On Thu, Sep 5, 2013 at 8:40 PM, David Miller <da...@davemloft.net> wrote:
> When doing comparisons, this structure is being accessed as a byte
> array in 'long' sized chunks, not by its members. Therefore, the
Like ... an optimized memcmp() does? Which handles all (un)alignment
well?
>> To completely honest, I think the correct alignment should be
>> sizeof(long) because I know that 'long' is not always 8 bytes on all
>> architectures. However, you made the point before that this could
>> break the alignment of the 64-bit values on architectures where 'long'
>> is 32 bits wide, so 8 bytes is the generic solution.
>
> Look at net/core/flow.c:flow_key_compare().
>
> And then we annotate struct flowi with
>
> } __attribute__((__aligned__(BITS_PER_LONG/8)));
>
> Don't reinvent the wheel, either mimick how existing code does
> this kind of thing or provide a justification for doing it differently
> and update the existing cases to match and be consistent.
This may still break the alignment of 64-bit values on 32-bit architectures
where __alignof__(u64) == 8.
Now let's look at the comparison function:
static bool __cmp_key(const struct sw_flow_key *key1,
const struct sw_flow_key *key2, int key_start, int key_end)
{
const long *cp1 = (long *)((u8 *)key1 + key_start);
const long *cp2 = (long *)((u8 *)key2 + key_start);
long diffs = 0;
int i;
for (i = key_start; i < key_end; i += sizeof(long))
diffs |= *cp1++ ^ *cp2++;
return diffs == 0;
}
Why don't you abort the loop if a difference is found?
Or is this a security-related struct where you want to protect against
timing attacks?
Furthermore, as you compare the raw bytes, I hope you always
initialize all gaps in the struct to zero.
E.g. there's a 2-byte gap immediately after "ip", as the next member
is 32-bit (except op m68k, where the 32-bit member will be 2-byte aligned).
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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