| Message ID | 20241121111506.4717-2-gpdev@gpost.dk |
|---|---|
| State | New |
| Headers | show |
| Series | Fix to allow more correct isatty() | expand |
On Thu, Nov 21, 2024 at 12:12:54PM +0100, Gil Pedersen wrote: > Userspace libc implementations of the isatty() POSIX system interface > are currently unable to reliably determine if a fd is really a tty when > it is hung. > > Specifically glibc libc returns the success status of a TCGETS ioctl. > This will return an incorrect result when the TTY is hung, since an EIO > is unconditionally returned. Ie. an isatty() will return 0, wrongly > indicating that something that definitely is a TTY, is not a TTY. > > Userspace implementations could potentially remap EIO errors to a > success to work around this. This will likely work in 99.99% of cases, > but there is no guarantee that a TCGETS ioctl on a non-TTY fd will not > also return EIO, making the isatty() call return a false positive! > > This commit enables a specific non-driver, non-ldisc, ioctl to continue > working after the TTY is hung. The TIOCGWINSZ ioctl was chosen since it > is readonly, and only access tty_struct.winsize (and its mutex), and is > already used for the isatty() implementation in musl. The glibc > implementation will need to be updated to use the TIOCGWINSZ ioctl, > either as a direct replacement, or more conservatively, as a fallback > test when the TCGETS ioctl fails with EIO. This is a fun "hack", yes, but now you are encoding an odd "side affect" into the system that everyone is going to rely on, well, eventually rely on. What code needs to be changed in userspace to determine this? Why not just have a new ioctl that tells you if the tty really is hung or not? Why does isatty() need to know this, does POSIX require it? And if it does, what does it say the ioctl command should be? thanks, greg k-h
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index 9771072da177..678fcc9b8264 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -157,6 +157,8 @@ static long tty_compat_ioctl(struct file *file, unsigned int cmd, static int __tty_fasync(int fd, struct file *filp, int on); static int tty_fasync(int fd, struct file *filp, int on); static void release_tty(struct tty_struct *tty, int idx); +static long hung_up_tty_ioctl(struct file *file, unsigned int cmd, + unsigned long arg); /** * free_tty_struct - free a disused tty @@ -433,16 +435,10 @@ static __poll_t hung_up_tty_poll(struct file *filp, poll_table *wait) return EPOLLIN | EPOLLOUT | EPOLLERR | EPOLLHUP | EPOLLRDNORM | EPOLLWRNORM; } -static long hung_up_tty_ioctl(struct file *file, unsigned int cmd, - unsigned long arg) -{ - return cmd == TIOCSPGRP ? -ENOTTY : -EIO; -} - static long hung_up_tty_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { - return cmd == TIOCSPGRP ? -ENOTTY : -EIO; + return hung_up_tty_ioctl(file, cmd, (unsigned long)compat_ptr(arg)); } static int hung_up_tty_fasync(int fd, struct file *file, int on) @@ -2817,6 +2813,25 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg) return retval; } +static long hung_up_tty_ioctl(struct file *file, unsigned int cmd, + unsigned long arg) +{ + struct tty_struct *tty = file_tty(file); + struct tty_struct *real_tty; + void __user *p = (void __user *)arg; + + real_tty = tty_pair_get_tty(tty); + + switch (cmd) { + case TIOCGWINSZ: + return tiocgwinsz(real_tty, p); + case TIOCSPGRP: + return -ENOTTY; + } + + return -EIO; +} + #ifdef CONFIG_COMPAT struct serial_struct32 {
Userspace libc implementations of the isatty() POSIX system interface are currently unable to reliably determine if a fd is really a tty when it is hung. Specifically glibc libc returns the success status of a TCGETS ioctl. This will return an incorrect result when the TTY is hung, since an EIO is unconditionally returned. Ie. an isatty() will return 0, wrongly indicating that something that definitely is a TTY, is not a TTY. Userspace implementations could potentially remap EIO errors to a success to work around this. This will likely work in 99.99% of cases, but there is no guarantee that a TCGETS ioctl on a non-TTY fd will not also return EIO, making the isatty() call return a false positive! This commit enables a specific non-driver, non-ldisc, ioctl to continue working after the TTY is hung. The TIOCGWINSZ ioctl was chosen since it is readonly, and only access tty_struct.winsize (and its mutex), and is already used for the isatty() implementation in musl. The glibc implementation will need to be updated to use the TIOCGWINSZ ioctl, either as a direct replacement, or more conservatively, as a fallback test when the TCGETS ioctl fails with EIO. Note that TCGETS is not available to use for this, since it is implemented at the ldisc level, which can not be called into once the TTY is hung. Signed-off-by: Gil Pedersen <gpdev@gpost.dk> --- drivers/tty/tty_io.c | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-)