Stream: git-wasmtime

Topic: wasmtime / PR #4314 Change how unwind information is stor...

Wasmtime GitHub notifications bot (Jun 24 2022 at 14:17):

alexcrichton opened PR #4314 from xdata-pdata to main:

Unwind information on Windows is stored in two separate locations. The
first location is the unwind information itself which corresponds to
UNWIND_INFO. The second location is a list of RUNTIME_INFO
structures which point to function bodes and UNWIND_INFO structures.

Currently in Wasmtime the UNWIND_INFO structures are stored just after
functions themselves with a somewhat cryptic comment indicating that
Windows prefers this (I'm unsure as to the provenance of this comment).
The RUNTIME_INFO data is then stored in a separate section which has
the custom name of _wasmtime_winx64_unwind.

After my recent foray into trying to debug windows-2022 bad unwind
information again I realized though that Windows actually has official
sections for these two unwind information items. The .xdata section is
used to store the UNWIND_INFO structures and the .pdata section
stores the RUNTIME_INFO list. To try to be somewhat idiomatic and
perhaps one day even hook into standard Windows debugging tools I went
ahead and refactored how our unwind information is stored to match this.

Perhaps the main benefit of this is that it reduces the size of the
read/execute section of the binary. Previously the unwind information
was executable since it was stored in the .text section, but
unnecessarily so. Now it's in a read-only section which is in theory a
small amount of hardening.

Otherwise though I don't think this will really help all that much to
hook up in to standard debugging tools like objdump because it's all
still stored in an ELF file rather than a COFF file.

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Wasmtime GitHub notifications bot (Jun 24 2022 at 14:17):

alexcrichton requested peterhuene for a review on PR #4314.

Wasmtime GitHub notifications bot (Jun 28 2022 at 02:46):

peterhuene created PR review comment:

It's fairer to say that every address in the RUNTIME_FUNCTION structure must be start-of-text-section relative (thus necessitating that .xdata comes _after_ the text section) because we're using RtlAddFunctionTable to register the .pdata section contents and using the loaded text section's address as the "image base address".

I think mentioning "relocations" here might be confusing since technically no fixups are being recorded in the artifact to apply at load time (unlike what a real .pdata section would have in a PE since RtlAddFunctionTable wouldn't be used in that case).

Wasmtime GitHub notifications bot (Jun 28 2022 at 02:46):

peterhuene submitted PR review.

Wasmtime GitHub notifications bot (Jun 28 2022 at 02:46):

peterhuene created PR review comment:

Nit: append_data argument was removed but still documented above.

Wasmtime GitHub notifications bot (Jun 28 2022 at 02:46):

peterhuene submitted PR review.

Wasmtime GitHub notifications bot (Jun 28 2022 at 02:53):

peterhuene edited PR review comment.

Wasmtime GitHub notifications bot (Jun 28 2022 at 14:41):

alexcrichton updated PR #4314 from xdata-pdata to main.

Wasmtime GitHub notifications bot (Jun 28 2022 at 14:41):

alexcrichton has enabled auto merge for PR #4314.

Wasmtime GitHub notifications bot (Jun 28 2022 at 15:40):

alexcrichton merged PR #4314.

Last updated: Dec 23 2024 at 12:05 UTC