From c8c03f1858331e85d397bacccd34ef409aae993c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 16 Aug 2017 17:08:07 -0700 Subject: 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//fd/. In particular, he wanted to use readlink() on /proc/self/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 Cc: Al Viro Acked-by: Eric Biederman Signed-off-by: Linus Torvalds --- include/linux/devpts_fs.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/devpts_fs.h') diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h index 277ab9af9ac2..7883e901f65c 100644 --- a/include/linux/devpts_fs.h +++ b/include/linux/devpts_fs.h @@ -19,7 +19,7 @@ struct pts_fs_info; -struct pts_fs_info *devpts_acquire(struct file *); +struct pts_fs_info *devpts_acquire(struct file *, struct vfsmount **ptsmnt); void devpts_release(struct pts_fs_info *); int devpts_new_index(struct pts_fs_info *); -- cgit v1.2.3 From 143c97cc652949893c8056c679012f0aeccb80e5 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Wed, 23 Aug 2017 18:16:11 -0700 Subject: Revert "pty: fix the cached path of the pty slave file descriptor in the master" This reverts commit c8c03f1858331e85d397bacccd34ef409aae993c. It turns out that while fixing the ptmx file descriptor to have the correct 'struct path' to the associated slave pty is a really good thing, it breaks some user space tools for a very annoying reason. The problem is that /dev/ptmx and its associated slave pty (/dev/pts/X) are on different mounts. That was what caused us to have the wrong path in the first place (we would mix up the vfsmount of the 'ptmx' node, with the dentry of the pty slave node), but it also means that now while we use the right vfsmount, having the pty master open also keeps the pts mount busy. And it turn sout that that makes 'pbuilder' very unhappy, as noted by Stefan Lippers-Hollmann: "This patch introduces a regression for me when using pbuilder 0.228.7[2] (a helper to build Debian packages in a chroot and to create and update its chroots) when trying to umount /dev/ptmx (inside the chroot) on Debian/ unstable (full log and pbuilder configuration file[3] attached). [...] Setting up build-essential (12.3) ... Processing triggers for libc-bin (2.24-15) ... I: unmounting dev/ptmx filesystem W: Could not unmount dev/ptmx: umount: /var/cache/pbuilder/build/1340/dev/ptmx: target is busy (In some cases useful info about processes that use the device is found by lsof(8) or fuser(1).)" apparently pbuilder tries to unmount the /dev/pts filesystem while still holding at least one master node open, which is arguably not very nice, but we don't break user space even when fixing other bugs. So this commit has to be reverted. I'll try to figure out a way to avoid caching the path to the slave pty in the master pty. The only thing that actually wants that slave pty path is the "TIOCGPTPEER" ioctl, and I think we could just recreate the path at that time. Reported-by: Stefan Lippers-Hollmann Cc: Eric W Biederman Cc: Christian Brauner Cc: Al Viro Signed-off-by: Linus Torvalds --- include/linux/devpts_fs.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/devpts_fs.h') diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h index 7883e901f65c..277ab9af9ac2 100644 --- a/include/linux/devpts_fs.h +++ b/include/linux/devpts_fs.h @@ -19,7 +19,7 @@ struct pts_fs_info; -struct pts_fs_info *devpts_acquire(struct file *, struct vfsmount **ptsmnt); +struct pts_fs_info *devpts_acquire(struct file *); void devpts_release(struct pts_fs_info *); int devpts_new_index(struct pts_fs_info *); -- cgit v1.2.3 From 311fc65c9fb9c966bca8e6f3ff8132ce57344ab9 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Thu, 24 Aug 2017 15:13:29 -0500 Subject: pty: Repair TIOCGPTPEER The implementation of TIOCGPTPEER has two issues. When /dev/ptmx (as opposed to /dev/pts/ptmx) is opened the wrong vfsmount is passed to dentry_open. Which results in the kernel displaying the wrong pathname for the peer. The second is simply by caching the vfsmount and dentry of the peer it leaves them open, in a way they were not previously Which because of the inreased reference counts can cause unnecessary behaviour differences resulting in regressions. To fix these move the ioctl into tty_io.c at a generic level allowing the ioctl to have access to the struct file on which the ioctl is being called. This allows the path of the slave to be derived when opening the slave through TIOCGPTPEER instead of requiring the path to the slave be cached. Thus removing the need for caching the path. A new function devpts_ptmx_path is factored out of devpts_acquire and used to implement a function devpts_mntget. The new function devpts_mntget takes a filp to perform the lookup on and fsi so that it can confirm that the superblock that is found by devpts_ptmx_path is the proper superblock. v2: Lots of fixes to make the code actually work v3: Suggestions by Linus - Removed the unnecessary initialization of filp in ptm_open_peer - Simplified devpts_ptmx_path as gotos are no longer required [ This is the fix for the issue that was reverted in commit 143c97cc6529, but this time without breaking 'pbuilder' due to increased reference counts - Linus ] Fixes: 54ebbfb16034 ("tty: add TIOCGPTPEER ioctl") Reported-by: Christian Brauner Reported-and-tested-by: Stefan Lippers-Hollmann Signed-off-by: "Eric W. Biederman" Signed-off-by: Linus Torvalds --- include/linux/devpts_fs.h | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'include/linux/devpts_fs.h') diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h index 277ab9af9ac2..100cb4343763 100644 --- a/include/linux/devpts_fs.h +++ b/include/linux/devpts_fs.h @@ -19,6 +19,7 @@ struct pts_fs_info; +struct vfsmount *devpts_mntget(struct file *, struct pts_fs_info *); struct pts_fs_info *devpts_acquire(struct file *); void devpts_release(struct pts_fs_info *); @@ -32,6 +33,15 @@ void *devpts_get_priv(struct dentry *); /* unlink */ void devpts_pty_kill(struct dentry *); +/* in pty.c */ +int ptm_open_peer(struct file *master, struct tty_struct *tty, int flags); + +#else +static inline int +ptm_open_peer(struct file *master, struct tty_struct *tty, int flags) +{ + return -EIO; +} #endif -- cgit v1.2.3