SchOS/kernel/dev/pty.c

// SPDX-License-Identifier: GPL-2.0-or-later
//
// Pseudo-terminals.
//
// A terminal emulator, tmux, sshd and script(1) all want the same thing: a
// terminal with a program on one side and *themselves* on the other, instead of
// a keyboard and a screen. Unix98 spells that /dev/ptmx -- each open makes a new
// pair and hands back its master -- and /dev/pts/N, the slave, which the program
// gets as its stdin, stdout and controlling terminal. There were none of these,
// so xterm had nothing to run a shell in and tmux could not start a server.
//
// Between the two sides sits a line discipline, the console's in miniature
// (kernel/core/console.c): what the master writes is the slave's *input* --
// canonical editing, echo, ^C as SIGINT to the foreground group -- and what the
// slave writes is its *output*, NL turned into CR NL, read back by the master.
//
// Every pair shares one lock. The paths are short and a second lock per pair
// would only be one more thing to take in the wrong order.

#include "core/types.h"
#include "core/defs.h"
#include "core/param.h"
#include "core/spinlock.h"
#include "core/sleeplock.h"
#include "core/proc.h"
#include "fs/fs.h"
#include "fs/file.h"
#include "core/uaccess.h"
#include "uapi/posix.h"
#include "uapi/errno.h"
#include "dev/pty.h"

#define PTY_BUF 4096

#define TIOCGPTN    0x80045430
#define TIOCSPTLCK  0x40045431
#define TIOCGPTPEER 0x5441
#define TIOCNOTTY   0x5422
#define TIOCGSID    0x5429
#define TIOCOUTQ    0x5411
#define TIOCPKT     0x5420

#define POLLIN_  0x001
#define POLLOUT_ 0x004
#define POLLHUP_ 0x010

struct pty {
  int used;
  int mopen, sopen;          // open file descriptions on each side
  int slave_seen;            // the slave has been opened at least once
  struct termios tio;
  struct winsize ws;
  int fg_pgrp, sid;

  // The slave's input, after the line discipline. [r, w) is ready to read;
  // [w, e) is the line still being edited in canonical mode.
  char in[PTY_BUF];
  uint ir, iw, ie;
  int  eof;                  // ^D on an empty line: the next read returns 0
  int  abort_read;           // ^C: wake the reader with EINTR

  // The slave's output, for the master to read.
  char out[PTY_BUF];
  uint or, ow;
};

static struct spinlock ptylock;
static struct pty ptys[NPTY];

#define CTRL(x) ((x) & 0x1f)

static void
termios_defaults(struct termios *t)
{
  memset(t, 0, sizeof(*t));
  t->c_iflag = ICRNL | IUTF8;
  t->c_oflag = OPOST | ONLCR;
  t->c_cflag = CS8 | CREAD;
  t->c_lflag = ISIG | ICANON | ECHO | ECHOE | IEXTEN;
  t->c_cc[VINTR]  = CTRL('C');
  t->c_cc[VQUIT]  = CTRL('\\');
  t->c_cc[VERASE] = 0x7f;
  t->c_cc[VKILL]  = CTRL('U');
  t->c_cc[VEOF]   = CTRL('D');
  t->c_cc[VSUSP]  = CTRL('Z');
  t->c_cc[VWERASE]= CTRL('W');
  t->c_cc[VMIN]   = 1;
  t->c_ispeed = 15;
  t->c_ospeed = 15;
}

void
pty_init(void)
{
  initlock(&ptylock, "pty");
}

int
pty_alloc(void)
{
  acquire(&ptylock);
  for (int i = 0; i < NPTY; i++) {
    struct pty *p = &ptys[i];
    if (p->used)
      continue;
    memset(p, 0, sizeof(*p));
    p->used = 1;
    termios_defaults(&p->tio);
    p->ws.ws_row = 24;
    p->ws.ws_col = 80;
    release(&ptylock);
    return i;
  }
  release(&ptylock);
  return -1;
}

int
pty_used(int n)
{
  return n >= 0 && n < NPTY && ptys[n].used;
}

static struct pty *
pty_of(struct inode *ip)
{
  if (ip->minor < 0 || ip->minor >= NPTY)
    return 0;
  return &ptys[ip->minor];
}

// /dev/ptmx, opened: a new pair, and the master in place of the ptmx inode.
// Called with ip locked, as open() holds it; hands back the master the same way.
struct inode *
ptmx_open(struct inode *ip)
{
  int n = pty_alloc();
  struct inode *m;

  if (n < 0)
    return 0;
  m = devfs_pty_master(n);
  if (m == 0) {
    ptys[n].used = 0;
    return 0;
  }
  devfs_pty_claim(n, myproc() ? myproc()->euid : 0);
  iunlockput(ip);
  ilock(m);
  return m;
}

static void
maybe_free(struct pty *p)
{
  if (p->mopen == 0 && p->sopen == 0)
    p->used = 0;
}

// --- output: the slave writes, the master reads ------------------------------

static int
out_room(struct pty *p)
{
  return PTY_BUF - (int)(p->ow - p->or);
}

// One byte of output, with OPOST processing. Caller holds the lock and has made
// sure there is room for two.
static void
out_put(struct pty *p, int c)
{
  if ((p->tio.c_oflag & OPOST) && (p->tio.c_oflag & ONLCR) && c == '\n')
    p->out[p->ow++ % PTY_BUF] = '\r';
  p->out[p->ow++ % PTY_BUF] = (char)c;
}

// Echo goes into the same queue, and is dropped rather than waited for when
// the master has stopped reading: the writer here is whoever typed, and
// blocking it on its own echo would deadlock a master that writes and reads
// from one thread.
static void
echo_put(struct pty *p, int c)
{
  if (out_room(p) < 2)
    return;
  out_put(p, c);
}

static void
echo(struct pty *p, int c)
{
  if (c == '\n') {
    if (p->tio.c_lflag & (ECHO | ECHONL))
      echo_put(p, '\n');
    return;
  }
  if (!(p->tio.c_lflag & ECHO))
    return;
  if (c < 32 && c != '\t' && c != '\r') {
    echo_put(p, '^');
    echo_put(p, c + '@');
  } else {
    echo_put(p, c);
  }
}

static void
erase_one(struct pty *p)
{
  if (p->ie == p->iw)
    return;
  p->ie--;
  if ((p->tio.c_lflag & ECHO) && (p->tio.c_lflag & ECHOE)) {
    echo_put(p, '\b');
    echo_put(p, ' ');
    echo_put(p, '\b');
  }
}

// --- input: the master writes, the slave reads -------------------------------

// One byte through the line discipline. Returns a signal to send to the
// foreground group, or 0. Caller holds the lock.
static int
input_byte(struct pty *p, int c)
{
  struct termios *t = &p->tio;

  if (t->c_iflag & ISTRIP)
    c &= 0x7f;
  if (c == '\r') {
    if (t->c_iflag & IGNCR)
      return 0;
    if (t->c_iflag & ICRNL)
      c = '\n';
  } else if (c == '\n' && (t->c_iflag & INLCR)) {
    c = '\r';
  }

  if (t->c_lflag & ISIG) {
    int sig = 0;
    if (c == t->c_cc[VINTR])      sig = SIGINT;
    else if (c == t->c_cc[VQUIT]) sig = SIGQUIT;
    else if (c == t->c_cc[VSUSP]) sig = SIGTSTP;
    if (sig && c != 0) {
      echo(p, c);
      if (!(t->c_lflag & NOFLSH))
        p->ie = p->iw = p->ir;
      p->abort_read = 1;
      return sig;
    }
  }

  if (t->c_lflag & ICANON) {
    if (c == t->c_cc[VERASE] || c == CTRL('H')) {
      erase_one(p);
      return 0;
    }
    if (c == t->c_cc[VKILL]) {
      while (p->ie != p->iw && p->in[(p->ie - 1) % PTY_BUF] != '\n')
        erase_one(p);
      return 0;
    }
    if (c == t->c_cc[VWERASE] && (t->c_lflag & IEXTEN)) {
      while (p->ie != p->iw && p->in[(p->ie - 1) % PTY_BUF] == ' ')
        erase_one(p);
      while (p->ie != p->iw && p->in[(p->ie - 1) % PTY_BUF] != ' ')
        erase_one(p);
      return 0;
    }
    if (c == t->c_cc[VEOF]) {
      if (p->ie == p->iw)
        p->eof = 1;
      p->iw = p->ie;
      return 0;
    }
  }

  if (p->ie - p->ir >= PTY_BUF)
    return 0;                           // full: dropped, as a console does
  p->in[p->ie++ % PTY_BUF] = (char)c;
  echo(p, c);
  if (!(t->c_lflag & ICANON) || c == '\n' || p->ie - p->ir >= PTY_BUF)
    p->iw = p->ie;
  return 0;
}

int
ptm_write(struct inode *ip, uint64 off, char *src, int n)
{
  struct pty *p = pty_of(ip);
  char kbuf[128];
  int done = 0;

  (void)off;
  if (p == 0 || !p->used)
    return -EIO;
  iunlock(ip);
  while (done < n) {
    int chunk = n - done < (int)sizeof(kbuf) ? n - done : (int)sizeof(kbuf);
    if (copy_from_user(kbuf, (addr_t)(src + done), chunk) < 0) {
      ilock(ip);
      return done ? done : -EFAULT;
    }
    acquire(&ptylock);
    for (int i = 0; i < chunk; i++) {
      // Raw-mode input waits for room rather than vanishing: a paste into an
      // editor is a burst of thousands of bytes, and dropping the tail of it
      // is worse than making the writer wait for the reader.
      while (!(p->tio.c_lflag & ICANON) && p->ie - p->ir >= PTY_BUF &&
             p->sopen > 0) {
        if (myproc()->killed) {
          release(&ptylock);
          ilock(ip);
          return done + i ? done + i : -EINTR;
        }
        sleep(&p->ie, &ptylock);
      }
      int sig = input_byte(p, kbuf[i] & 0xff);
      if (sig) {
        int pg = p->fg_pgrp;
        wakeup(&p->ir);
        release(&ptylock);
        if (pg > 0)
          sigsend_pgrp(pg, sig);
        acquire(&ptylock);
      }
    }
    wakeup(&p->ir);
    wakeup(&p->or);                   // the echo
    release(&ptylock);
    done += chunk;
  }
  poll_kick();
  ilock(ip);
  return n;
}

int
ptm_read(struct inode *ip, uint64 off, char *dst, int n)
{
  struct pty *p = pty_of(ip);
  char kbuf[256];
  int got = 0;

  (void)off;
  if (p == 0 || !p->used)
    return -EIO;
  iunlock(ip);
  acquire(&ptylock);
  while (p->or == p->ow) {
    // Every slave descriptor closed, after there was one: the program on the
    // other side has gone. Linux says EIO, and that is what terminal
    // emulators and tmux take as "the shell exited".
    if (p->slave_seen && p->sopen == 0) {
      release(&ptylock);
      ilock(ip);
      return -EIO;
    }
    if (myproc()->killed) {
      release(&ptylock);
      ilock(ip);
      return -EINTR;
    }
    sleep(&p->or, &ptylock);
  }
  while (got < n && got < (int)sizeof(kbuf) && p->or != p->ow)
    kbuf[got++] = p->out[p->or++ % PTY_BUF];
  wakeup(&p->ow);
  release(&ptylock);
  poll_kick();
  ilock(ip);
  if (copy_to_user((addr_t)dst, kbuf, got) < 0)
    return -EFAULT;
  return got;
}

int
ptm_ready(struct inode *ip)
{
  struct pty *p = pty_of(ip);
  return p && (p->or != p->ow || (p->slave_seen && p->sopen == 0));
}

void
ptm_open(struct inode *ip)
{
  struct pty *p = pty_of(ip);
  if (p) {
    acquire(&ptylock);
    p->mopen++;
    release(&ptylock);
  }
}

// The master is gone: the slave side has been hung up on. Its readers see end
// of file, its writers EIO, and its session SIGHUP -- which is how closing an
// xterm window ends the shell inside it.
void
ptm_close(struct inode *ip)
{
  struct pty *p = pty_of(ip);
  int sid = 0, pg = 0;

  if (p == 0)
    return;
  acquire(&ptylock);
  if (p->mopen > 0)
    p->mopen--;
  if (p->mopen == 0) {
    sid = p->sid;
    pg = p->fg_pgrp;
    wakeup(&p->ir);
    wakeup(&p->ow);
    wakeup(&p->ie);
  }
  maybe_free(p);
  release(&ptylock);
  if (sid > 0)
    sigsend_pgrp(sid, SIGHUP);
  if (pg > 0 && pg != sid)
    sigsend_pgrp(pg, SIGHUP);
  poll_kick();
}

// --- the slave ---------------------------------------------------------------

int
pts_read(struct inode *ip, uint64 off, char *dst, int n)
{
  struct pty *p = pty_of(ip);
  char kbuf[256];
  int got = 0, c;

  (void)off;
  if (p == 0 || !p->used)
    return -EIO;
  if (n > (int)sizeof(kbuf))
    n = sizeof(kbuf);
  iunlock(ip);
  acquire(&ptylock);
  while (got < n) {
    while (p->ir == p->iw) {
      if (p->eof) {
        p->eof = 0;
        goto done;
      }
      if (p->mopen == 0)
        goto done;                      // hung up: end of file
      // VMIN 0 is a poll, and a program that asked for one must not block.
      if (!(p->tio.c_lflag & ICANON) && p->tio.c_cc[VMIN] == 0)
        goto done;
      if (got > 0 && !(p->tio.c_lflag & ICANON))
        goto done;
      if (myproc()->killed || p->abort_read) {
        p->abort_read = 0;
        release(&ptylock);
        ilock(ip);
        return got ? got : -EINTR;
      }
      if (sig_attention(myproc())) {
        release(&ptylock);
        int e = sig_blocking_check();
        if (e) {
          ilock(ip);
          return got ? got : e;
        }
        acquire(&ptylock);
        continue;
      }
      sleep(&p->ir, &ptylock);
    }
    c = p->in[p->ir++ % PTY_BUF];
    kbuf[got++] = (char)c;
    if ((p->tio.c_lflag & ICANON) && c == '\n')
      break;
    if (!(p->tio.c_lflag & ICANON) &&
        got >= (p->tio.c_cc[VMIN] ? p->tio.c_cc[VMIN] : 1) && p->ir == p->iw)
      break;
  }
done:
  wakeup(&p->ie);
  release(&ptylock);
  poll_kick();
  ilock(ip);
  if (got && copy_to_user((addr_t)dst, kbuf, got) < 0)
    return -EFAULT;
  return got;
}

int
pts_write(struct inode *ip, uint64 off, char *src, int n)
{
  struct pty *p = pty_of(ip);
  char kbuf[128];
  int done = 0;

  (void)off;
  if (p == 0 || !p->used)
    return -EIO;
  iunlock(ip);
  while (done < n) {
    int chunk = n - done < (int)sizeof(kbuf) ? n - done : (int)sizeof(kbuf);
    if (copy_from_user(kbuf, (addr_t)(src + done), chunk) < 0) {
      ilock(ip);
      return done ? done : -EFAULT;
    }
    acquire(&ptylock);
    for (int i = 0; i < chunk; i++) {
      while (out_room(p) < 2) {
        if (p->mopen == 0 || myproc()->killed) {
          release(&ptylock);
          ilock(ip);
          return done + i ? done + i : (p->mopen == 0 ? -EIO : -EINTR);
        }
        wakeup(&p->or);
        sleep(&p->ow, &ptylock);
      }
      if (p->mopen == 0) {
        release(&ptylock);
        ilock(ip);
        return done + i ? done + i : -EIO;
      }
      out_put(p, kbuf[i] & 0xff);
    }
    wakeup(&p->or);
    release(&ptylock);
    done += chunk;
  }
  poll_kick();
  ilock(ip);
  return n;
}

int
pts_ready(struct inode *ip)
{
  struct pty *p = pty_of(ip);
  return p && (p->ir != p->iw || p->eof || p->mopen == 0);
}

// Opening a terminal makes it the caller's controlling terminal when the
// caller leads a session that has none -- the System V rule, which Linux keeps
// and which xterm relies on: its child calls setsid() and opens the slave, and
// expects ^C to work from then on without a TIOCSCTTY.
void
pts_open(struct inode *ip)
{
  struct pty *p = pty_of(ip);
  struct proc *me = myproc();

  if (p == 0)
    return;
  acquire(&ptylock);
  p->sopen++;
  p->slave_seen = 1;
  if (me && me->sid == me->pid && me->ctty == 0 && p->sid == 0) {
    me->ctty = PTY_CTTY + ip->minor;
    p->sid = me->sid;
    p->fg_pgrp = me->pgid;
  }
  release(&ptylock);
}

void
pts_close(struct inode *ip)
{
  struct pty *p = pty_of(ip);

  if (p == 0)
    return;
  acquire(&ptylock);
  if (p->sopen > 0)
    p->sopen--;
  if (p->sopen == 0) {
    p->sid = 0;
    p->fg_pgrp = 0;
    wakeup(&p->or);                     // the master reads EIO now
  }
  maybe_free(p);
  release(&ptylock);
  poll_kick();
}

// --- ioctl and poll ----------------------------------------------------------

int
pty_ioctl(struct inode *ip, uint request, addr_t arg)
{
  struct pty *p = pty_of(ip);
  int master = ip->major == PTM_MAJOR;
  int v;

  if (p == 0 || !p->used)
    return -EIO;

  switch (request) {
  case TIOCGPTN:
    if (!master)
      return -ENOTTY;
    v = ip->minor;
    return put_user(v, arg) < 0 ? -EFAULT : 0;
  case TIOCSPTLCK:
  case TIOCPKT:
    // Slaves are never locked here, so unlockpt() has nothing to undo; packet
    // mode is not offered, and nothing in the image asks for it.
    return master ? 0 : -ENOTTY;
  case TCGETS: {
    struct termios t;
    acquire(&ptylock);
    t = p->tio;
    release(&ptylock);
    return copy_to_user(arg, &t, sizeof(t)) < 0 ? -EFAULT : 0;
  }
  case TCSETS: case TCSETSW: case TCSETSF: {
    struct termios t;
    if (copy_from_user(&t, arg, sizeof(t)) < 0)
      return -EFAULT;
    acquire(&ptylock);
    p->tio = t;
    if (request == TCSETSF)
      p->ie = p->iw = p->ir;
    // Leaving canonical mode hands over the line being edited, as on the
    // console: a shell that switches to raw mid-line must not lose it.
    if (!(t.c_lflag & ICANON))
      p->iw = p->ie;
    wakeup(&p->ir);
    release(&ptylock);
    poll_kick();
    return 0;
  }
  case TCFLSH:
    acquire(&ptylock);
    if (arg == TCIFLUSH || arg == TCIOFLUSH) {
      p->ie = p->iw = p->ir;
      p->eof = 0;
    }
    if (arg == TCOFLUSH || arg == TCIOFLUSH)
      p->or = p->ow;
    wakeup(&p->ie);
    wakeup(&p->ow);
    release(&ptylock);
    return 0;
  case TIOCGWINSZ:
    return copy_to_user(arg, &p->ws, sizeof(p->ws)) < 0 ? -EFAULT : 0;
  case TIOCSWINSZ: {
    struct winsize ws;
    int changed, pg;
    if (copy_from_user(&ws, arg, sizeof(ws)) < 0)
      return -EFAULT;
    acquire(&ptylock);
    changed = ws.ws_row != p->ws.ws_row || ws.ws_col != p->ws.ws_col;
    p->ws = ws;
    pg = p->fg_pgrp;
    release(&ptylock);
    // The program inside learns of a resize the way it would on Linux.
    if (changed && pg > 0)
      sigsend_pgrp(pg, SIGWINCH);
    return 0;
  }
  case FIONREAD:
    v = master ? (int)(p->ow - p->or) : (int)(p->iw - p->ir);
    return put_user(v, arg) < 0 ? -EFAULT : 0;
  case TIOCOUTQ:
    v = master ? 0 : (int)(p->ow - p->or);
    return put_user(v, arg) < 0 ? -EFAULT : 0;
  case TIOCGPGRP:
    v = p->fg_pgrp;
    return put_user(v, arg) < 0 ? -EFAULT : 0;
  case TIOCSPGRP:
    if (copy_from_user(&v, arg, sizeof(v)) < 0)
      return -EFAULT;
    if (v <= 0)
      return -EINVAL;
    p->fg_pgrp = v;
    return 0;
  case TIOCGSID:
    if (p->sid == 0)
      return -ENOTTY;
    v = p->sid;
    return put_user(v, arg) < 0 ? -EFAULT : 0;
  case TIOCSCTTY:
    if (master)
      return -EINVAL;
    myproc()->ctty = PTY_CTTY + ip->minor;
    p->sid = myproc()->sid;
    p->fg_pgrp = myproc()->pgid;
    return 0;
  case TIOCNOTTY:
    if (myproc()->ctty == PTY_CTTY + ip->minor)
      myproc()->ctty = 0;
    return 0;
  }
  return -ENOTTY;
}

int
pty_poll(struct inode *ip, int events, int readable, int writable)
{
  struct pty *p = pty_of(ip);
  int rev = 0;

  if (p == 0 || !p->used)
    return POLLHUP_;
  if (ip->major == PTM_MAJOR) {
    if (readable && (events & POLLIN_) && p->or != p->ow)
      rev |= POLLIN_;
    if (p->slave_seen && p->sopen == 0)
      rev |= POLLHUP_ | (events & POLLIN_);
  } else {
    if (readable && (events & POLLIN_) && (p->ir != p->iw || p->eof))
      rev |= POLLIN_;
    if (p->mopen == 0)
      rev |= POLLHUP_ | (events & POLLIN_);
  }
  if (writable && (events & POLLOUT_))
    rev |= POLLOUT_;
  return rev;
}