On 9/29/22 08:39, Stefan Metzmacher wrote:

I hit a very bad problem during my tests of SENDMSG_ZC. BUG(); in first_iovec_segment() triggered very easily. The problem was io_setup_async_msg() in the partial retry case, which seems to happen more often with _ZC.

Reviewed-by: Pavel Begunkov asml.silence@gmail.com

And tested with liburing patches I sent yesterday

iov_iter_iovec_advance() may change i->iov in order to have i->iov_offset being only relative to the first element.

Which means kmsg->msg.msg_iter.iov is no longer the same as kmsg->fast_iov.

But this would rewind the copy to be the start of async_msg->fast_iov, which means the internal state of sync_msg->msg.msg_iter is inconsitent.

I tested with 5 vectors with length like this 4, 0, 64, 20, 8388608 and got a short writes with:

• ret=2675244 min_ret=8388692 => remaining 5713448 sr->done_io=2675244
• ret=-EAGAIN => io_uring_poll_arm
• ret=4911225 min_ret=5713448 => remaining 802223 sr->done_io=7586469
• ret=-EAGAIN => io_uring_poll_arm
• ret=802223 min_ret=802223 => res=8388692

While this was easily triggered with SENDMSG_ZC (queued for 6.1), it was a potential problem starting with 7ba89d2af17aa879dda30f5d5d3f152e587fc551 in 5.18 for IORING_OP_RECVMSG. And also with 4c3c09439c08b03d9503df0ca4c7619c5842892e in 5.19 for IORING_OP_SENDMSG.

However 257e84a5377fbbc336ff563833a8712619acce56 introduced the critical code into io_setup_async_msg() in 5.11.

Fixes: 7ba89d2af17aa ("io_uring: ensure recv and recvmsg handle MSG_WAITALL correctly") Fixes: 257e84a5377fb ("io_uring: refactor sendmsg/recvmsg iov managing") Cc: stable@vger.kernel.org Signed-off-by: Stefan Metzmacher metze@samba.org

---

io_uring/net.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/io_uring/net.c b/io_uring/net.c index 60e392f7f2dc..a81fccd38ae4 100644 --- a/io_uring/net.c +++ b/io_uring/net.c @@ -165,8 +165,10 @@ static int io_setup_async_msg(struct io_kiocb *req, memcpy(async_msg, kmsg, sizeof(*kmsg)); async_msg->msg.msg_name = &async_msg->addr; /* if were using fast_iov, set it to the new one */

• if (!async_msg->free_iov)

• async_msg->msg.msg_iter.iov = async_msg->fast_iov;
  

• if (!kmsg->free_iov) {

• size_t fast_idx = kmsg->msg.msg_iter.iov - kmsg->fast_iov;
  

• async_msg->msg.msg_iter.iov = &async_msg->fast_iov[fast_idx];
  

• }

return -EAGAIN; }