[release/7.0] Fix RVA field alignment (#60) #61
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This ensures that section starts are aligned by adjusting the previous Range's length to ensure the computed start of a new Range meets the alignment requirements. It was done this way rather than just computing an aligned start for the new Range, because the TextMap assumes that the Ranges are contiguous - see for example GetNextRVA. GetNextRVA was used to compute the Code RVA, before adding that segment to the TextMap. This meant that the alignment (only added later) wasn't taken into account when writing out the code, but then later the TextMap was modified to include the alignment. This is fixed by first inserting an aligned zero-length Code segment before writing the code, then inserting it again once the length is known. Removing the Code alignment altogether would work too, but the alignment constants are kept in there, because it looks like they were added intentionally, even though the reason is unknown. Note that before this change, the start of the Code segment (at least on x64) was not 16-byte aligned in the first place, so this change will actually move the beginning of the Code segment.
vitek-karas
approved these changes
Jan 11, 2023
jeffschwMSFT
approved these changes
Jan 11, 2023
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@agocke @vitek-karas now that we have over a month of this fix shipping in .NET 8 Preview 1, I think we should take it into the next 7.0 servicing release. |
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Sounds good to me. I'll send this to tactics. |
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Sounds good! |
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Approved. |
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Backporting #60 to the release/7.0 branch (which I created for this purpose, pointing at the commit currently referenced by the dotnet/linker 7.0 branch: https://github.com/dotnet/linker/tree/release/7.0/external).
This implements a cecil fix required to support the alignment requirements of the Roslyn
CreateSpansupport.Fixes dotnet/runtime#79477
Customer Impact
Soon (once we ship a Roslyn that supports the CreateSpan APIs introduced in 7.0), trimmed 7.0 apps using the latest Roslyn may start crashing if they use constant arrays of
longorulongthat are cast toReadOnlySpan<T>. The failure mode we have observed isArgumentExceptionduringCreateSpan. The crash happens if the constant data representing the array happens to be misaligned after trimming (which is likely, because the 7.0 trimmer did not respect the new alignment requirements).Note that this hasn't actually been observed externally yet, but we anticipate that we will once the new Roslyn is shipped.
Testing
CreateSpansupport is being used in Use Roslyn support for RuntimeHelpers.CreateSpan (or field caching downlevel) runtime#79461, where we originally discovered the issue that this fixes. We will get validation of the same fix on 8.0 bits there.We might want to wait to service this until we have released the fix in an 8.0 preview to get extra validation in the wild.
Risk
Medium to high risk. This changes the alignment of the code section for all assemblies written by cecil on x64, and can also change the alignment of later sections, even for code which doesn't use
CreateSpanhelpers. The change is significant at the binary level, but we have no reason to believe that this will break anything downstream. If the binary change is problematic, we would expect to see catastrophic failures during the testing, but this was not the case. (An earlier incomplete fix did result in catastrophic failures, validating this expectation).