recvmmsg.2, sendmmsg.2: point out that error handling is unreliable

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 <[email protected]>

---
 man2/recvmmsg.2 | 12 ++++++++++++
 man2/sendmmsg.2 |  8 ++++++++
 2 files changed, 20 insertions(+)

man2/recvmmsg.2 +12 -0

index 3ff1b63a3..ca05b1d3a 100644
@@ -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

man2/sendmmsg.2 +1 -0

index e61ed0b4e..2f4bbeb01 100644
@@ -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.

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 <[email protected]>

---
 man2/recvmmsg.2 | 12 ++++++++++++
 man2/sendmmsg.2 |  8 ++++++++
 2 files changed, 20 insertions(+)

man2/recvmmsg.2 +2 -0

index 3ff1b63a3..ca05b1d3a 100644
@@ -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 ()

man2/recvmmsg.2 +6 -0

+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

man2/sendmmsg.2 +1 -0

index e61ed0b4e..2f4bbeb01 100644
@@ -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

I am not sure that I understand that last sentence. Do you mean something like:

The caller can retry the transmission, starting at first failed message, but there is no guarantee that, if an error is returned, it will be the same error as the one that was lost on the previous call.

?