On 15.12.2016 16:41, David Laight wrote:
> Try (retyped):
>
> echo 'struct { long a; long long b; } s; int bar { return sizeof s; }' >foo.c
> gcc [-m32] -O2 -S foo.c; cat foo.s
>
> And look at what is generated.
I used __alignof__(unsigned long long) with -m32.
>> Right now ipv6 addresses have an alignment of 4. So we couldn't even
>> naturally pass them to siphash but would need to copy them around, which
>> I feel like a source of bugs.
>
> That is more of a problem on systems that don't support misaligned accesses.
> Reading the 64bit values with two explicit 32bit reads would work.
> I think you can get gcc to do that by adding an aligned(4) attribute to the
> structure member.
Yes, and that is actually my fear, because we support those
architectures. I can't comment on that as I don't understand enough of this.
If someone finds a way to cause misaligned reads on a small box this
seems (maybe depending on sysctls they get fixed up or panic) to be a
much bigger issue than having a hash DoS.
Thanks,
Hannes
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