linux-kernel - Does SCM_RIGHTS limit on fd array apply to a particular cmsghdr or the whole ancillary data?

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Date:   Wed, 9 Dec 2020 20:23:58 +0100
From:   Wolfgang Draxinger <list@...enwolf.net>
To:     LKML Mailing List <linux-kernel@...r.kernel.org>
Subject: Does SCM_RIGHTS limit on fd array apply to a particular cmsghdr or
 the whole ancillary data?

Hi,

in the documentation on SCM_RIGHTS the maximum number of file
descriptors that can be passed is described as (release 5.09 of the
Linux man-pages project):

| SCM_RIGHTS
|   Send or receive a set of open file descriptors from another
|   process. The data portion contains an integer array of the file
|   descriptors.
| (...)
|   The kernel constant SCM_MAX_FD defines a limit on the number of
        |   file descriptors in the array.  Attempting to send an array
            |   larger than this limit causes sendmsg(2) to fail with
the error              |   EINVAL.

In my reading this might be interpreted as SCM_MAX_FD per SCM_RIGHTS
cmsghdr, xor SCM_MAX_FD per sendmsg. It would be good to know, which one
it is. So which is the proper strategy if I have to pass more than
SCM_MAX_FD file descriptors between processes?

Cheers,
Wolfgang