tty: fix sending SIGTTOU on tty write

How to reproduce:
  $ echo "timeout 10 ls" > foo.sh
  $ chmod +x foo.sh
  $ ./foo.sh
  (will hang here for 10 secs, and the output of ls does not show)

When "ls" process writes to stdout, it receives SIGTTOU signal, and
hangs there. Until "timeout" process timeouts, and kills "ls" process.

The expected result is: "ls" writes its output into tty, and terminates
immdedately, then "timeout" process receives SIGCHLD and terminates.

The reason for this failure is that we missed the check for TOSTOP (if
set, background processes will receive the SIGTTOU signal when they do
write).

We use drivers/tty/n_tty.c:n_tty_write() as a reference.

Fixes: #862

Reported-by: chris.zn <chris.zn@antfin.com>
Signed-off-by: Jianfeng Tan <henry.tjf@antfin.com>
Signed-off-by: chenglang.hy <chenglang.hy@antfin.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/pkg/sentry/fs/host/tty.go b/pkg/sentry/fs/host/tty.go
index 2526412a4..90331e3b2 100644
--- a/pkg/sentry/fs/host/tty.go
+++ b/pkg/sentry/fs/host/tty.go
@@ -43,12 +43,15 @@ type TTYFileOperations struct {
 	// fgProcessGroup is the foreground process group that is currently
 	// connected to this TTY.
 	fgProcessGroup *kernel.ProcessGroup
+
+	termios linux.KernelTermios
 }
 
 // newTTYFile returns a new fs.File that wraps a TTY FD.
 func newTTYFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags, iops *inodeOperations) *fs.File {
 	return fs.NewFile(ctx, dirent, flags, &TTYFileOperations{
 		fileOperations: fileOperations{iops: iops},
+		termios:        linux.DefaultSlaveTermios,
 	})
 }
 
@@ -97,9 +100,12 @@ func (t *TTYFileOperations) Write(ctx context.Context, file *fs.File, src userme
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
-	// Are we allowed to do the write?
-	if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
-		return 0, err
+	// Check whether TOSTOP is enabled. This corresponds to the check in
+	// drivers/tty/n_tty.c:n_tty_write().
+	if t.termios.LEnabled(linux.TOSTOP) {
+		if err := t.checkChange(ctx, linux.SIGTTOU); err != nil {
+			return 0, err
+		}
 	}
 	return t.fileOperations.Write(ctx, file, src, offset)
 }
@@ -144,6 +150,9 @@ func (t *TTYFileOperations) Ioctl(ctx context.Context, _ *fs.File, io usermem.IO
 			return 0, err
 		}
 		err := ioctlSetTermios(fd, ioctl, &termios)
+		if err == nil {
+			t.termios.FromTermios(termios)
+		}
 		return 0, err
 
 	case linux.TIOCGPGRP: