Pseudo-terminals, and what tmux and htop needed around them

- /dev/ptmx and /dev/pts/N: Unix98 pairs with a full line discipline,
  window sizes, job control and hangup (kernel/dev/pty.c).
- The console speaks scroll regions, insert/delete line, SU/SD, RI/IND/NEL,
  DECSC/DECRC; swallows charset designations and private CSI sequences;
  and LF only returns the carriage under ONLCR.
- setsid() drops the controlling terminal, so a pane's pty becomes its ctty.
- pipe2() honours O_NONBLOCK and O_CLOEXEC (libevent's signal pipe).
- File modes are stored and enforced: chmod, fchmod(at), umask, creation
  modes; /tmp is 1777.
- /proc/stat has real user/system/idle; /proc/<pid>/task; kthread flag.
- SO_PEERCRED, IOV_MAX 1024.
- /proc/strace: name a program and its syscalls go to the kernel log.
- World gains htop, btop, tmux, mc, ncdu, fastfetch, games, X toys, NetSurf.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/net/unix.c

@@ -125,6 +125,11 @@ alloc_locked(int type)
       s->ref   = 1;
       s->type  = type;
       s->state = SS_UNBOUND;
+      if(myproc()){
+        s->cred_pid = myproc()->pid;
+        s->cred_uid = myproc()->euid;
+        s->cred_gid = myproc()->egid;
+      }
       return s;
     }
   }
@@ -852,6 +857,9 @@ sockconnect(struct socket *s, uint dev, uint64 ino)
 
   peer->peer  = s;
   peer->state = SS_CONNECTED;
+  peer->cred_pid = srv->cred_pid;
+  peer->cred_uid = srv->cred_uid;
+  peer->cred_gid = srv->cred_gid;
   s->peer     = peer;
   s->state    = SS_CONNECTED;
 
@@ -1016,3 +1024,21 @@ socksendto(struct socket *s, char *buf, int n, uint addr, ushort port)
     return -EAFNOSUPPORT;      // an AF_INET destination on an AF_UNIX socket
   return sockwrite(s, buf, n);
 }
+
+// SO_PEERCRED: the credentials recorded for the far end of a connected Unix
+// socket. -1 if there is no far end.
+int
+sock_peercred(struct socket *s, int *pid, uint *uid, uint *gid)
+{
+  int r = -1;
+
+  acquire(&stab.lock);
+  if(s->domain != AF_INET && s->peer){
+    *pid = s->peer->cred_pid;
+    *uid = s->peer->cred_uid;
+    *gid = s->peer->cred_gid;
+    r = 0;
+  }
+  release(&stab.lock);
+  return r;
+}