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([2a01:e0a:e10:ef90:4c84:58cb:a1ef:8b78]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42e969a8bd5sm8732435e9.0.2024.09.24.23.40.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Sep 2024 23:40:28 -0700 (PDT) Message-ID: Date: Wed, 25 Sep 2024 08:40:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 4/4] tcp: Update TCP checksum using an iovec array To: David Gibson References: <20240924154642.182857-1-lvivier@redhat.com> <20240924154642.182857-5-lvivier@redhat.com> From: Laurent Vivier Autocrypt: addr=lvivier@redhat.com; keydata= xsFNBFYFJhkBEAC2me7w2+RizYOKZM+vZCx69GTewOwqzHrrHSG07MUAxJ6AY29/+HYf6EY2 WoeuLWDmXE7A3oJoIsRecD6BXHTb0OYS20lS608anr3B0xn5g0BX7es9Mw+hV/pL+63EOCVm SUVTEQwbGQN62guOKnJJJfphbbv82glIC/Ei4Ky8BwZkUuXd7d5NFJKC9/GDrbWdj75cDNQx UZ9XXbXEKY9MHX83Uy7JFoiFDMOVHn55HnncflUncO0zDzY7CxFeQFwYRbsCXOUL9yBtqLer Ky8/yjBskIlNrp0uQSt9LMoMsdSjYLYhvk1StsNPg74+s4u0Q6z45+l8RAsgLw5OLtTa+ePM JyS7OIGNYxAX6eZk1+91a6tnqfyPcMbduxyBaYXn94HUG162BeuyBkbNoIDkB7pCByed1A7q q9/FbuTDwgVGVLYthYSfTtN0Y60OgNkWCMtFwKxRaXt1WFA5ceqinN/XkgA+vf2Ch72zBkJL RBIhfOPFv5f2Hkkj0MvsUXpOWaOjatiu0fpPo6Hw14UEpywke1zN4NKubApQOlNKZZC4hu6/ 8pv2t4HRi7s0K88jQYBRPObjrN5+owtI51xMaYzvPitHQ2053LmgsOdN9EKOqZeHAYG2SmRW LOxYWKX14YkZI5j/TXfKlTpwSMvXho+efN4kgFvFmP6WT+tPnwARAQABzSNMYXVyZW50IFZp dmllciA8bHZpdmllckByZWRoYXQuY29tPsLBeAQTAQIAIgUCVgVQgAIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQ8ww4vT8vvjwpgg//fSGy0Rs/t8cPFuzoY1cex4limJQfReLr SJXCANg9NOWy/bFK5wunj+h/RCFxIFhZcyXveurkBwYikDPUrBoBRoOJY/BHK0iZo7/WQkur 6H5losVZtrotmKOGnP/lJYZ3H6OWvXzdz8LL5hb3TvGOP68K8Bn8UsIaZJoeiKhaNR0sOJyI YYbgFQPWMHfVwHD/U+/gqRhD7apVysxv5by/pKDln1I5v0cRRH6hd8M8oXgKhF2+rAOL7gvh jEHSSWKUlMjC7YwwjSZmUkL+TQyE18e2XBk85X8Da3FznrLiHZFHQ/NzETYxRjnOzD7/kOVy gKD/o7asyWQVU65mh/ECrtjfhtCBSYmIIVkopoLaVJ/kEbVJQegT2P6NgERC/31kmTF69vn8 uQyW11Hk8tyubicByL3/XVBrq4jZdJW3cePNJbTNaT0d/bjMg5zCWHbMErUib2Nellnbg6bc 2HLDe0NLVPuRZhHUHM9hO/JNnHfvgiRQDh6loNOUnm9Iw2YiVgZNnT4soUehMZ7au8PwSl4I KYE4ulJ8RRiydN7fES3IZWmOPlyskp1QMQBD/w16o+lEtY6HSFEzsK3o0vuBRBVp2WKnssVH qeeV01ZHw0bvWKjxVNOksP98eJfWLfV9l9e7s6TaAeySKRRubtJ+21PRuYAxKsaueBfUE7ZT 7zfOwU0EVgUmGQEQALxSQRbl/QOnmssVDxWhHM5TGxl7oLNJms2zmBpcmlrIsn8nNz0rRyxT 460k2niaTwowSRK8KWVDeAW6ZAaWiYjLlTunoKwvF8vP3JyWpBz0diTxL5o+xpvy/Q6YU3BN efdq8Vy3rFsxgW7mMSrI/CxJ667y8ot5DVugeS2NyHfmZlPGE0Nsy7hlebS4liisXOrN3jFz asKyUws3VXek4V65lHwB23BVzsnFMn/bw/rPliqXGcwl8CoJu8dSyrCcd1Ibs0/Inq9S9+t0 VmWiQWfQkz4rvEeTQkp/VfgZ6z98JRW7S6l6eophoWs0/ZyRfOm+QVSqRfFZdxdP2PlGeIFM C3fXJgygXJkFPyWkVElr76JTbtSHsGWbt6xUlYHKXWo+xf9WgtLeby3cfSkEchACrxDrQpj+ Jt/JFP+q997dybkyZ5IoHWuPkn7uZGBrKIHmBunTco1+cKSuRiSCYpBIXZMHCzPgVDjk4viP brV9NwRkmaOxVvye0vctJeWvJ6KA7NoAURplIGCqkCRwg0MmLrfoZnK/gRqVJ/f6adhU1oo6 z4p2/z3PemA0C0ANatgHgBb90cd16AUxpdEQmOCmdNnNJF/3Zt3inzF+NFzHoM5Vwq6rc1JP jfC3oqRLJzqAEHBDjQFlqNR3IFCIAo4SYQRBdAHBCzkM4rWyRhuVABEBAAHCwV8EGAECAAkF AlYFJhkCGwwACgkQ8ww4vT8vvjwg9w//VQrcnVg3TsjEybxDEUBm8dBmnKqcnTBFmxN5FFtI WlEuY8+YMiWRykd8Ln9RJ/98/ghABHz9TN8TRo2b6WimV64FmlVn17Ri6FgFU3xNt9TTEChq AcNg88eYryKsYpFwegGpwUlaUaaGh1m9OrTzcQy+klVfZWaVJ9Nw0keoGRGb8j4XjVpL8+2x OhXKrM1fzzb8JtAuSbuzZSQPDwQEI5CKKxp7zf76J21YeRrEW4WDznPyVcDTa+tz++q2S/Bp P4W98bXCBIuQgs2m+OflERv5c3Ojldp04/S4NEjXEYRWdiCxN7ca5iPml5gLtuvhJMSy36gl U6IW9kn30IWuSoBpTkgV7rLUEhh9Ms82VWW/h2TxL8enfx40PrfbDtWwqRID3WY8jLrjKfTd R3LW8BnUDNkG+c4FzvvGUs8AvuqxxyHbXAfDx9o/jXfPHVRmJVhSmd+hC3mcQ+4iX5bBPBPM oDqSoLt5w9GoQQ6gDVP2ZjTWqwSRMLzNr37rJjZ1pt0DCMMTbiYIUcrhX8eveCJtY7NGWNyx FCRkhxRuGcpwPmRVDwOl39MB3iTsRighiMnijkbLXiKoJ5CDVvX5yicNqYJPKh5MFXN1bvsB kmYiStMRbrD0HoY1kx5/VozBtc70OU0EB8Wrv9hZD+Ofp0T3KOr1RUHvCZoLURfFhSQ= In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Message-ID-Hash: XTJMXT3EB5HZQU25TJLTSZ6SPREACKON X-Message-ID-Hash: XTJMXT3EB5HZQU25TJLTSZ6SPREACKON X-MailFrom: lvivier@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 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 25/09/2024 03:12, David Gibson wrote: > On Tue, Sep 24, 2024 at 05:46:42PM +0200, Laurent Vivier wrote: >> TCP header and payload are supposed to be in the same buffer, >> and tcp_update_check_tcp4()/tcp_update_check_tcp6() compute >> the checksum from the base address of the header using the >> length of the IP payload. >> >> In the future (for vhost-user) we need to dispatch the TCP header and >> the TCP payload through several buffers. To be able to manage that, we >> provide an iovec array that points to the data of the TCP frame. >> We provide also an offset to be able to provide an array that contains >> the TCP frame embedded in an lower level frame, and this offset points >> to the TCP header inside the iovec array. >> >> Signed-off-by: Laurent Vivier >> --- >> checksum.c | 1 - >> tcp.c | 100 ++++++++++++++++++++++++++++++++++++++--------------- >> 2 files changed, 72 insertions(+), 29 deletions(-) >> >> diff --git a/checksum.c b/checksum.c >> index f80db4d309a2..96ccfe2af50b 100644 >> --- a/checksum.c >> +++ b/checksum.c >> @@ -503,7 +503,6 @@ uint16_t csum(const void *buf, size_t len, uint32_t init) >> * >> * Return: 16-bit folded, complemented checksum >> */ >> -/* cppcheck-suppress unusedFunction */ >> uint16_t csum_iov(const struct iovec *iov, size_t n, size_t offset, >> uint32_t init) >> { >> diff --git a/tcp.c b/tcp.c >> index c9472d905520..efd4037ed008 100644 >> --- a/tcp.c >> +++ b/tcp.c >> @@ -755,36 +755,65 @@ static void tcp_sock_set_bufsize(const struct ctx *c, int s) >> } >> >> /** >> - * tcp_update_check_tcp4() - Update TCP checksum from stored one >> - * @iph: IPv4 header >> - * @bp: TCP header followed by TCP payload >> - */ >> -static void tcp_update_check_tcp4(const struct iphdr *iph, >> - struct tcp_payload_t *bp) >> + * tcp_update_check_tcp4() - Calculate TCP checksum for IPv6 >> + * @src: IPv4 source address >> + * @dst: IPv4 destination address >> + * @iov: Pointer to the array of IO vectors >> + * @iov_cnt: Length of the array >> + * @payload_offset: IPv4 payload offset in the iovec array > > You explain it here, but "payload_offset" is a bit unclear if you're > not sure which layer it's talking about. "l4offset" maybe? > >> + */ >> +void tcp_update_check_tcp4(struct in_addr src, >> + struct in_addr dst, >> + const struct iovec *iov, int iov_cnt, >> + size_t payload_offset) >> { >> - uint16_t l4len = ntohs(iph->tot_len) - sizeof(struct iphdr); >> - struct in_addr saddr = { .s_addr = iph->saddr }; >> - struct in_addr daddr = { .s_addr = iph->daddr }; >> - uint32_t sum = proto_ipv4_header_psum(l4len, IPPROTO_TCP, saddr, daddr); >> + size_t check_ofs; >> + __sum16 *check; > > What's a __sum16? It's the type of "check" in struct tcphdr. > >> + int check_idx; >> + uint32_t sum; >> + >> + sum = proto_ipv4_header_psum(iov_size(iov, iov_cnt) - payload_offset, >> + IPPROTO_TCP, src, dst); >> + >> + check_idx = iov_skip_bytes(iov, iov_cnt, >> + payload_offset + offsetof(struct tcphdr, check), >> + &check_ofs); >> + >> + check = (__sum16 *)((char *)iov[check_idx].iov_base + check_ofs); > > So.. it's not likely, but it's possible for the first byte of the > checksum to be in one iovec and the second byte in another. This > whole construction is a bit awkward too. I'm guessing iov_base and iov_len are 32bit aligned, and address of "check" too (as it is from tcphdr in memory, that should be 32bit aligned, and offset of "check" is 16), so a 16 bit value cannot be shared between two iovecs. I'm guessing that any 32bit value we take from a structure will not cross boundary of an iovec. > > I think we want another helper on top of iov_skip_bytes(). It would > retreive a pointer to a field of a given length and offset within the > IOV, returning NULL if that can't be found contiguously. It could > have a macro wrapper that fills in some of the details based on a > type. For now I'd imagine we just give up if it returns NULL, but > that's enough to reduce a potential out of bounds memory access to > merely breaking one connection. If we ever need it, we can add a slow > path to handle that case. > > There are a couple of other curly cases to consider too, alas: what if > the field you request does exist contiguously, but isn't properly > aligned for the type we want to access it as? Then there's the > question of whether doing this will run afoul of the type-based > aliasing rules. > In our case, "check" is a field of "struct tcphdr", I think it's sane to think it is correctly aligned. I don't want to write complicated code only to write the checksum of the tcp header. Thanks, Laurent