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    Safe explicit_bzero/timingsafe_bcmp for Linux ☶ show c security github.com
    mikejsavage avatar via mikejsavage 4 years ago | cached | 3 comments

Despite how trivial this is, I’m not 100% sure that it’s correct. Posting it here so it can have some more experienced eyes reading it.

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    tedu avatar tedu 4 years ago | link

    != is probably the wrong operator in bcmp. Most implementations use xor.

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      effdee avatar effdee 4 years ago | link

      Using xor is just a tiny optimization (makes it 3 bytes smaller and, in theory, saves a clock cycle per iteration):

      < cmp    %al,%dl
< setne  %al
---
> xor    %edx,%eax
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        tedu avatar tedu 4 years ago | link

        Until you use a different compiler/CPU and end up with the equivalent of:

        if (a[i] == b[i])
     rv |= 1;

        And suddenly you have data dependent branches again. That’s very unlikely, but since the point of the exercise is to avoid branching instructions, best to avoid potentially branching operators entirely.