[release/6.0] Use "read" to fetch misaligned 64bit values from bundle header

[github-actions]

Backport of #63431 to release/6.0

Fixes: #62273

Customer Impact

The problem was reported by a customer. Single file does not work on ARM32 devices configured to terminate processes on misaligned memory access (happens on some Raspbian systems, unclear how often they're configured in this way).

Testing

Regular test pass.

Manually tested the app built with singlefilehost that contains the fix on Raspbian (bullseye arm32). The app runs regardless of /proc/cpu/alignment settings.

Without the fix, when /proc/cpu/alignment is set to 4 (fault), the app crashes on misaligned reads while processing the bundle headers.

This is a regression introduced when we added support for compression. With compression, we introduced versioned fields into the header, so the header cannot be memcopied all-at-once and the code was changed to read individual fields from memmapped header, which introduced misaligned reads.

Note that setting /proc/cpu/alignment to fault is not typically the default setting. With other settings the kernel can trap the failures and silently handle misaligned reads, so apps run.
However, when configured to fail, no singlefile app would run. In such mode it is a regression from 5.0.

Risk

Low. Instead of direct reads of int64 values form mapped header, we call memcpy, which is alignment-aware.

[ghost]

Tagging subscribers to this area: @agocke, @vitek-karas, @VSadov
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Issue Details

---

Backport of #63431 to release/6.0

/cc @VSadov

Customer Impact

Testing

Risk

Author:	github-actions[bot]
Assignees:	-
Labels:	area-Single-File
Milestone:	-

[janvorli]

LGTM, thank you!

[danmoseley]

OpenDevices01 fix is just now ported.
Networking test failures on Win 11 has port PR open.

[VSadov]

This change only impacts the singlefilehost binary, so I assume the System.Net.Security.Tests failures in regular tests on win-x64 cannot be from this change.

[danmoseley]

Indeed, our comments crossed. I do not know about the ios failures. I can re-run.

[danmoseley]

Should we consider adjusting one of our CI test configurations to set /proc/cpu/alignment to fault? Perhaps it might catch a bug in future.

[VSadov]

set /proc/cpu/alignment to fault

Seems like a good idea.
Misaligned access is generally unexpected. Even silent handling via kernel emulation is a perf loss, which can add up depending on a scenario.

It would need to be on linux-arm32, I think.

[jeffschwMSFT]

Approved. We should take for consideration in 6.0.x

[danmoseley]

set /proc/cpu/alignment to fault

Seems like a good idea. Misaligned access is generally unexpected. Even silent handling via kernel emulation is a perf loss, which can add up depending on a scenario.

It would need to be on linux-arm32, I think.

I'll let you open the issue if you think it's worthwhile, as you have more context.

[agocke]

Issue created #63638

[mmitche]

@jeffschwMSFT This missed the check-in window. Is it critical for 6.0.2?

[agocke]

We only have customer who reported this — probably fine to get it in the next patch

[jeffschwMSFT]

I was talking with @safern to coordinate merge.