Re: [PATCH] recvmmsg.2, sendmmsg.2: point out that error handling is unreliable

["Nikola Forró" <nforro-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Hello,

any objections against the patch?

On Thu, 2017-10-26 at 15:06 +0200, Nikola Forró wrote:
> When an underlying recvmsg call fails after at least one message has
> been received, recvmmsg stores the error code into sock->sk->sk_err,
> and returns it on a subsequent call. But it doesn't take into account
> that the error can be owerwritten in the meantime, for example in ICMP
> error handler when an ICMP packet arrives.
> 
> When an underlying sendmsg call fails after at least one message has
> been sent, sendmmsg discards the error code and expects the caller to
> retry at first failed message. But the error code returned from the
> subsqeuent call can be different from the previously discarded one.
> 
> Reference:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/net/socket.c
> 
> Signed-off-by: Nikola Forró <nforro-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> ---
>  man2/recvmmsg.2 | 12 ++++++++++++
>  man2/sendmmsg.2 |  8 ++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/man2/recvmmsg.2 b/man2/recvmmsg.2
> index 3ff1b63a3..ca05b1d3a 100644
> --- a/man2/recvmmsg.2
> +++ b/man2/recvmmsg.2
> @@ -160,6 +160,8 @@ In addition, the following error can occur:
>  .B EINVAL
>  .I timeout
>  is invalid.
> +.PP
> +See also BUGS.
>  .SH VERSIONS
>  The
>  .BR recvmmsg ()
> @@ -179,6 +181,16 @@ so that if up to
>  .I vlen\-1
>  datagrams are received before the timeout expires,
>  but then no further datagrams are received, the call will block forever.
> +.PP
> +If an error occurs on an underlying
> +.BR recvmsg (2)
> +call after at least one message has been received, the call succeeds,
> +and returns the number of messages received. The error code is expected
> +to be returned on a subsequent call to
> +.BR recvmmsq ().
> +However, in the current implementation the error code can be overwritten
> +in the meantime by an unrelated network event on a socket,
> +for example an incoming ICMP packet.
>  .SH EXAMPLE
>  .PP
>  The following program uses
> diff --git a/man2/sendmmsg.2 b/man2/sendmmsg.2
> index e61ed0b4e..2f4bbeb01 100644
> --- a/man2/sendmmsg.2
> +++ b/man2/sendmmsg.2
> @@ -134,6 +134,7 @@ is set to indicate the error.
>  Errors are as for
>  .BR sendmsg (2).
>  An error is returned only if no datagrams could be sent.
> +See also BUGS.
>  .\" commit 728ffb86f10873aaf4abd26dde691ee40ae731fe
>  .\"     ... only return an error if no datagrams could be sent.
>  .\"     If less than the requested number of messages were sent, the application
> @@ -165,6 +166,13 @@ is capped to
>  .\"     For error handling an application using sendmmsg needs to retry at
>  .\"     the first unsent message, so capping is simpler and requires less
>  .\"     application logic than returning EINVAL.
> +.SH BUGS
> +If an error occurs on an underlying
> +.BR sendmsg (2)
> +call after at least one message has been sent, the call succeeds,
> +and returns the number of messages sent. The error code is lost.
> +The caller can retry starting at first failed message, but there is
> +no guarantee that the error returned will be the same as the previous one.
>  .SH EXAMPLE
>  The example below uses
>  .BR sendmmsg ()
--
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