linux ttyname{_r}: Don't bail prematurely [BZ #22145]

Commit 15e9a4f378 introduced logic for ttyname()
sending back ENODEV to signal that we can't get a name for the TTY because we
inherited it from a different mount namespace.

However, just because we inherited it from a different mount namespace and it
isn't available at its original path, doesn't mean that its name is unknowable;
we can still try to find it by allowing the normal fall back on iterating
through devices.

An example scenario where this happens is with "/dev/console" in containers.
It's a common practice among container managers to allocate a PTY master/slave
pair in the host's mount namespace (the slave having a path like "/dev/pty/$X"),
bind mount the slave to "/dev/console" in the container's mount namespace, and
send the slave FD to a process in the container. Inside of the
container, the slave-end isn't available at its original path ("/dev/pts/$X"),
since the container mount namespace has a separate devpts instance from the host
(that path may or may not exist in the container; if it does exist, it's not the
 same PTY slave device). Currently ttyname{_r} sees that the file at the
original "/dev/pts/$X" path doesn't match the FD passed to it, and fails early
and gives up, even though if it kept searching it would find the TTY at
"/dev/console". Fix that; don't have the ENODEV path force an early return
inhibiting the fall-back search.

This change is based on the previous patch that adds use of is_mytty in
getttyname and getttyname_r. Without that change, this effectively reverts
15e9a4f, which made us disregard the false similarity of file pointed to by
"/proc/self/fd/$Y", because if it doesn't bail prematurely then that file
("/dev/pts/$X") will just come up again anyway in the fall-back search.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
This commit is contained in:
Luke Shumaker 2017-11-15 20:36:44 +01:00 committed by Christian Brauner
parent 2fbce9c203
commit a09dfc19ed
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3 changed files with 29 additions and 15 deletions

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@ -1,5 +1,10 @@
2017-11-15 Luke Shumaker <lukeshu@parabola.nu>
[BZ #22145]
* sysdeps/unix/sysv/linux/ttyname.c (ttyname):
Defer is_pty check until end of the function.
* sysdeps/unix/sysv/linux/ttyname_r.c (__ttyname_r): Likewise.
[BZ #22145]
* sysdeps/unix/sysv/linux/ttyname.h (is_mytty): New function.
* sysdeps/unix/sysv/linux/ttyname.c (getttyname): Call is_mytty.

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@ -115,6 +115,7 @@ ttyname (int fd)
char procname[30];
struct stat64 st, st1;
int dostat = 0;
int doispty = 0;
char *name;
int save = errno;
struct termios term;
@ -165,13 +166,7 @@ ttyname (int fd)
&& is_mytty (&st, &st1))
return ttyname_buf;
/* If the link doesn't exist, then it points to a device in another
namespace. */
if (is_pty (&st))
{
__set_errno (ENODEV);
return NULL;
}
doispty = 1;
}
if (__xstat64 (_STAT_VER, "/dev/pts", &st1) == 0 && S_ISDIR (st1.st_mode))
@ -195,5 +190,15 @@ ttyname (int fd)
name = getttyname ("/dev", &st, save, &dostat);
}
if (!name && doispty && is_pty (&st))
{
/* We failed to figure out the TTY's name, but we can at least
signal that we did verify that it really is a PTY slave.
This happens when we have inherited the file descriptor from
a different mount namespace. */
__set_errno (ENODEV);
return NULL;
}
return name;
}

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@ -95,6 +95,7 @@ __ttyname_r (int fd, char *buf, size_t buflen)
char procname[30];
struct stat64 st, st1;
int dostat = 0;
int doispty = 0;
int save = errno;
/* Test for the absolute minimal size. This makes life easier inside
@ -149,14 +150,7 @@ __ttyname_r (int fd, char *buf, size_t buflen)
&& is_mytty (&st, &st1))
return 0;
/* If the link doesn't exist, then it points to a device in another
* namespace.
*/
if (is_pty (&st))
{
__set_errno (ENODEV);
return ENODEV;
}
doispty = 1;
}
/* Prepare the result buffer. */
@ -190,6 +184,16 @@ __ttyname_r (int fd, char *buf, size_t buflen)
save, &dostat);
}
if (ret && doispty && is_pty (&st))
{
/* We failed to figure out the TTY's name, but we can at least
signal that we did verify that it really is a PTY slave.
This happens when we have inherited the file descriptor from
a different mount namespace. */
__set_errno (ENODEV);
return ENODEV;
}
return ret;
}