[PATCH v2 4/8] man/man7/unix.7: SCM_SECURITY: Generalize to other LSMs, not just SELinux

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

Reword the description of SCM_SECURITY to not be specific to SELinux.

The security context is populated in scm_passec() in linux.git
net/core/scm.c, through the security_secid_to_secctx() LSM hook, which
is currently implemented by SELinux, Smack and AppArmor.

As clarified in [1], the returned security context is in the general
case a byte buffer, not a null-terminated string.  The only LSM that
returns a buffer with trailing null byte is SELinux, but even that is
not guaranteed in case that the message is truncated.

Link: [1] <https://lore.kernel.org/all/auZcwYWv0NeiIR8d@google.com/>
Fixes: ffad6a017f91 (2018-07-28; "unix.7: Document SCM_SECURITY ancillary data")
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Eric Dumazet <edumazet@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---

Notes:
    This feature is also available in other LSMs.  Unfortunately, they
    implemented it inconsistently and that complicates the description of
    how string boundaries are handled.  Linux does indeed neither
    guarantee that this terminates with a NUL byte, nor that NAME_MAX is
    large enough to hold it.
    
    After confirmation from Kuniyuki Iwashima, this documents it as
    returning a byte buffer.
    
    I prefer this variant over the previous longer one, also because it
    refers to the individual LSMs for the exact format.  A reader of the
    security context will require LSM-specific documentation anyway to
    interpret the security context.
    
    This patch is still LLM-assisted, but at this point, since everything
    is rewritten now, the only LLM-assisted part is the discovery that
    this part of the documentation could be improved.

 man/man7/unix.7 | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index 61f06a74359b..c0e2bdbacdd8 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -312,7 +312,7 @@ .SS Socket options
 is an integer boolean flag.
 .TP
 .B SO_PASSSEC
-Enables receiving of the SELinux security label of the peer socket
+Enables receiving of the security label of the peer socket
 in an ancillary message of type
 .B SCM_SECURITY
 (see below).
@@ -538,13 +538,16 @@ .SS Ancillary messages
 option must be enabled on the socket.
 .TP
 .B SCM_SECURITY
-Receive the SELinux security context (the security label)
-of the peer socket.
-The received ancillary data is a null-terminated string containing
-the security context.
-The receiver should allocate at least
-.B NAME_MAX
-bytes in the data portion of the ancillary message for this data.
+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.
+.IP
+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).
 .IP
 To receive the security context, the
 .B SO_PASSSEC
-- 
2.56.0