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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a8e3840bfsm2366511f8f.39.2025.01.09.09.17.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 09 Jan 2025 09:17:44 -0800 (PST) Date: Thu, 9 Jan 2025 18:17:42 +0100 From: Stefano Brivio To: Laurent Vivier Subject: Re: [PATCH] checksum: fix checksum with odd base address Message-ID: <20250109181742.4a97d717@elisabeth> In-Reply-To: <55777e39-4b17-47be-8fff-3dd1faa53e18@redhat.com> References: <20250109130648.326933-1-lvivier@redhat.com> <20250109163642.0a0bfcea@elisabeth> <55777e39-4b17-47be-8fff-3dd1faa53e18@redhat.com> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.41; x86_64-pc-linux-gnu) MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 2NFLS2z4EPZyGSDoZwmkjvsXwIl0P1J9ylATFY-EuFo_1736443067 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: R3R56R6ZZ37PNK4R6G2IULRBNGYKH6YV X-Message-ID-Hash: R3R56R6ZZ37PNK4R6G2IULRBNGYKH6YV X-MailFrom: sbrivio@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top, Mike Jones X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Thu, 9 Jan 2025 17:47:06 +0100 Laurent Vivier wrote: > On 09/01/2025 16:36, Stefano Brivio wrote: > > [Cc'ed Mike who reported this] > > > > On Thu, 9 Jan 2025 14:06:48 +0100 > > Laurent Vivier 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 > > > >> Link: https://bugs.passt.top/show_bug.cgi?id=108 > >> Signed-off-by: Laurent Vivier > >> --- > >> 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) > > Could you update the patch on merge accordingly to your comments? Ah, yes, sure. I'll just add that line. -- Stefano