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Zero the REPARSE_MOUNTPOINT_DATA_BUFFER header #107900

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Feb 11, 2023
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Zero the REPARSE_MOUNTPOINT_DATA_BUFFER header #107900

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2 changes: 2 additions & 0 deletions library/std/src/sys/windows/fs.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1393,6 +1393,8 @@ fn symlink_junction_inner(original: &Path, junction: &Path) -> io::Result<()> {
let mut data = Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
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Might MU::zeroed be clearer here?

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let mut data = Align8([MaybeUninit::<u8>::uninit(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);
let mut data = Align8([MaybeUninit::<u8>::zeroed(); c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE]);

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Hm, that would zero the entire buffer tho. Which I assume we're trying to avoid because it's large. Otherwise [0_u8, c::MAXIMUM_REPARSE_DATA_BUFFER_SIZE] would probably be ok because it's a plain old data struct.

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It probably doesn't matter too much if it's only used in tests but good point.

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It's apparently 16KB for the buffer, so we'd be touching 4 pages on x86. That doesn't thrash all of a standard 32KB cache but it's still a lot when compared to simply zeroing a few shorts and longs.

otoh, "just tests".

let data_ptr = data.0.as_mut_ptr();
let db = data_ptr.cast::<c::REPARSE_MOUNTPOINT_DATA_BUFFER>();
// Zero the header to ensure it's fully initialized, including reserved parameters.
*db = mem::zeroed();
let buf = ptr::addr_of_mut!((*db).ReparseTarget).cast::<c::WCHAR>();
let mut i = 0;
// FIXME: this conversion is very hacky
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