Fix aligned access in memrchr fallback #9
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The fallback did not compute the offset correctly. It was intentioned to
land on usize-aligned addresses but did not. This resulted in a SIGBUS
on an ARMv7 platform!
I think like this, if we have a slice with pointer `ptr` and length
`len`, we want to find the last usize-aligned offset in the slice.
The correct computation should be:
For example if ptr = 1 and len = 6, and size_of::<usize>() is 4:
[ x x x x x x ]
1 2 3 4 5 6
^-- last aligned address at offset 3 from the start.
The last aligned address is ptr + len - (ptr + len) % usize_size.
Compute offset from the start as:
offset = len - (ptr + len) % usize_size = 6 - (1 + 6) % 4 = 6 - 3 = 3.
I believe the answer was always correct previously, if the platform
supported unaligned addresses.
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Interesting! Do any of the newly added tests fail without the fix? (I'm assuming not.) |
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They do fail due to the new debug assertion. |
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This makes the critical calculation easier to understand.
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Thanks for this! Sorry for the tardiness, completely forgot this was sitting here. :-) |
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The fallback implementation is not computing the aligned offset correctly(!). This is UB unfortunately and does crash on platforms where aligned accesses are not allowed.
@tomaka found a crash in memrchr on an ARMv7 (using the memrchr in libstd) and I suspect this was the cause. I wanted to put the fix here first, so that we can review it together. I think the original calculation was just bogus. See the first commit's log for more explanation.