Hi Ard,
On Thu, Feb 02, 2017 at 03:56:28PM +0000, Ard Biesheuvel wrote:
> + const int size = sizeof(unsigned long);
> + int delta = ((unsigned long)dst ^ (unsigned long)src) & (size - 1);
> + int misalign = 0;
> +
> + if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && delta) {
> + misalign = 1 << __ffs(delta);
> +
> + /*
> + * If we care about alignment, process as many bytes as
> + * needed to advance dst and src to values whose alignments
> + * equal their relative misalignment. This will allow us to
> + * process the remainder of the input using optimal strides.
> + */
> + while (((unsigned long)dst & (misalign - 1)) && len > 0) {
> + *dst++ ^= *src++;
> + len--;
> + }
> + }
>
> + while (len >= size && !misalign) {
> + *(unsigned long *)dst ^= *(unsigned long *)src;
> + dst += size;
> + src += size;
> + len -= size;
> + }
>
Unfortunately this is still broken, for two different reasons. First, if the
pointers have the same relative misalignment, then 'delta' and 'misalign' will
be set to 0 and long accesses will be used, even though the pointers may
actually be misaligned, e.g. 0x80000001 and 0x90000001. Second, if the pointers
have a relative misalignent that is not a power-of-2, then 'misalign' will be
set to the wrong value. For example, with delta=3, it's actually only safe to
do byte accesses, but the code will set misalign=2 and do u16 accesses.
I kind of liked the version with put_unaligned/get_unaligned (and it seems to
perform okay on MIPS, though not on ARM which is probably more important). But
if the various cases with !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS are going to
be handled/optimized I think they will just need to be separated out, maybe
something like this:
void crypto_xor(u8 *dst, const u8 *src, unsigned int len)
{
#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
unsigned long delta;
if (((unsigned long)dst | (unsigned long)src | len) %
sizeof(unsigned long) == 0) {
/* fast path: everything is aligned, including len */
while (len >= sizeof(unsigned long)) {
*(unsigned long *)dst ^= *(unsigned long *)src;
dst += sizeof(unsigned long);
src += sizeof(unsigned long);
len -= sizeof(unsigned long);
}
return;
}
/* handle relative misalignment */
delta = (unsigned long)dst ^ (unsigned long)src;
if (delta % sizeof(unsigned long)) {
/* 1-byte relative misalignment: do byte accesses */
if (delta & 1) {
while (len--)
*dst++ ^= *src++;
return;
}
/* 2-byte relative misalignment: do u16 accesses */
if ((delta & 2) || sizeof(unsigned long) == 4) {
if ((unsigned long)dst % 2 && len) {
*dst++ ^= *src++;
len--;
}
while (len >= 2) {
*(u16 *)dst ^= *(u16 *)src;
dst += 2, src += 2, len -= 2;
}
if (len)
*dst ^= *src;
return;
}
/* 4-byte relative misalignment: do u32 accesses */
while ((unsigned long)dst % 4 && len) {
*dst++ ^= *src++;
len--;
}
while (len >= 4) {
*(u32 *)dst ^= *(u32 *)src;
dst += 4, src += 4, len -= 4;
}
while (len--)
*dst++ ^= *src++;
return;
}
/* handle absolute misalignment */
while ((unsigned long)dst % sizeof(unsigned long) && len) {
*dst++ ^= *src++;
len--;
}
#endif /* !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS */
while (len >= sizeof(unsigned long)) {
*(unsigned long *)dst ^= *(unsigned long *)src;
dst += sizeof(unsigned long);
src += sizeof(unsigned long);
len -= sizeof(unsigned long);
}
while (len--)
*dst++ ^= *src++;
}
