits March 2, 2026 15:57

Merge branch 'llvm:main' into Bug_Subscript-Overrun

e927a30

Delete flang-rt/test/Runtime/bad_subscript_overrun01.f90

eea96ea

I realized that this is an executable test so it needs to go into llvm-test-suite.

This was referenced Mar 3, 2026

[flang] [flang-rt] Adding a runtime test that demonstrates a subscript overrun correction. llvm/llvm-test-suite#360

Closed

[flang] Adding a runtime test that demonstrates a subscript overrun correction. llvm/llvm-test-suite#361

Merged

kwyatt-ext mentioned this pull request Mar 5, 2026

[flang] [flang-rt] Subscript overrun occurs in namelists during a READ command. #184884

Closed

klausler reviewed Mar 17, 2026

View reviewed changes

Changed variable name/initialization to match flang coding standard.

bebdc6d

klausler merged commit 2a89e24 into llvm:main Mar 18, 2026
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kwyatt-ext mentioned this pull request Mar 19, 2026

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