[PATCH v3] recv.2: Document MSG_CMSG_CLOEXEC as returned in msg_flags

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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.

Cc: linux-api@xxxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: Ulrich Drepper <drepper@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Matthew House <mattlloydhouse@xxxxxxxxx>
Alright, I've summarized the history in the commit message, and I've added
the CCs you requested.

Also, for future reference, Drepper gave a reply to the last email, which
did not make it onto the list:

On Tue, Jul 18, 2023 at 9:24 AM Ulrich Drepper <drepper@xxxxxxxxxx> wrote:
> On Tue, Jul 18, 2023 at 2:10 PM Alejandro Colomar <alx@xxxxxxxxxx> 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.

Thank you,
Matthew House

 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.
 indicates that no data was received but an extended error from the socket
 error queue.
+.BR MSG_CMSG_CLOEXEC " (since Linux 2.6.23)"
+.\" commit 4a19542e5f694cd408a32c3d9dc593ba9366e2d7
+indicates that
+was specified in the
+.I flags
+argument of
+.BR recvmsg ().
 These calls return the number of bytes received, or \-1
 if an error occurred.

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