[Cc'ed Mike who reported this]
On Thu, 9 Jan 2025 14:06:48 +0100
Laurent Vivier <lvivier(a)redhat.com> wrote:
csum_unfolded() must call csum_avx2() with a
32byte aligned base address.
To be able to do that if the buffer is not correctly aligned,
it splits the buffers in 2 parts, the second part is 32byte aligned and
can be used with csum_avx2(), the first part is the remaining part, that
is not 32byte aligned and we use sum_16b() to compute the checksum.
A problem appears if the length of the first part is odd because
the checksum is using 16bit words to do the checksum.
If the length is odd, when the second part is computed, all words are
shifted by 1 byte, meaning weight of upper and lower byte is swapped.
For instance a 13 bytes buffer:
bytes:
aa AA bb BB cc CC dd DD ee EE ff FF gg
16bit words:
AAaa BBbb CCcc DDdd EEee FFff 00gg
If we don't split the sequence, the checksum is:
AAaa + BBbb + CCcc + DDdd + EEee + FFff + 00gg
If we split the sequence with an even length for the first part:
(AAaa + BBbb) + (CCcc + DDdd + EEee + FFff + 00gg)
But if the first part has an odd length:
(AAaa + BBbb + 00cc) + (ddCC + eeDD + ffEE + ggFF)
Thanks, this description is really helpful.
To avoid the problem, do not call csum_avx2() if
the first part cannot
have an even length, and compute the checksum of all the buffer using
sum_16b().
This is slower but it can only happen if the buffer base address is odd,
and this can only happen if the binary is built using '-Os', and that
means we have chosen to prioritize size over speed.
Reported-by: Mike Jones <mike(a)mjones.io>
Link:
https://bugs.passt.top/show_bug.cgi?id=108
Signed-off-by: Laurent Vivier <lvivier(a)redhat.com>
---
checksum.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/checksum.c b/checksum.c
index 1c4354d35734..2fd6867cdf75 100644
--- a/checksum.c
+++ b/checksum.c
@@ -452,7 +452,7 @@ uint32_t csum_unfolded(const void *buf, size_t len, uint32_t init)
intptr_t align = ROUND_UP((intptr_t)buf, sizeof(__m256i));
unsigned int pad = align - (intptr_t)buf;
- if (len < pad)
+ if (pad & 1 || len < pad)
I'm fine applying this as it is, because the issue is quite nasty and we
have this great commit message anyway, but for clarity, could we have a
comment mentioning why we're doing this? Something like:
/* Don't mix sum_16b() and csum_avx2() with odd padding lengths */
(I'm not quite satisfied with it but I find it better than nothing).
> pad = len;
>
> if (pad)