debug/elf: use saferio.SliceCap when decoding ELF sections

[ZekeLu]

This avoids allocating an overly large slice for corrupt input.

No test case because the problem can only happen for invalid data. Let
the fuzzer find cases like this.

Updates #33121.

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Message from Ian Lance Taylor:

Patch Set 1: Run-TryBot+1

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Patch Set 1:

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[rockdaboot]

👍

I confirm that this change fixes the issue for me.

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This PR is being closed because golang.org/cl/445076 has been merged.

[rockdaboot]

@ZekeLu Just out of curiosity (and for planning): Will the fix go into 1.19.3 or only into 1.20 ?
Because without the fix we can not always recover. Depending on the shnum value, there is either a recoverable panic or a fatal error (fatal error: runtime: out of memory). The error can not be recover()'ed from.

[ZekeLu]

Hi @rockdaboot, no backport request is created, so it won't go into 1.19.x.

If you want to make it capable for fuzzing, I think the following commits should be backported too:

• dbd0ce8
• b6d5831
• fa13731

I'm not sure whether they are qualified for backporting to the 1.19 branch. Maybe @ianlancetaylor can answer your question.

[ianlancetaylor]

I don't think these kinds of changes meet our backport criteria (https://go.dev/wiki/MinorReleases). Sorry.

[rockdaboot]

@ZekeLu Thanks GTN. I fuzz using latest master. But I think we have to roll out our own code to parse ELF files (we are mostly interested in getting symbols).

[ianlancetaylor]

it shouldn't be too hard to use your own copy of the master version of debug/elf for now.