On Wed, Sep 25, 2024 at 08:40:28AM +0200, Laurent Vivier wrote:On 25/09/2024 03:12, David Gibson wrote:Huh, ok.On Tue, Sep 24, 2024 at 05:46:42PM +0200, Laurent Vivier wrote:It's the type of "check" in struct tcphdr.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 <lvivier(a)redhat.com> --- 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 arrayYou 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?That's probably true.. but I think we should actually verify / assert that somewhere.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.+ 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.It will be correctly aligned as long as the struct tcphdr itself is correctly aligned. We can probably count on that being 2-byte aligned w.r.t. the start of the frame, but probably no more since the L2 header is 14 bytes long. If there were odd-length iovs, and the TCP header wasn't in the first, that could destroy its alignment though. Actually... the same thing could happen in the first IOV if iov_base wasn't aligned, which is admittedly probably even less likely.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.I agree, but I think we should at least test and bail with an error message if our assumptions about the alignments of the IOVs were given aren't true. -- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson