Hi, Adding valgrind-developers to CC. On Wed, 2018-04-18 at 21:03 +0200, Florian Weimer wrote: > Since this commit: > > commit 28a94d8fb35b3a75b802f368ae6f4a9f6b0d435a > Author: Tom Herbert <tom@xxxxxxxxxxxxxxx> > Date: Mon Mar 7 14:11:02 2016 -0800 > > net: Allow MSG_EOR in each msghdr of sendmmsg > > This patch allows setting MSG_EOR in each individual msghdr passed > in sendmmsg. This allows a sendmmsg to send multiple messages when > using SOCK_SEQPACKET. > > Signed-off-by: Tom Herbert <tom@xxxxxxxxxxxxxxx> > Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> > > the msg_flags argument in individual msghdr arguments is longer > completely ignored for SOCK_SEQPACKET sockets. msg_flags was and is > still documented as ignored for sendmsg(2), so by analogy for > sendmmsg(2) as well. > > It seems that valgrind does not know about this yet, and due to > limited use of SCTP, this userspace ABI change has not been noticed so > far. That is correct. If you look at coregrind/m_syswrap/syswrap-generic.c (msghdr_foreachfield), it explicitly says: /* msg_flags is completely ignored for send_mesg, recv_mesg doesn't read the field, but does write to it. */ So the various valgrind syscall wrappers will not be called to inspect the msg_flags field. This means you don't get warned if the msg_flags field contains undefined bits for any syscall send_msg variant. > What are the plans in this area? Will other kinds of sockets start > using the msghdr flags for sending? > > A fully backwards-compatibility way to achieve this would be to > specify that you have to pass a new flag to sendmmsg (MSG_PERHDR?), in > its flags argument, to activate the per-msghdr flags. > > The glibc DNS stub resolver relies on the previously documented > behavior, and I wonder how widely we should backport the change: > > https://sourceware.org/bugzilla/show_bug.cgi?id=23037 > > If the MSG_PERHDR route will be taken, we can skip this work, and > valgrind can flag uninitialized bits in msg_flags only if MSG_PERHDR > is passed. (I believe it would be difficult for valgrind to look at > the socket type to determine whether undefined bits need reporting.) We have abstracted the checking a bit so it can be reused for the different ways a msg can be send, through socketcall, sendmsg and sendmmsg. I am not sure we can easily determine the underlying socket type. It seems to mean we would have to track the creation of all socketfds. So having a flag bit for sendmsg and sendmmsg would be much nicer. Thanks, Mark -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
