Re: [PATCH 2/7] man/man7/unix.7: Multiple same-typed SCM_* messages may be sent in one message

["Günther Noack" <gnoack3000@gmail.com>]

Hello Alejandro!

On Thu, Oct 22, 2026 at 11:28:12PM +0200, Alejandro Colomar wrote:
> > Date: 2026-10-21 23:22:51+0200
> > From: Günther Noack <gnoack3000@gmail.com>
> >
> > Verified in Linux kernel code:
> 
> I think 'verified' in a commit message seems inappropriate.  It should
> describe, rather than verify, and the verification would come only from
> the review tags.  Maybe say something like "Relevant kernel code:".

Good point, replaced.  I'll double check the other commits as well.


> > __scm_send() in linux.git net/core/scm.c walks all control messages
> > in a loop like this:
> > 
> > 	for_each_cmsghdr(cmsg, msg) {
> > 		// ...
> > 		switch (cmsg->cmsg_type)
> > 		{
> > 		case SCM_RIGHTS:
> > 			// Append(!) file descriptors
> > 			// up to the SCM_MAX_FD limit
> > 			break;
> > 		case SCM_CREDENTIALS:
> > 			// Replace previously set credentials
> 
> Since these are your comments, and not actual comments in the source
> code (are they yours, or from the LLM?), I'd format them clearly as
> such using invalid code rather than comments:
> 
> 			... replace previously set credentials ...

Done.

Yes, I transcribed the source code by hand here, including these
comments.

> > 			break;
> > 		default:
> > 			goto error;
> > 		}
> > 	}
> > 
> > So it is possible to pass multiple SCM_* messages -- they will compose
> > if possible and override the previous value if not.  Since their
> > contents are being conflated that way, they will appear as one message
> > again on the receiving side.
> > 
> > Keep the recommendation as portability advice, and describe the Linux
> > behavior.
> > 
> > Fixes: 5b5cb1958062 (2018-07-10; "unix.7: When sending ancillary data, only one item of each type may be sent")
> > Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> > Cc: Kuniyuki Iwashima <kuniyu@google.com>
> > Assisted-by: LLM
> > Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> > ---
> > 
> > Notes:
> >     The factual analysis in the commit message is correct.
> >     
> >     I kept the LLM phrasing; it did not seem overly wordy to me and the
> >     style is in line with the paragraph after it, which also documents
> >     Linux-only vs. portable APIs that way.
> > 
> >  man/man7/unix.7 | 19 ++++++++++++++++++-
> >  1 file changed, 18 insertions(+), 1 deletion(-)
> > 
> > diff --git a/man/man7/unix.7 b/man/man7/unix.7
> > index 0d2ea8bda2ae..a1dce8c47246 100644
> > --- a/man/man7/unix.7
> > +++ b/man/man7/unix.7
> > @@ -543,7 +543,24 @@ option must be enabled on the socket (see above).
> >  .P
> >  When sending ancillary data with
> >  .BR sendmsg (2),
> > -only one item of each of the above types may be included in the sent message.
> > +portable applications should include
> > +only one item of each of the above types in the sent message.
> > +On Linux,
> > +if several
> > +.B SCM_RIGHTS
> > +items are included,
> > +their file descriptors are combined into a single list,
> > +to which the
> > +.B SCM_MAX_FD
> > +limit applies as a whole,
> > +and the receiver gets them in a single
> > +.B SCM_RIGHTS
> > +item;
> > +if several
> > +.B SCM_CREDENTIALS
> > +items are included,
> > +each of them is checked,
> > +and the last one takes effect.
> 
> Is 'takes effect' appropriate?  I think 'is sent' might be more
> appropriate, right?  There's nothing taking effect, I think (but I'm not
> an expert in SCM_* stuff).

You are right, "takes effect" is maybe too much described from the
angle of the implementation - I replaced it with "is sent".

What's happening is that the sender process specifies the "struct
ucred", which contains a pid, uid and gid in the SCM_CREDENTIALS
control message.  When the kernel deals with these, it *checks* that
they are valid (they describe the sending process *OR* the sending
process is privileged and can send what it wants).  Only the last of
these SCM_CREDENTIALS messages survives that processing though and the
receiving process then only sees that last one.

Thanks for the review!
–Günther