Re: [PATCH 3/7] man/man7/unix.7: Autobind: Clarify when it happens, and mention SO_PASSPIDFD

[Kuniyuki Iwashima <kuniyu@google.com>]

On Wed, 21 Oct 2026 23:22:52 +0200, Günther Noack wrote:
> Autobind happens if either one of SO_PASSCRED/SO_PASSPIDFD is set and
> either connect() or sendmsg() happens.  Mention SO_PASSPIDFD and
> clarify that autobind does not happen immediately when setting these
> socket options.
> 
> In linux.git net/unix/af_unix.c, unix_autobind() is called by
> unix_dgram_connect(), unix_stream_connect() and unix_dgram_sendmsg(),
> if the socket has no address and unix_may_passcred() is true.
> 
> unix_may_passcred() is also true when SO_PASSPIDFD is set, which was
> added in linux.git 5e2ff6704a27 (2023-06-12; "scm: add SO_PASSPIDFD and
> SCM_PIDFD"), Linux 6.5.
> 
> Also, the old text said "not yet connected", but the condition is
> about the socket not being bound.
> 
> Fixes: 9d1fd1157306 (1999-06-15, 2022-12-19; "man-pages 1.24")
> Fixes: 0cf2caa41ea6 (2010-10-10; "unix.7: Document the autobind feature")
> Cc: Michael Kerrisk <mtk.manpages@gmail.com>
> Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
> Cc: Kuniyuki Iwashima <kuniyu@google.com>
> Assisted-by: LLM
> Signed-off-by: Günther Noack <gnoack3000@gmail.com>
> ---
> 
> Notes:
>     Rephrased the LLM-generated text so that the canonical description is
>     in the "Autobind feature" section and the other place just refers to
>     it.
>     
>     SO_PASSPIDFD is unfortunately not documented yet at this point in
>     time, but it's still correct to mention it here.
> 
>  man/man7/unix.7 | 25 ++++++++++++++++++++-----
>  1 file changed, 20 insertions(+), 5 deletions(-)
> 
> diff --git a/man/man7/unix.7 b/man/man7/unix.7
> index a1dce8c47246..ded82768322b 100644
> --- a/man/man7/unix.7
> +++ b/man/man7/unix.7
> @@ -301,8 +301,14 @@ if the sender did not specify
>  .B SCM_CREDENTIALS
>  ancillary data.
>  .IP
> -When this option is set and the socket is not yet connected,
> -a unique name in the abstract namespace will be generated automatically.
> +When this option is set on a socket that is not bound to an address,
> +the socket is autobound to a unique name in the abstract namespace
> +when
> +.BR connect (2)
> +or
> +.BR sendmsg (2)
> +is called on it;
> +see "Autobind feature", below.
>  .IP
>  The value given as an argument to
>  .BR setsockopt (2)
> @@ -370,10 +376,19 @@ call specifies
>  as
>  .IR sizeof(sa_family_t) ,
>  .\" i.e., sizeof(short)
> -or the
> +or if
> +.BR connect (2)
> +or
> +.BR sendmsg (2)
> +is called on a socket that was
> +not explicitly bound to an address
> +and that has the
>  .B SO_PASSCRED
> -socket option was specified for a socket that was
> -not explicitly bound to an address,
> +or
> +.\" commit 5e2ff6704a275be009be8979af17c52361b79b89

Do we really need this SHA1 here ?


> +.B SO_PASSPIDFD
> +(since Linux 6.5)
> +socket option enabled,

You will need to update here with SO_PASSPIDFD_THREAD,
https://lore.kernel.org/netdev/20261005-work-unix-passpidfd-v4-3-350183b6e02a@amutable.com/

but I'm not sure to what extent you want to clarify things.

For example, is "the socket is autobound when it passes
credentials or pidfd but is not yet bound." too vague ?


>  then the socket is autobound to an abstract address.
>  The address consists of a null byte
>  followed by 5 bytes in the character set
> -- 
> 2.56.0
>