Date:   Thu, 18 Mar 2021 01:15:30 +0100
From:   Stefan Metzmacher <metze@...ba.org>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 0/2] send[msg]()/recv[msg]() fixes/improvements

Hi Pavel,

>>>>> here're patches which fix linking of send[msg]()/recv[msg]() calls
>>>>> and make sure io_uring_enter() never generate a SIGPIPE.
>>>
>>> 1/2 breaks userspace.
>>
>> Can you explain that a bit please, how could some application ever
>> have a useful use of IOSQE_IO_LINK with these socket calls?
> 
> Packet delivery of variable size, i.e. recv(max_size). Byte stream
> that consumes whatever you've got and links something (e.g. notification
> delivery, or poll). Not sure about netlink, but maybe. Or some
> "create a file via send" crap, or some made-up custom protocols

Ok, then we need a flag or a new opcode to provide that behavior?

For recv() and recvmsg() MSG_WAITALL might be usable.

It's not defined in 'man 2 sendmsg', but should we use it anyway
for IORING_OP_SEND[MSG] in order to activate the short send check
as the low level sock_sendmsg() call seem to ignore unused flags,
which seems to be the reason for the following logic in tcp_sendmsg_locked:

if (flags & MSG_ZEROCOPY && size && sock_flag(sk, SOCK_ZEROCOPY)) {

You need to set SOCK_ZEROCOPY in the socket in order to give a meaning
to MSG_ZEROCOPY.

Should I prepare an add-on patch to make the short send/recv logic depend
on MSG_WAITALL?

I'm cc'ing netdev@...r.kernel.org in order to more feedback of
MSG_WAITALL can be passed to sendmsg without fear to trigger
-EINVAL.

The example for io_sendmsg() would look like this:

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4383,7 +4383,7 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
        struct io_async_msghdr iomsg, *kmsg;
        struct socket *sock;
        unsigned flags;
-       int expected_ret;
+       int min_ret = 0;
        int ret;

        sock = sock_from_file(req->file);
@@ -4404,9 +4404,11 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
        else if (issue_flags & IO_URING_F_NONBLOCK)
                flags |= MSG_DONTWAIT;

-       expected_ret = iov_iter_count(&kmsg->msg.msg_iter);
-       if (unlikely(expected_ret == MAX_RW_COUNT))
-               expected_ret += 1;
+       if (flags & MSG_WAITALL) {
+               min_ret = iov_iter_count(&kmsg->msg.msg_iter);
+               if (unlikely(min_ret == MAX_RW_COUNT))
+                       min_ret += 1;
+       }
        ret = __sys_sendmsg_sock(sock, &kmsg->msg, flags);
        if ((issue_flags & IO_URING_F_NONBLOCK) && ret == -EAGAIN)
                return io_setup_async_msg(req, kmsg);
@@ -4417,7 +4419,7 @@ static int io_sendmsg(struct io_kiocb *req, unsigned int issue_flags)
        if (kmsg->free_iov)
                kfree(kmsg->free_iov);
        req->flags &= ~REQ_F_NEED_CLEANUP;
-       if (ret != expected_ret)
+       if (ret < min_ret)
                req_set_fail_links(req);
        __io_req_complete(req, issue_flags, ret, 0);
        return 0;

Which means the default of min_ret = 0 would result in:

        if (ret < 0)
                req_set_fail_links(req);

again...

>>> Sounds like 2/2 might too, does it?
>>
>> Do you think any application really expects to get a SIGPIPE
>> when calling io_uring_enter()?
> 
> If it was about what I think I would remove lots of old garbage :)
> I doubt it wasn't working well before, e.g. because of iowq, but
> who knows

Yes, it was inconsistent before and now it's reliable.

metze

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