Tiny patch (w/ discussion) to provide the peer information on unixdomain sockets

[Erkki Seppala]

Hello,

Attached you will find a patch that appends the field 'Peer' to
/proc/net/unix indicating the peer of a connected unix domain
socket. It shows its kernel address, as this is how unix domain
sockets are identified in the same file. The preceding optional file
name (actual address of the socket) field is replaced with "-" if it
is not set. If there is no peer, 0 is used in its place.

The purpose of the patch is to provide information on onto which
services processes are connected. This information is readily
available for TCP/IP sockets but not for unix domain sockets. The
information can be retrieved with relative ease by using the following
scripts for GDB, if you have GDB 7.1 and debugging symbols around:

  http://www.modeemi.fi/~flux/software/list-unix-sockets.tar.gz

But obviously this is not optimal :). With the patch, this is how you
can find out the peers of the sockets of the X server:

  # grep -E $(lsof -p `pidof X` | grep unix | awk '{print s $6; s="|"}' |
              tr -d '\n' | sed 's/0x//g;s/.*/(&)$/') /proc/net/unix
  d913b000: 00000003 00000000 00000000 0001 03  5494 - d913b200
  d9085600: 00000003 00000000 00000000 0001 03  5487 - d9085800
  ..

The number in the first column is the address, for which the process
can be most easily found with lsof: 

  # lsof | grep d913b000 
  gnome-pow 1245        gdm    3u     unix 0xd913b000      0t0       5494 -

Hopefully this kind of work would be integrated into lsof to display
the information more easily.

The patch does break the current API, but the impact seems rather
light. I'm happy to incorporate alternative approaches if this seems
to severe.

Originally the last field in the field was fully optional: if it was
not known, the line would terminate immediately after the field
preceding it. Because we append a new field (instead of inserting it
before Path, which causes more severe problems), the missing field
value is indicated with a hyphen instead. So this (where $ indicates
EOL):

  de38f800: 00000003 00000000 00000000 0001 03  2323$

becomes this:

  de38f800: 00000003 00000000 00000000 0001 03  2323 - de2e4400$

This changes the output of netstat -x for unix domain sockets that
don't have a known address. I'm pretty sure nobody cares,
though.. Similarly this changes the behavior of lsof, which before the
patch can display:

  rxvt      17693         ese    6u     unix 0xffff880050ba8900      0t0     746261 socket

but after the patch:

  gconf-hel 1377        gdm    3u     unix 0xd9197800      0t0       5796 -

Pretty sure this is pretty much the same for everyone as well
:). Although that could likely be worked around by using "@" in place
of "-". Infact this might be the proper thing to do: I'm not sure what
the meaning of UNIX_ABSTRACT used in af_unix.c:unix_seq_show is,
perhaps just that there is no name to be found? I cannot use the
macro, though, because it uses the nondefined field addr.

Btw, I noticed that af_unix.c:unix_seq_show does not quote the unix
domain socket path (possibly due to lacking convenient quoting
function in seq_file.c), which allows spoofing the contents of the
file into a certain extent. Atleast spaces should be quoted for this
patch to work properly in the cases when socket names have spaces in
them.

Here is the patch, also available at 

  http://www.modeemi.fi/~flux/software/linux-2.6.31-procfs-unix_peer_information.patch

--- linux-2.6.31/net/unix/af_unix.c	2010-03-30 16:19:52.306372568 +0300
+++ linux-2.6.31-flux/net/unix/af_unix.c	2010-03-30 14:32:47.000000000 +0300
@@ -2154,7 +2154,7 @@
 
 	if (v == SEQ_START_TOKEN)
 		seq_puts(seq, "Num       RefCount Protocol Flags    Type St "
-			 "Inode Path\n");
+			 "Inode Path Peer\n");
 	else {
 		struct sock *s = v;
 		struct unix_sock *u = unix_sk(s);
@@ -2185,7 +2185,14 @@
 			}
 			for ( ; i < len; i++)
 				seq_putc(seq, u->addr->name->sun_path[i]);
-		}
+		} else
+			seq_puts(seq, " -");
+
+		if (u->peer)
+			seq_printf(seq, " %p", u->peer);
+		else
+			seq_puts(seq, " 0");
+		
 		unix_state_unlock(s);
 		seq_putc(seq, '\n');
 	}

-- 
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