[PATCH 0/3 v4] devpts: handle bind-mounts

[PATCH 0/3 v4] devpts: handle bind-mounts

[Christian Brauner]

Hey everyone,

This is the fourth iteration of this patch. Relevant changes are.
ChangeLog v3->v4:
* small logical simplifications
* add test that  bind-mounts of /dev/pts/ptmx to locations that do not
  resolve to a valid slave pty path under the originating devpts mount
  fail
ChangeLog v2->v3:
* rewritten commit message to thoroughly analyse the problem for
  posterity in Subject: [PATCH 2/3 v3] devpts: resolve devpts bind-mounts
  and in this cover letter.
* extended selftests to test for correct handling of /dev/pts/ptmx
  bind-mounts to /dev/ptmx and non-standard devpts mounts such as
  mount -t devpts devpts /mnt

Most libcs will still look at /dev/ptmx when opening the master fd of a pty
device. When /dev/ptmx is a bind-mount of /dev/pts/ptmx and the TIOCGPTPEER
ioctl() is used to safely retrieve a file descriptor for the slave side of
the pty based on the master fd, the /proc/self/fd/{0,1,2} symlinks will
point to /.

When the kernel tries to look up the root mount of the dentry for the slave
file descriptor it will detect that the dentry is escaping its bind-mount
since the root mount of the dentry is /dev/pts where the devpts is mounted
but the root mount of /dev/ptmx is /dev.

Having bind-mounts of /dev/pts/ptmx to /dev/ptmx not working correctly is a
regression. In addition, it is also a fairly common scenario in containers
employing user namespaces.

To handle bind-mounts of /dev/pts/ptmx to /dev/ptmx correctly we need to
walk up the bind-mounts for /dev/ptmx in devpts_mntget(). Since the
contents of /proc/<pid>/fd/<nr> symlinks attached to the slave side of a
file descriptor will always point to a path under the devpts mount we need
to try and ensure that the kernel doesn't falsely get the impression that a
pty slave file descriptor retrieved via TIOCGPTPEER based on a pty master
file descriptor opened via a bind-mount of the ptmx device escapes its
bind-mount. To clarify in pseudo code:

* bind-mount /dev/pts/ptmx to /dev/ptmx
* master = open("/dev/ptmx", O_RDWR | O_NOCTTY | O_CLOEXEC);
* slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY | O_CLOEXEC);

would cause the kernel to think that slave is escaping its bind-mount. The
reason is that while the devpts mounted at /dev/pts has devtmpfs mounted at
/dev as its parent mount:

21 -- -- / /dev
-- 21 -- / /dev/pts

they are on different devices

-- -- 0:6  / /dev
-- -- 0:20 / /dev/pts

which has the consequence that the pathname of the directory which forms
the root of the /dev/pts mount is /. So if we bind-mount /dev/pts/ptmx to
/dev/ptmx we will end up on the same device as the devtmpfs mount at
/dev/pts

-- -- 0:20 /ptmx /dev/ptmx

Without the bind-mount resolution patch here the kernel will now perform
the bind-mount escape check directly on /dev/ptmx. When it hits
devpts_ptmx_path()  calls pts_path() which in turn calls
path_parent_directory(). While one would expect that
path_parent_directory() *should* yield /dev it will yield / since
/dev and /dev/pts are on different devices. This will cause path_pts() to
fail finding a "pts" directory since there is none under /. Thus, the
kernel detects that /dev/ptmx is escaping its bind-mount and will set
/proc/<pid>/fd/<nr> to /.

This patch changes the logic to do bind-mount resolution and after the
bind-mount has been resolved (i.e. we have traced it back to the devpts
mount) we can safely perform devpts_ptmx_path() and check whether we find a
"pts" directory in the parent directory of the devpts mount. Since
path_parent_directory() will now correctly yield /dev as parent directory
for the devpts mount at /dev/pts.

However, we can only perform devpts_ptmx_path() devpts_mntget() if we
either did resolve a bind-mount or did not find a suitable devpts
filesystem. The reason is that we want and need to support non-standard
mountpoints for the devpts filesystem. If we call devpts_ptmx_path()
although we did already find a devpts filesystem and did not resolve
bind-mounts we will fail on devpts mounts such as:

mount -t devpts devpts /mnt

where no "pts" directory will be under /. So change the logic to account
for this.

Here's a little reproducer that presupposes a libc that uses TIOCGPTPEER in
its openpty() implementation:

unshare --mount
mount --bind /dev/pts/ptmx /dev/ptmx
chmod 666 /dev/ptmx
script
ls -al /proc/self/fd/0

with output:

lrwx------ 1 chb chb 64 Mar  7 16:41 /proc/self/fd/0 -> /

Christian Brauner (3):
  devpts: hoist out check for DEVPTS_SUPER_MAGIC
  devpts: resolve devpts bind-mounts
  selftests: add devpts selftests

 fs/devpts/inode.c                                |  33 ++-
 tools/testing/selftests/Makefile                 |   1 +
 tools/testing/selftests/filesystems/.gitignore   |   1 +
 tools/testing/selftests/filesystems/Makefile     |   2 +-
 tools/testing/selftests/filesystems/devpts_pts.c | 313 +++++++++++++++++++++++
 5 files changed, 338 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/devpts_pts.c

-- 
2.15.1

[PATCH 1/3 v4] devpts: hoist out check for DEVPTS_SUPER_MAGIC

[Christian Brauner]

Hoist the check whether we have already found a suitable devpts filesystem
out of devpts_ptmx_path() in preparation for the devpts bind-mount
resolution patch. This is a non-functional change.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
 ---
ChangeLog v3->v4:
* patch unchanged
ChangeLog v2->v3:
* patch unchanged
ChangeLog v1->v2:
* patch added
ChangeLog v0->v1:
* patch not present
---
 fs/devpts/inode.c | 35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index e31d6ed3ec32..d3ddbb888874 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -138,10 +138,6 @@ static int devpts_ptmx_path(struct path *path)
 	struct super_block *sb;
 	int err;
 
-	/* Has the devpts filesystem already been found? */
-	if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
-		return 0;
-
 	/* Is a devpts filesystem at "pts" in the same directory? */
 	err = path_pts(path);
 	if (err)
@@ -159,17 +155,22 @@ static int devpts_ptmx_path(struct path *path)
 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
 {
 	struct path path;
-	int err;
 
 	path = filp->f_path;
 	path_get(&path);
 
-	err = devpts_ptmx_path(&path);
-	dput(path.dentry);
-	if (err) {
-		mntput(path.mnt);
-		return ERR_PTR(err);
+	/* Has the devpts filesystem already been found? */
+	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
+		int err;
+
+		err = devpts_ptmx_path(&path);
+		dput(path.dentry);
+		if (err) {
+			mntput(path.mnt);
+			return ERR_PTR(err);
+		}
 	}
+
 	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
 		mntput(path.mnt);
 		return ERR_PTR(-ENODEV);
@@ -182,15 +183,19 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
 	struct pts_fs_info *result;
 	struct path path;
 	struct super_block *sb;
-	int err;
 
 	path = filp->f_path;
 	path_get(&path);
 
-	err = devpts_ptmx_path(&path);
-	if (err) {
-		result = ERR_PTR(err);
-		goto out;
+	/* Has the devpts filesystem already been found? */
+	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
+		int err;
+
+		err = devpts_ptmx_path(&path);
+		if (err) {
+			result = ERR_PTR(err);
+			goto out;
+		}
 	}
 
 	/*
-- 
2.15.1

Re: [PATCH 1/3 v4] devpts: hoist out check for DEVPTS_SUPER_MAGIC

[Eric W. Biederman]

Christian Brauner <christian.brauner@ubuntu.com> writes:

> Hoist the check whether we have already found a suitable devpts filesystem
> out of devpts_ptmx_path() in preparation for the devpts bind-mount
> resolution patch. This is a non-functional change.
>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>

>  ---
> ChangeLog v3->v4:
> * patch unchanged
> ChangeLog v2->v3:
> * patch unchanged
> ChangeLog v1->v2:
> * patch added
> ChangeLog v0->v1:
> * patch not present
> ---
>  fs/devpts/inode.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index e31d6ed3ec32..d3ddbb888874 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -138,10 +138,6 @@ static int devpts_ptmx_path(struct path *path)
>  	struct super_block *sb;
>  	int err;
>  
> -	/* Has the devpts filesystem already been found? */
> -	if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
> -		return 0;
> -
>  	/* Is a devpts filesystem at "pts" in the same directory? */
>  	err = path_pts(path);
>  	if (err)
> @@ -159,17 +155,22 @@ static int devpts_ptmx_path(struct path *path)
>  struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
>  {
>  	struct path path;
> -	int err;
>  
>  	path = filp->f_path;
>  	path_get(&path);
>  
> -	err = devpts_ptmx_path(&path);
> -	dput(path.dentry);
> -	if (err) {
> -		mntput(path.mnt);
> -		return ERR_PTR(err);
> +	/* Has the devpts filesystem already been found? */
> +	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
> +		int err;
> +
> +		err = devpts_ptmx_path(&path);
> +		dput(path.dentry);
> +		if (err) {
> +			mntput(path.mnt);
> +			return ERR_PTR(err);
> +		}
>  	}
> +
>  	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
>  		mntput(path.mnt);
>  		return ERR_PTR(-ENODEV);
> @@ -182,15 +183,19 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
>  	struct pts_fs_info *result;
>  	struct path path;
>  	struct super_block *sb;
> -	int err;
>  
>  	path = filp->f_path;
>  	path_get(&path);
>  
> -	err = devpts_ptmx_path(&path);
> -	if (err) {
> -		result = ERR_PTR(err);
> -		goto out;
> +	/* Has the devpts filesystem already been found? */
> +	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
> +		int err;
> +
> +		err = devpts_ptmx_path(&path);
> +		if (err) {
> +			result = ERR_PTR(err);
> +			goto out;
> +		}
>  	}
>  
>  	/*

Re: [PATCH 1/3 v4] devpts: hoist out check for DEVPTS_SUPER_MAGIC

[Eric W. Biederman]

Christian Brauner <christian.brauner@ubuntu.com> writes:

> Hoist the check whether we have already found a suitable devpts filesystem
> out of devpts_ptmx_path() in preparation for the devpts bind-mount
> resolution patch. This is a non-functional change.

Sigh.  Scratch my review-by.


>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
>  ---
> ChangeLog v3->v4:
> * patch unchanged
> ChangeLog v2->v3:
> * patch unchanged
> ChangeLog v1->v2:
> * patch added
> ChangeLog v0->v1:
> * patch not present
> ---
>  fs/devpts/inode.c | 35 ++++++++++++++++++++---------------
>  1 file changed, 20 insertions(+), 15 deletions(-)
>
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index e31d6ed3ec32..d3ddbb888874 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -138,10 +138,6 @@ static int devpts_ptmx_path(struct path *path)
>  	struct super_block *sb;
>  	int err;
>  
> -	/* Has the devpts filesystem already been found? */
> -	if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
> -		return 0;
> -
>  	/* Is a devpts filesystem at "pts" in the same directory? */
>  	err = path_pts(path);
>  	if (err)
> @@ -159,17 +155,22 @@ static int devpts_ptmx_path(struct path *path)
>  struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
>  {
>  	struct path path;
> -	int err;
>  
>  	path = filp->f_path;
>  	path_get(&path);
>  
> -	err = devpts_ptmx_path(&path);
> -	dput(path.dentry);
> -	if (err) {
> -		mntput(path.mnt);
> -		return ERR_PTR(err);
> +	/* Has the devpts filesystem already been found? */
> +	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
> +		int err;
> +
> +		err = devpts_ptmx_path(&path);
> +		dput(path.dentry);
> +		if (err) {
> +			mntput(path.mnt);
> +			return ERR_PTR(err);
> +		}
>  	}

Indenting the dput results in a dentry leak.  You catch this
in your next patch but this is still wrong in this one.

>  	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
>  		mntput(path.mnt);
>  		return ERR_PTR(-ENODEV);
> @@ -182,15 +183,19 @@ struct pts_fs_info *devpts_acquire(struct file *filp)
>  	struct pts_fs_info *result;
>  	struct path path;
>  	struct super_block *sb;
> -	int err;
>  
>  	path = filp->f_path;
>  	path_get(&path);
>  
> -	err = devpts_ptmx_path(&path);
> -	if (err) {
> -		result = ERR_PTR(err);
> -		goto out;
> +	/* Has the devpts filesystem already been found? */
> +	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
> +		int err;
> +
> +		err = devpts_ptmx_path(&path);
> +		if (err) {
> +			result = ERR_PTR(err);
> +			goto out;
> +		}
>  	}
>  
>  	/*

Eric

[PATCH 2/3 v4] devpts: resolve devpts bind-mounts

[Christian Brauner]

Most libcs will still look at /dev/ptmx when opening the master fd of a pty
device. When /dev/ptmx is a bind-mount of /dev/pts/ptmx and the TIOCGPTPEER
ioctl() is used to safely retrieve a file descriptor for the slave side of
the pty based on the master fd, the /proc/self/fd/{0,1,2} symlinks will
point to /.

When the kernel tries to look up the root mount of the dentry for the slave
file descriptor it will detect that the dentry is escaping its bind-mount
since the root mount of the dentry is /dev/pts where the devpts is mounted
but the root mount of /dev/ptmx is /dev.

Having bind-mounts of /dev/pts/ptmx to /dev/ptmx not working correctly is a
regression. In addition, it is also a fairly common scenario in containers
employing user namespaces.

To handle bind-mounts of /dev/pts/ptmx to /dev/ptmx correctly we need to
walk up the bind-mounts for /dev/ptmx in devpts_mntget(). Since the
contents of /proc/<pid>/fd/<nr> symlinks attached to the slave side of a
file descriptor will always point to a path under the devpts mount we need
to try and ensure that the kernel doesn't falsely get the impression that a
pty slave file descriptor retrieved via TIOCGPTPEER based on a pty master
file descriptor opened via a bind-mount of the ptmx device escapes its
bind-mount. To clarify in pseudo code:

* bind-mount /dev/pts/ptmx to /dev/ptmx
* master = open("/dev/ptmx", O_RDWR | O_NOCTTY | O_CLOEXEC);
* slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY | O_CLOEXEC);

would cause the kernel to think that slave is escaping its bind-mount. The
reason is that while the devpts mounted at /dev/pts has devtmpfs mounted at
/dev as its parent mount:

21 -- -- / /dev
-- 21 -- / /dev/pts

they are on different devices

-- -- 0:6  / /dev
-- -- 0:20 / /dev/pts

which has the consequence that the pathname of the directory which forms
the root of the /dev/pts mount is /. So if we bind-mount /dev/pts/ptmx to
/dev/ptmx we will end up on the same device as the devtmpfs mount at
/dev/pts

-- -- 0:20 /ptmx /dev/ptmx

Without the bind-mount resolution patch here the kernel will now perform
the bind-mount escape check directly on /dev/ptmx. When it hits
devpts_ptmx_path()  calls pts_path() which in turn calls
path_parent_directory(). While one would expect that
path_parent_directory() *should* yield /dev it will yield / since
/dev and /dev/pts are on different devices. This will cause path_pts() to
fail finding a "pts" directory since there is none under /. Thus, the
kernel detects that /dev/ptmx is escaping its bind-mount and will set
/proc/<pid>/fd/<nr> to /.

This patch changes the logic to do bind-mount resolution and after the
bind-mount has been resolved (i.e. we have traced it back to the devpts
mount) we can safely perform devpts_ptmx_path() and check whether we find a
"pts" directory in the parent directory of the devpts mount. Since
path_parent_directory() will now correctly yield /dev as parent directory
for the devpts mount at /dev/pts.

However, we can only perform devpts_ptmx_path() devpts_mntget() if we
either did resolve a bind-mount or did not find a suitable devpts
filesystem. The reason is that we want and need to support non-standard
mountpoints for the devpts filesystem. If we call devpts_ptmx_path()
although we did already find a devpts filesystem and did not resolve
bind-mounts we will fail on devpts mounts such as:

mount -t devpts devpts /mnt

where no "pts" directory will be under /. So change the logic to account
for this.

Here's a little reproducer that presupposes a libc that uses TIOCGPTPEER in
its openpty() implementation:

unshare --mount
mount --bind /dev/pts/ptmx /dev/ptmx
chmod 666 /dev/ptmx
script
ls -al /proc/self/fd/0

with output:

lrwx------ 1 chb chb 64 Mar  7 16:41 /proc/self/fd/0 -> /

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Suggested-by: Eric Biederman <ebiederm@xmission.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
---
ChangeLog v3->v4:
* simplify if condition
ChangeLog v2->v3:
* rework logic to account for non-standard devpts mounts such as
  mount -t devpts devpts /mnt
ChangeLog v1->v2:
* move removal of if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
  condition to separate patch with non-functional changes
ChangeLog v0->v1:
* remove
        /* Has the devpts filesystem already been found? */
        if (path->mnt->mnt_sb->s_magic == DEVPTS_SUPER_MAGIC)
	                return 0
  from devpts_ptmx_path()
* check superblock after devpts_ptmx_path() returned
---
 fs/devpts/inode.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index d3ddbb888874..4a94f0d2d4c8 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -155,26 +155,32 @@ static int devpts_ptmx_path(struct path *path)
 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
 {
 	struct path path;
+	int err = 0;
 
 	path = filp->f_path;
 	path_get(&path);
 
-	/* Has the devpts filesystem already been found? */
-	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
-		int err;
+	/* Walk upward while the start point is a bind mount of
+	 * a single file.
+	 */
+	while (path.mnt->mnt_root == path.dentry)
+		if (follow_up(&path) == 0)
+			break;
 
+	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
+	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
 		err = devpts_ptmx_path(&path);
-		dput(path.dentry);
-		if (err) {
-			mntput(path.mnt);
-			return ERR_PTR(err);
-		}
+	dput(path.dentry);
+	if (err) {
+		mntput(path.mnt);
+		return ERR_PTR(err);
 	}
 
 	if (DEVPTS_SB(path.mnt->mnt_sb) != fsi) {
 		mntput(path.mnt);
 		return ERR_PTR(-ENODEV);
 	}
+
 	return path.mnt;
 }
 
-- 
2.15.1

Re: [PATCH 2/3 v4] devpts: resolve devpts bind-mounts

[Linus Torvalds]

On Mon, Mar 12, 2018 at 5:01 PM, Christian Brauner
<christian.brauner@ubuntu.com> wrote:
>
> ChangeLog v3->v4:
> * simplify if condition

Ok, I definitely prefer this simpler version.

We could simplify it even more, though. That end could be just

        ...
        dput(path.dentry);
        if (!err) {
                if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
                        return path.mnt;
                err = -ENODEV;
        }

        mntput(path.mnt);
        return ERR_PTR(err);

instead.

And it might be worth adding a comment that devpts_ptmx_path() always
returns a DEVPTS filesystem of an error. I already wrote out "that's
not right, you can't do DEVPTS_SB() without checking s_magic", but
devpts_ptmx_path() will have done that already.

             Linus

[PATCH 3/3 v4] selftests: add devpts selftests

[Christian Brauner]

This adds tests to check:
- bind-mounts from /dev/pts/ptmx to /dev/ptmx work
- non-standard mounts of devpts work
- bind-mounts of /dev/pts/ptmx to locations that do not resolve to a valid
  slave pty path under the originating devpts mount fail

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
ChangeLog v3->v4:
* patch unchanged
ChangeLog v2->v3:
* extend test for non-standard devpts mounts such as
  mount -t devpts e devpts /mnt
ChangeLog v1->v2:
* patch added
ChangeLog v0->v1:
* patch not present
---
 tools/testing/selftests/Makefile                 |   1 +
 tools/testing/selftests/filesystems/.gitignore   |   1 +
 tools/testing/selftests/filesystems/Makefile     |   2 +-
 tools/testing/selftests/filesystems/devpts_pts.c | 313 +++++++++++++++++++++++
 4 files changed, 316 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/filesystems/devpts_pts.c

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 7442dfb73b7f..dbda89c9d9b9 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -7,6 +7,7 @@ TARGETS += cpufreq
 TARGETS += cpu-hotplug
 TARGETS += efivarfs
 TARGETS += exec
+TARGETS += filesystems
 TARGETS += firmware
 TARGETS += ftrace
 TARGETS += futex
diff --git a/tools/testing/selftests/filesystems/.gitignore b/tools/testing/selftests/filesystems/.gitignore
index 31d6e426b6d4..8449cf6716ce 100644
--- a/tools/testing/selftests/filesystems/.gitignore
+++ b/tools/testing/selftests/filesystems/.gitignore
@@ -1 +1,2 @@
 dnotify_test
+devpts_pts
diff --git a/tools/testing/selftests/filesystems/Makefile b/tools/testing/selftests/filesystems/Makefile
index 13a73bf725b5..4e6d09fb166f 100644
--- a/tools/testing/selftests/filesystems/Makefile
+++ b/tools/testing/selftests/filesystems/Makefile
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := dnotify_test
+TEST_PROGS := dnotify_test devpts_pts
 all: $(TEST_PROGS)
 
 include ../lib.mk
diff --git a/tools/testing/selftests/filesystems/devpts_pts.c b/tools/testing/selftests/filesystems/devpts_pts.c
new file mode 100644
index 000000000000..c5b2eb9eac01
--- /dev/null
+++ b/tools/testing/selftests/filesystems/devpts_pts.c
@@ -0,0 +1,313 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/ioctl.h>
+#include <sys/mount.h>
+#include <sys/wait.h>
+
+static bool terminal_dup2(int duplicate, int original)
+{
+	int ret;
+
+	ret = dup2(duplicate, original);
+	if (ret < 0)
+		return false;
+
+	return true;
+}
+
+static int terminal_set_stdfds(int fd)
+{
+	int i;
+
+	if (fd < 0)
+		return 0;
+
+	for (i = 0; i < 3; i++)
+		if (!terminal_dup2(fd, (int[]){STDIN_FILENO, STDOUT_FILENO,
+					       STDERR_FILENO}[i]))
+			return -1;
+
+	return 0;
+}
+
+static int login_pty(int fd)
+{
+	int ret;
+
+	setsid();
+
+	ret = ioctl(fd, TIOCSCTTY, NULL);
+	if (ret < 0)
+		return -1;
+
+	ret = terminal_set_stdfds(fd);
+	if (ret < 0)
+		return -1;
+
+	if (fd > STDERR_FILENO)
+		close(fd);
+
+	return 0;
+}
+
+static int wait_for_pid(pid_t pid)
+{
+	int status, ret;
+
+again:
+	ret = waitpid(pid, &status, 0);
+	if (ret == -1) {
+		if (errno == EINTR)
+			goto again;
+		return -1;
+	}
+	if (ret != pid)
+		goto again;
+
+	if (!WIFEXITED(status) || WEXITSTATUS(status) != 0)
+		return -1;
+
+	return 0;
+}
+
+static int resolve_procfd_symlink(int fd, char *buf, size_t buflen)
+{
+	int ret;
+	char procfd[4096];
+
+	ret = snprintf(procfd, 4096, "/proc/self/fd/%d", fd);
+	if (ret < 0 || ret >= 4096)
+		return -1;
+
+	ret = readlink(procfd, buf, buflen);
+	if (ret < 0 || (size_t)ret >= buflen)
+		return -1;
+
+	buf[ret] = '\0';
+
+	return 0;
+}
+
+static int do_tiocgptpeer(char *ptmx, char *expected_procfd_contents)
+{
+	int ret;
+	int master = -1, slave = -1, fret = -1;
+
+	master = open(ptmx, O_RDWR | O_NOCTTY | O_CLOEXEC);
+	if (master < 0) {
+		fprintf(stderr, "Failed to open \"%s\": %s\n", ptmx,
+			strerror(errno));
+		return -1;
+	}
+
+	/*
+	 * grantpt() makes assumptions about /dev/pts/ so ignore it. It's also
+	 * not really needed.
+	 */
+	ret = unlockpt(master);
+	if (ret < 0) {
+		fprintf(stderr, "Failed to unlock terminal\n");
+		goto do_cleanup;
+	}
+
+#ifdef TIOCGPTPEER
+	slave = ioctl(master, TIOCGPTPEER, O_RDWR | O_NOCTTY | O_CLOEXEC);
+#endif
+	if (slave < 0) {
+		if (errno == EINVAL) {
+			fprintf(stderr, "TIOCGPTPEER is not supported. "
+					"Skipping test.\n");
+			fret = EXIT_SUCCESS;
+		}
+
+		fprintf(stderr, "Failed to perform TIOCGPTPEER ioctl\n");
+		goto do_cleanup;
+	}
+
+	pid_t pid = fork();
+	if (pid < 0)
+		goto do_cleanup;
+
+	if (pid == 0) {
+		char buf[4096];
+
+		ret = login_pty(slave);
+		if (ret < 0) {
+			fprintf(stderr, "Failed to setup terminal\n");
+			_exit(EXIT_FAILURE);
+		}
+
+		ret = resolve_procfd_symlink(STDIN_FILENO, buf, sizeof(buf));
+		if (ret < 0) {
+			fprintf(stderr, "Failed to retrieve pathname of pts "
+					"slave file descriptor\n");
+			_exit(EXIT_FAILURE);
+		}
+
+		if (strncmp(expected_procfd_contents, buf,
+			    strlen(expected_procfd_contents)) != 0) {
+			fprintf(stderr, "Received invalid contents for "
+					"\"/proc/<pid>/fd/%d\" symlink: %s\n",
+					STDIN_FILENO, buf);
+			_exit(-1);
+		}
+
+		fprintf(stderr, "Contents of \"/proc/<pid>/fd/%d\" "
+				"symlink are valid: %s\n", STDIN_FILENO, buf);
+
+		_exit(EXIT_SUCCESS);
+	}
+
+	ret = wait_for_pid(pid);
+	if (ret < 0)
+		goto do_cleanup;
+
+	fret = EXIT_SUCCESS;
+
+do_cleanup:
+	if (master >= 0)
+		close(master);
+	if (slave >= 0)
+		close(slave);
+
+	return fret;
+}
+
+static int verify_non_standard_devpts_mount(void)
+{
+	char *mntpoint;
+	int ret = -1;
+	char devpts[] = P_tmpdir "/devpts_fs_XXXXXX";
+	char ptmx[] = P_tmpdir "/devpts_fs_XXXXXX/ptmx";
+
+	ret = umount("/dev/pts");
+	if (ret < 0) {
+		fprintf(stderr, "Failed to unmount \"/dev/pts\": %s\n",
+				strerror(errno));
+		return -1;
+	}
+
+	(void)umount("/dev/ptmx");
+
+	mntpoint = mkdtemp(devpts);
+	if (!mntpoint) {
+		fprintf(stderr, "Failed to create temporary mountpoint: %s\n",
+				 strerror(errno));
+		return -1;
+	}
+
+	ret = mount("devpts", mntpoint, "devpts", MS_NOSUID | MS_NOEXEC,
+		    "newinstance,ptmxmode=0666,mode=0620,gid=5");
+	if (ret < 0) {
+		fprintf(stderr, "Failed to mount devpts fs to \"%s\" in new "
+				"mount namespace: %s\n", mntpoint,
+				strerror(errno));
+		unlink(mntpoint);
+		return -1;
+	}
+
+	ret = snprintf(ptmx, sizeof(ptmx), "%s/ptmx", devpts);
+	if (ret < 0 || (size_t)ret >= sizeof(ptmx)) {
+		unlink(mntpoint);
+		return -1;
+	}
+
+	ret = do_tiocgptpeer(ptmx, mntpoint);
+	unlink(mntpoint);
+	if (ret < 0)
+		return -1;
+
+	return 0;
+}
+
+static int verify_ptmx_bind_mount(void)
+{
+	int ret;
+
+	ret = mount("/dev/pts/ptmx", "/dev/ptmx", NULL, MS_BIND, NULL);
+	if (ret < 0) {
+		fprintf(stderr, "Failed to bind mount \"/dev/pts/ptmx\" to "
+				"\"/dev/ptmx\" mount namespace\n");
+		return -1;
+	}
+
+	ret = do_tiocgptpeer("/dev/ptmx", "/dev/pts/");
+	if (ret < 0)
+		return -1;
+
+	return 0;
+}
+
+static int verify_invalid_ptmx_bind_mount(void)
+{
+	int ret;
+	char mntpoint_fd;
+	char ptmx[] = "/devpts_ptmx_XXXXXX";
+
+	mntpoint_fd = mkstemp(ptmx);
+	if (mntpoint_fd < 0) {
+		fprintf(stderr, "Failed to create temporary directory: %s\n",
+				 strerror(errno));
+		return -1;
+	}
+
+	ret = mount("/dev/pts/ptmx", ptmx, NULL, MS_BIND, NULL);
+	close(mntpoint_fd);
+	if (ret < 0) {
+		fprintf(stderr, "Failed to bind mount \"/dev/pts/ptmx\" to "
+				"\"/dev/ptmx\" mount namespace\n");
+		return -1;
+	}
+
+	ret = do_tiocgptpeer(ptmx, "/dev/pts/");
+	if (ret == 0)
+		return -1;
+
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	int ret;
+
+	if (!isatty(STDIN_FILENO)) {
+		fprintf(stderr, "Standard input file desciptor is not attached "
+				"to a terminal. Skipping test\n");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = unshare(CLONE_NEWNS);
+	if (ret < 0) {
+		fprintf(stderr, "Failed to unshare mount namespace\n");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
+	if (ret < 0) {
+		fprintf(stderr, "Failed to make \"/\" MS_PRIVATE in new mount "
+				"namespace\n");
+		exit(EXIT_FAILURE);
+	}
+
+	ret = verify_ptmx_bind_mount();
+	if (ret < 0)
+		exit(EXIT_FAILURE);
+
+	ret = verify_invalid_ptmx_bind_mount();
+	if (ret < 0)
+		exit(EXIT_FAILURE);
+
+	ret = verify_non_standard_devpts_mount();
+	if (ret < 0)
+		exit(EXIT_FAILURE);
+
+	exit(EXIT_SUCCESS);
+}
-- 
2.15.1