Re: [PATCH 4/7] man/man7/unix.7: SCM_SECURITY: Generalize to other LSMs, not just SELinux

[Alejandro Colomar <alx@kernel.org>]

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Hi Günther,

> Date: 2026-10-27 09:54:51+0100
> From: Günther Noack <gnoack3000@gmail.com>
>
> On Mon, Oct 26, 2026 at 11:41:13PM +0100, Günther Noack wrote:
> > I'm still leaning towards documenting it as a buffer with
> > implementation-defined contents which are provided by a LSM.  How
> > these buffer contents are interpreted is depending on the LSM in use
> > anyway.
> 
> To make a more concrete and constructive wording proposal:
> 
> 
> SCM_SECURITY
> 
>   Receive the security context (the security label) of the peer
>   socket, as provided by the active Linux Security Module (LSM).  The
>   received ancillary data is a byte buffer.
> 
>   The exact format of the security context buffer is defined by the
>   active major LSM.  For example, SELinux returns a SELinux label as a
>   string (including a terminating null byte).

Let's pause the discussion of the specific text for now.

I have questions about the exact format used by each LSM.  For example,
you say that SELinux guarantees a string.  Does it count the '\0' as
part of the data size?  Or is it guaranteed to exist but not counted?

Same question for the other LSMs.


Have a lovely night!
Alex

> Does that sound reasonable? Let me know what you think.
> 
> I removed the wording "There is no fixed limit ...", because it should
> be a given that callers allocate enough buffer space for receiving and
> check the truncation flag.  (The more important part is that we remove
> the previous wording which suggested that it would fit into NAME_MAX.)
> 
> Given that SCM_SECURITY on "real" networks seems to have the same
> "buffer, not string" semantics, it feels fair to express it that way?
> People who want to read the security context will anyway need to read
> up on the exact format used by the LSM in order to attribute any
> meaning to it.
> 
> –Günther
> 
> P.S.: I had a hard time finding many usage examples.  The main user of
> SCM_SECURITY seems to be systemd's journald, which uses it to log the
> SELinux security label.  That code can deal with both null terminated
> strings and ones where it is missing (due to truncation), and there is
> a comment indicating that they are aware of possible truncation.

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

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