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authorLinus Torvalds <torvalds@linux-foundation.org>2017-08-16 17:08:07 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-08-17 09:10:48 -0700
commitc8c03f1858331e85d397bacccd34ef409aae993c (patch)
tree02e36380538407e87ebd5b523c79851f7549d6c9 /fs
parentac9a40905a610fb02086a37b11ff4bf046825a88 (diff)
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pty: fix the cached path of the pty slave file descriptor in the master
Christian Brauner reported that if you use the TIOCGPTPEER ioctl() to get a slave pty file descriptor, the resulting file descriptor doesn't look right in /proc/<pid>/fd/<fd>. In particular, he wanted to use readlink() on /proc/self/fd/<fd> to get the pathname of the slave pty (basically implementing "ptsname{_r}()"). The reason for that was that we had generated the wrong 'struct path' when we create the pty in ptmx_open(). In particular, the dentry was correct, but the vfsmount pointed to the mount of the ptmx node. That _can_ be correct - in case you use "/dev/pts/ptmx" to open the master - but usually is not. The normal case is to use /dev/ptmx, which then looks up the pts/ directory, and then the vfsmount of the ptmx node is obviously the /dev directory, not the /dev/pts/ directory. We actually did have the right vfsmount available, but in the wrong place (it gets looked up in 'devpts_acquire()' when we get a reference to the pts filesystem), and so ptmx_open() used the wrong mnt pointer. The end result of this confusion was that the pty worked fine, but when if you did TIOCGPTPEER to get the slave side of the pty, end end result would also work, but have that dodgy 'struct path'. And then when doing "d_path()" on to get the pathname, the vfsmount would not match the root of the pts directory, and d_path() would return an empty pathname thinking that the entry had escaped a bind mount into another mount. This fixes the problem by making devpts_acquire() return the vfsmount for the pts filesystem, allowing ptmx_open() to trivially just use the right mount for the pts dentry, and create the proper 'struct path'. Reported-by: Christian Brauner <christian.brauner@ubuntu.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Acked-by: Eric Biederman <ebiederm@xmission.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
-rw-r--r--fs/devpts/inode.c4
1 files changed, 3 insertions, 1 deletions
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 108df2e3602c..44dfbca9306f 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -133,7 +133,7 @@ static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
return sb->s_fs_info;
}
-struct pts_fs_info *devpts_acquire(struct file *filp)
+struct pts_fs_info *devpts_acquire(struct file *filp, struct vfsmount **ptsmnt)
{
struct pts_fs_info *result;
struct path path;
@@ -142,6 +142,7 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
path = filp->f_path;
path_get(&path);
+ *ptsmnt = NULL;
/* Has the devpts filesystem already been found? */
sb = path.mnt->mnt_sb;
@@ -165,6 +166,7 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
* pty code needs to hold extra references in case of last /dev/tty close
*/
atomic_inc(&sb->s_active);
+ *ptsmnt = mntget(path.mnt);
result = DEVPTS_SB(sb);
out: