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Transforming &[u8; 16] to [u64; 2] using transmute_copy causes Bus errors on aarch64 #55044

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newpavlov opened this issue Oct 13, 2018 · 3 comments · Fixed by #55052
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Transforming &[u8; 16] to [u64; 2] using transmute_copy causes Bus errors on aarch64 #55044

newpavlov opened this issue Oct 13, 2018 · 3 comments · Fixed by #55052

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@newpavlov
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newpavlov commented Oct 13, 2018 •
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See this issue: RustCrypto/stream-ciphers#5

The line in question is here. Does it contain UB or is it a miscompilation? If it's the former, then I think transmute_copy documentation should be updated accordingly.
@matthewjasper
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It's definitely UB somewhere: try running this with MIRI https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=794f6a68cf5009fe2c9c8c1277fd42f0.

@newpavlov
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newpavlov commented Oct 13, 2018 •
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Looks like replacing transmute_copy with read_unaligned solves the problem. But is it intended for transmute_copy to use aligned load? Currently it uses ptr::read, shouldn't it be replaced with ptr::read_unaligned?

@newpavlov
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cc rust-lang/rust-memory-model#14

newpavlov added a commit to newpavlov/rust that referenced this issue Oct 13, 2018
@newpavlov
Use read_unaligned instead of read in transmute_copy
4210cca 
Closes: rust-lang#55044

This change could result in performance regression on non-x86 platforms. Alternative would be to update `transmute_copy` with alignment requirements.
@newpavlov newpavlov mentioned this issue Oct 13, 2018
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bors added a commit that referenced this issue Nov 13, 2018
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Auto merge of #55052 - newpavlov:patch-2, r=alexcrichton
c4371c8 
Use read_unaligned instead of read in transmute_copy

Closes: #55044

This change could result in performance regression on non-x86 platforms. (but it also can fix some of UB which lurks in existing programs) An alternative would be to update `transmute_copy` documentation with alignment requirements.
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Use read_unaligned instead of read in transmute_copy newpavlov/rust
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@newpavlov @matthewjasper