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

[Alejandro Colomar <alx@kernel.org>]

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Hi Günther,

> 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:".

> __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 ...

> 			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).


Have a lovely night!
Alex

>  .P
>  At least one byte of real data should be sent when sending ancillary data.
>  On Linux, this is required to successfully send ancillary data over
> -- 
> 2.56.0
> 

-- 
<https://www.alejandro-colomar.es>

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