[PATCH v2 14/24] man/man7/unix.7: Several items of the same type may be sent in one message

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

__scm_send() in linux.git net/core/scm.c walks all control messages.
Each SCM_RIGHTS item is passed to scm_fp_copy(), which appends the file
descriptors to the existing scm_fp_list, and fails with EINVAL only if
the combined count exceeds SCM_MAX_FD.  On the receiving side,
scm_detach_fds() emits a single SCM_RIGHTS control message.  Each
SCM_CREDENTIALS item is validated, and overwrites the credentials of the
previous one.

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>
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 4d431ec6deaf..480a565f7d04 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -528,7 +528,24 @@ 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.
 .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