From c654cde5ec9e9fa975d5d041aa069489ab12a4ce Mon Sep 17 00:00:00 2001 From: Matthew House Date: Tue, 18 Jul 2023 13:26:08 -0400 Subject: recv.2: Document MSG_CMSG_CLOEXEC as returned in msg_flags MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Ever since commit 4a19542e5f69 ("O_CLOEXEC for SCM_RIGHTS") added the MSG_CMSG_CLOEXEC flag to recvmsg(2), the flag has also been copied into the returned msg->msg_flags when specified, regardless of whether any file descriptors were actually received, or whether the protocol supports receiving file descriptors at all. This behavior was primarily an implementation artifact: by copying MSG_CMSG_CLOEXEC into the msg_flags, scm_detach_fds() in net/core/scm.c (and its _compat() counterpart in net/compat.c) could determine whether it was set without having to receive a copy of the recvmsg(2) flags. This mechanism was closely modeled after the internal MSG_CMSG_COMPAT flag, which is passed by the compat versions of the send[m]msg(2) and recv[m]msg(2) syscalls to inform various functions that user space expects a compat layout. When the flag was first implemented by commits 3225fc8a85f4 ("[NET]: Simplify scm handling and sendmsg/recvmsg invocation, consolidate net compat syscalls.") and 7e8d06bc1d90 ("[COMPAT]: Fix MSG_CMSG_COMPAT flag passing, kill cmsg_compat_recvmsg_fixup.") (in history/history.git), the behavior was very similar: recvmsg(2) would add MSG_CMSG_COMPAT to the msg_flags, and put_cmsg() and scm_detach_fds() in net/core/scm.c would read the flag to determine whether to delegate to their _compat() counterparts. However, after the initial implementation, more work was done to hide MSG_CMSG_COMPAT from user space. First, commit 37f7f421cce1 ("[NET]: Do not leak MSG_CMSG_COMPAT into userspace.") started scrubbing the bit from msg_flags right before copying it back into user space. Then, since passing the MSG_CMSG_COMPAT flag into the syscalls from non-compat code could confuse the kernel, commits 1be374a0518a ("net: Block MSG_CMSG_COMPAT in send(m)msg and recv(m)msg") and a7526eb5d06b ("net: Unbreak compat_sys_{send,recv}msg") made them return -EINVAL if user space attempted to pass the flag. But to reduce breakage, commit d720d8cec563 ("net: compat: Ignore MSG_CMSG_COMPAT in compat_sys_{send, recv}msg") rolled that back somewhat, making MSG_CMSG_COMPAT an error for the non-compat syscalls and a no-op for the compat syscalls, which is the current status quo. Even though MSG_CMSG_CLOEXEC was implemented after the kernel started scrubbing MSG_CMSG_COMPAT from the returned msg_flags, the newer flag never received the same treatment. At this point, this behavior has effectively become part of the user-space API, to the extent that io_uring has been careful in commit 9bb66906f23e ("io_uring: support multishot in recvmsg") to replicate the behavior in its multishot IORING_OP_RECVMSG operation. Therefore, document this behavior to avoid confusion when user space sees MSG_CMSG_CLOEXEC returned in msg->msg_flags. On 2023-07-18 15:24, Ulrich Drepper wrote: > On Tue, Jul 18, 2023 at 2:10 PM Alejandro Colomar wrote: >> On 2023-07-18 08:00, Matthew House wrote: >>> >>> As for the original purpose of the behavior, it's not really clear, >>> and it may well have been an implementation artifact that got >>> enshrined in the user space ABI. >>> (Even io_uring is careful to replicate this behavior!) >> >> This is what worries me. I've CCd a bunch of people to see if they can >> bring some light. > > It definitely was an artifact of the implementation. I haven't tested > getting the close-on-exec flag information for all interfaces. The > assumption was that the information about the close-on-exec flag is > received with the universal fcntl() call. Cc: linux-api@vger.kernel.org Cc: netdev@vger.kernel.org Cc: Ulrich Drepper Cc: "David S. Miller" Cc: Andrew Morton Signed-off-by: Matthew House Signed-off-by: Alejandro Colomar --- man2/recv.2 | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/man2/recv.2 b/man2/recv.2 index 660c103fb..1cd9f3e1b 100644 --- a/man2/recv.2 +++ b/man2/recv.2 @@ -412,6 +412,15 @@ is returned to indicate that expedited or out-of-band data was received. .B MSG_ERRQUEUE indicates that no data was received but an extended error from the socket error queue. +.TP +.BR MSG_CMSG_CLOEXEC " (since Linux 2.6.23)" +.\" commit 4a19542e5f694cd408a32c3d9dc593ba9366e2d7 +indicates that +.B MSG_CMSG_CLOEXEC +was specified in the +.I flags +argument of +.BR recvmsg (). .SH RETURN VALUE These calls return the number of bytes received, or \-1 if an error occurred. -- cgit v1.2.3