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

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Hi Matthew, Ulrich,

On 2023-07-18 19:26, Matthew House wrote:
> 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.

Patch applied.  I've included Drepper's quote in the commit message too.
Thank you both!


> 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.
> +.TP
> +.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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