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checksum: fix checksum with odd base address

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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)

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@mjones.io>
Link: https://bugs.passt.top/show_bug.cgi?id=108
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Added comment explaining why we check for pad & 1]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
Laurent Vivier 2025-01-09 14:06:48 +01:00 committed by Stefano Brivio
parent 725acd111b
commit 2c174f1fe8
1 changed files with 2 additions and 1 deletions
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checksum.c
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@ -452,7 +452,8 @@ 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)
/* Don't mix sum_16b() and csum_avx2() with odd padding lengths */
if (pad & 1 || len < pad)
pad = len;
if (pad)