[PATCH 18/24] man/man7/unix.7: SCM_SECURITY: This is not specific to 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.

Also clarify the properties of the returned string:

* There is no NAME_MAX limit on the length; SELinux contexts can be
  longer than that.  If the control buffer is too small, put_cmsg()
  truncates the data and sets MSG_CTRUNC.

* Whether the string is null-terminated also depends on the LSM:
  SELinux includes the null byte in the length, but smack_to_secctx()
  in security/smack/smack_lsm.c uses strlen() and doesn't.

Assisted-by: LLM
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---

Notes:
    I reworded the LLM phrasing slightly and rewrote the commit message.
    
    I'm open for feedback about whether this is going into too much detail
    about the properties of the returned string.  Linux does indeed not
    guarantee that this terminates with a NUL byte, nor that NAME_MAX is
    large enough to hold it.

 man/man7/unix.7 | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/man/man7/unix.7 b/man/man7/unix.7
index 787a48baa97a..ff5a90863378 100644
--- a/man/man7/unix.7
+++ b/man/man7/unix.7
@@ -307,7 +307,7 @@ and returned as the result of
 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).
@@ -534,13 +534,26 @@ message, the
 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
+Receive the security context (the security label)
+of the peer socket,
+as provided by the active Linux Security Module (LSM),
+such as SELinux or Smack.
+The received ancillary data is a string containing
 the security context.
-The receiver should allocate at least
+Whether the string includes a terminating null byte
+depends on the LSM;
+the length of the data is given by the
+.I cmsg_len
+field.
+There is no fixed limit on the length of the security context;
+allocating
 .B NAME_MAX
-bytes in the data portion of the ancillary message for this data.
+bytes in the data portion of the ancillary message
+is usually enough.
+If the buffer is too small,
+the data is truncated and the
+.B MSG_CTRUNC
+flag is set (see below).
 .IP
 To receive the security context, the
 .B SO_PASSSEC
-- 
2.56.0