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/proc/proc.c

@@ -25,6 +25,10 @@ struct {
   struct proc proc[NPROC];
 } ptable;
 
+// CPU time of every process already reaped, so that /proc/stat's busy time
+// does not drop each time one is. Under ptable.lock, like the processes.
+static uint64 reaped_utime, reaped_stime;
+
 // Kernel stacks, one per ptable slot, statically.
 //
 // **They used to be a single kalloc() page**, and a measurement said that was
@@ -313,6 +317,8 @@ found:
   p->pending_signals = 0;
   p->alarm_at = p->alarm_every = 0;   // a child's timers start disarmed
   p->utime = p->stime = p->cutime = p->cstime = 0;
+  p->strace = 0;
+  p->umask = 022;
 
   // A recycled slot must not inherit a dead process's place in the queue --
   // in either direction. sched_init_proc starts it at min_vruntime, which is
@@ -509,6 +515,8 @@ fork(void)
   np->fs_base = myproc()->fs_base;
 
   safestrcpy(np->name, myproc()->name, sizeof(myproc()->name));
+  np->strace = myproc()->strace;
+  np->umask = myproc()->umask;
 
   // The child inherits the parent's place in the queue, priority and all.
   //
@@ -746,6 +754,8 @@ waitpid(int pid, int *status, int options, uint64 *ut, uint64 *st)
         // of processes rather than one generation deep.
         myproc()->cutime += p->utime + p->cutime;
         myproc()->cstime += p->stime + p->cstime;
+        reaped_utime += p->utime;
+        reaped_stime += p->stime;
         // wait4()'s fourth argument wants the *child's* usage, not the
         // parent's running total, so it has to be read here -- three lines
         // further down the slot is UNUSED and these are zero.
@@ -1786,6 +1796,11 @@ proc_setsid(void)
   }
   myproc()->sid  = myproc()->pid;
   myproc()->pgid = myproc()->pid;
+  // A new session starts with no controlling terminal; that is what lets the
+  // first terminal it opens become one. Keeping the old one meant a tmux pane
+  // still had tty1 as its /dev/tty, and zsh and htop drew straight onto the
+  // console underneath tmux instead of into their pane.
+  myproc()->ctty = 0;
   r = myproc()->sid;
   release(&ptable.lock);
   return r;
@@ -1913,6 +1928,7 @@ proc_snapshot(int idx, struct procinfo *out)
     out->nivcsw   = p->nivcsw;
     safestrcpy(out->name, p->name, sizeof(out->name));
     safestrcpy(out->exe, p->exe, sizeof(out->exe));
+    out->kthread = p->kfn != 0;
     release(&ptable.lock);
     return 0;
   }
@@ -2012,3 +2028,24 @@ proc_kill_init(void)
   if (initproc)
     initproc->killed = 1;
 }
+
+// Ticks spent in user and kernel mode by every process, living or reaped.
+// What /proc/stat's busy columns are made of; the rest of the time was idle.
+void
+proc_cputime(uint64 *user, uint64 *sys)
+{
+  struct proc *p;
+  uint64 u, s;
+
+  acquire(&ptable.lock);
+  u = reaped_utime;
+  s = reaped_stime;
+  for (p = ptable.proc; p < &ptable.proc[NPROC]; p++)
+    if (p->state != UNUSED) {
+      u += p->utime;
+      s += p->stime;
+    }
+  release(&ptable.lock);
+  *user = u;
+  *sys = s;
+}