The /dev/ptmx device node is changed to lookup the directory entry "pts"
in the same directory as the /dev/ptmx device node was opened in. If
there is a "pts" entry and that entry is a devpts filesystem /dev/ptmx
uses that filesystem. Otherwise the open of /dev/ptmx fails.
The DEVPTS_MULTIPLE_INSTANCES configuration option is removed, so that
userspace can now safely depend on each mount of devpts creating a new
instance of the filesystem.
Each mount of devpts is now a separate and equal filesystem.
Reserved ttys are now available to all instances of devpts where the
mounter is in the initial mount namespace.
A new vfs helper path_pts is introduced that finds a directory entry
named "pts" in the directory of the passed in path, and changes the
passed in path to point to it. The helper path_pts uses a function
path_parent_directory that was factored out of follow_dotdot.
In the implementation of devpts:
- devpts_mnt is killed as it is no longer meaningful if all mounts of
devpts are equal.
- pts_sb_from_inode is replaced by just inode->i_sb as all cached
inodes in the tty layer are now from the devpts filesystem.
- devpts_add_ref is rolled into the new function devpts_ptmx. And the
unnecessary inode hold is removed.
- devpts_del_ref is renamed devpts_release and reduced to just a
deacrivate_super.
- The newinstance mount option continues to be accepted but is now
ignored.
In devpts_fs.h definitions for when !CONFIG_UNIX98_PTYS are removed as
they are never used.
Documentation/filesystems/devices.txt is updated to describe the current
situation.
This has been verified to work properly on openwrt-15.05, centos5,
centos6, centos7, debian-6.0.2, debian-7.9, debian-8.2, ubuntu-14.04.3,
ubuntu-15.10, fedora23, magia-5, mint-17.3, opensuse-42.1,
slackware-14.1, gentoo-
20151225 (13.0?), archlinux-2015-12-01. With the
caveat that on centos6 and on slackware-14.1 that there wind up being
two instances of the devpts filesystem mounted on /dev/pts, the lower
copy does not end up getting used.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg KH <greg@kroah.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Jann Horn <jann@thejh.net>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
+Each mount of the devpts filesystem is now distinct such that ptys
+and their indicies allocated in one mount are independent from ptys
+and their indicies in all other mounts.
-To support containers, we now allow multiple instances of devpts filesystem,
-such that indices of ptys allocated in one instance are independent of indices
-allocated in other instances of devpts.
+All mounts of the devpts filesystem now create a /dev/pts/ptmx node
+with permissions 0000.
-To preserve backward compatibility, this support for multiple instances is
-enabled only if:
+To retain backwards compatibility the a ptmx device node (aka any node
+created with "mknod name c 5 2") when opened will look for an instance
+of devpts under the name "pts" in the same directory as the ptmx device
+node.
- - CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, and
- - '-o newinstance' mount option is specified while mounting devpts
-
-IOW, devpts now supports both single-instance and multi-instance semantics.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there is no change in behavior and
-this referred to as the "legacy" mode. In this mode, the new mount options
-(-o newinstance and -o ptmxmode) will be ignored with a 'bogus option' message
-on console.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and devpts is mounted without the
-'newinstance' option (as in current start-up scripts) the new mount binds
-to the initial kernel mount of devpts. This mode is referred to as the
-'single-instance' mode and the current, single-instance semantics are
-preserved, i.e PTYs are common across the system.
-
-The only difference between this single-instance mode and the legacy mode
-is the presence of new, '/dev/pts/ptmx' node with permissions 0000, which
-can safely be ignored.
-
-If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and 'newinstance' option is specified,
-the mount is considered to be in the multi-instance mode and a new instance
-of the devpts fs is created. Any ptys created in this instance are independent
-of ptys in other instances of devpts. Like in the single-instance mode, the
-/dev/pts/ptmx node is present. To effectively use the multi-instance mode,
-open of /dev/ptmx must be a redirected to '/dev/pts/ptmx' using a symlink or
-bind-mount.
-
-Eg: A container startup script could do the following:
-
- $ chmod 0666 /dev/pts/ptmx
- $ rm /dev/ptmx
- $ ln -s pts/ptmx /dev/ptmx
- $ ns_exec -cm /bin/bash
-
- # We are now in new container
-
- $ umount /dev/pts
- $ mount -t devpts -o newinstance lxcpts /dev/pts
- $ sshd -p 1234
-
-where 'ns_exec -cm /bin/bash' calls clone() with CLONE_NEWNS flag and execs
-/bin/bash in the child process. A pty created by the sshd is not visible in
-the original mount of /dev/pts.
+As an option instead of placing a /dev/ptmx device node at /dev/ptmx
+it is possible to place a symlink to /dev/pts/ptmx at /dev/ptmx or
+to bind mount /dev/ptx/ptmx to /dev/ptmx. If you opt for using
+the devpts filesystem in this manner devpts should be mounted with
+the ptmxmode=0666, or chmod 0666 /dev/pts/ptmx should be called.
Total count of pty pairs in all instances is limited by sysctls:
kernel.pty.max = 4096 - global limit
-kernel.pty.reserve = 1024 - reserve for initial instance
+kernel.pty.reserve = 1024 - reserved for filesystems mounted from the initial mount namespace
kernel.pty.nr - current count of ptys
Per-instance limit could be set by adding mount option "max=<count>".
This feature was added in kernel 3.4 together with sysctl kernel.pty.reserve.
In kernels older than 3.4 sysctl kernel.pty.max works as per-instance limit.
-
-User-space changes
-------------------
-
-In multi-instance mode (i.e '-o newinstance' mount option is specified at least
-once), following user-space issues should be noted.
-
-1. If -o newinstance mount option is never used, /dev/pts/ptmx can be ignored
- and no change is needed to system-startup scripts.
-
-2. To effectively use multi-instance mode (i.e -o newinstance is specified)
- administrators or startup scripts should "redirect" open of /dev/ptmx to
- /dev/pts/ptmx using either a bind mount or symlink.
-
- $ mount -t devpts -o newinstance devpts /dev/pts
-
- followed by either
-
- $ rm /dev/ptmx
- $ ln -s pts/ptmx /dev/ptmx
- $ chmod 666 /dev/pts/ptmx
- or
- $ mount -o bind /dev/pts/ptmx /dev/ptmx
-
-3. The '/dev/ptmx -> pts/ptmx' symlink is the preferred method since it
- enables better error-reporting and treats both single-instance and
- multi-instance mounts similarly.
-
- But this method requires that system-startup scripts set the mode of
- /dev/pts/ptmx correctly (default mode is 0000). The scripts can set the
- mode by, either
-
- - adding ptmxmode mount option to devpts entry in /etc/fstab, or
- - using 'chmod 0666 /dev/pts/ptmx'
-
-4. If multi-instance mode mount is needed for containers, but the system
- startup scripts have not yet been updated, container-startup scripts
- should bind mount /dev/ptmx to /dev/pts/ptmx to avoid breaking single-
- instance mounts.
-
- Or, in general, container-startup scripts should use:
-
- mount -t devpts -o newinstance -o ptmxmode=0666 devpts /dev/pts
- if [ ! -L /dev/ptmx ]; then
- mount -o bind /dev/pts/ptmx /dev/ptmx
- fi
-
- When all devpts mounts are multi-instance, /dev/ptmx can permanently be
- a symlink to pts/ptmx and the bind mount can be ignored.
-
-5. A multi-instance mount that is not accompanied by the /dev/ptmx to
- /dev/pts/ptmx redirection would result in an unusable/unreachable pty.
-
- mount -t devpts -o newinstance lxcpts /dev/pts
-
- immediately followed by:
-
- open("/dev/ptmx")
-
- would create a pty, say /dev/pts/7, in the initial kernel mount.
- But /dev/pts/7 would be invisible in the new mount.
-
-6. The permissions for /dev/pts/ptmx node should be specified when mounting
- /dev/pts, using the '-o ptmxmode=%o' mount option (default is 0000).
-
- mount -t devpts -o newinstance -o ptmxmode=0644 devpts /dev/pts
-
- The permissions can be later be changed as usual with 'chmod'.
-
- chmod 666 /dev/pts/ptmx
-
-7. A mount of devpts without the 'newinstance' option results in binding to
- initial kernel mount. This behavior while preserving legacy semantics,
- does not provide strict isolation in a container environment. i.e by
- mounting devpts without the 'newinstance' option, a container could
- get visibility into the 'host' or root container's devpts.
-
- To workaround this and have strict isolation, all mounts of devpts,
- including the mount in the root container, should use the newinstance
- option.
All modern Linux systems use the Unix98 ptys. Say Y unless
you're on an embedded system and want to conserve memory.
-config DEVPTS_MULTIPLE_INSTANCES
- bool "Support multiple instances of devpts"
- depends on UNIX98_PTYS
- default n
- ---help---
- Enable support for multiple instances of devpts filesystem.
- If you want to have isolated PTY namespaces (eg: in containers),
- say Y here. Otherwise, say N. If enabled, each mount of devpts
- filesystem with the '-o newinstance' option will create an
- independent PTY namespace.
-
config LEGACY_PTYS
bool "Legacy (BSD) PTY support"
default y
else
fsi = tty->link->driver_data;
devpts_kill_index(fsi, tty->index);
- devpts_put_ref(fsi);
+ devpts_release(fsi);
}
static const struct tty_operations ptm_unix98_ops = {
if (retval)
return retval;
- fsi = devpts_get_ref(inode, filp);
- retval = -ENODEV;
- if (!fsi)
+ fsi = devpts_acquire(filp);
+ if (IS_ERR(fsi)) {
+ retval = PTR_ERR(fsi);
goto out_free_file;
+ }
/* find a device that is not in use. */
mutex_lock(&devpts_mutex);
retval = index;
if (index < 0)
- goto out_put_ref;
+ goto out_put_fsi;
mutex_lock(&tty_mutex);
return retval;
out:
devpts_kill_index(fsi, index);
-out_put_ref:
- devpts_put_ref(fsi);
+out_put_fsi:
+ devpts_release(fsi);
out_free_file:
tty_free_file(filp);
return retval;
static DEFINE_MUTEX(allocated_ptys_lock);
-static struct vfsmount *devpts_mnt;
-
struct pts_mount_opts {
int setuid;
int setgid;
kgid_t gid;
umode_t mode;
umode_t ptmxmode;
- int newinstance;
+ int reserve;
int max;
};
{Opt_uid, "uid=%u"},
{Opt_gid, "gid=%u"},
{Opt_mode, "mode=%o"},
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
{Opt_ptmxmode, "ptmxmode=%o"},
{Opt_newinstance, "newinstance"},
{Opt_max, "max=%d"},
-#endif
{Opt_err, NULL}
};
return sb->s_fs_info;
}
-static inline struct super_block *pts_sb_from_inode(struct inode *inode)
+struct pts_fs_info *devpts_acquire(struct file *filp)
{
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
- if (inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
- return inode->i_sb;
-#endif
- if (!devpts_mnt)
- return NULL;
- return devpts_mnt->mnt_sb;
+ struct pts_fs_info *result;
+ struct path path;
+ struct super_block *sb;
+ int err;
+
+ path = filp->f_path;
+ path_get(&path);
+
+ /* Has the devpts filesystem already been found? */
+ sb = path.mnt->mnt_sb;
+ if (sb->s_magic != DEVPTS_SUPER_MAGIC) {
+ /* Is a devpts filesystem at "pts" in the same directory? */
+ err = path_pts(&path);
+ if (err) {
+ result = ERR_PTR(err);
+ goto out;
+ }
+
+ /* Is the path the root of a devpts filesystem? */
+ result = ERR_PTR(-ENODEV);
+ sb = path.mnt->mnt_sb;
+ if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
+ (path.mnt->mnt_root != sb->s_root))
+ goto out;
+ }
+
+ /*
+ * pty code needs to hold extra references in case of last /dev/tty close
+ */
+ atomic_inc(&sb->s_active);
+ result = DEVPTS_SB(sb);
+
+out:
+ path_put(&path);
+ return result;
+}
+
+void devpts_release(struct pts_fs_info *fsi)
+{
+ deactivate_super(fsi->sb);
}
#define PARSE_MOUNT 0
/*
* parse_mount_options():
* Set @opts to mount options specified in @data. If an option is not
- * specified in @data, set it to its default value. The exception is
- * 'newinstance' option which can only be set/cleared on a mount (i.e.
- * cannot be changed during remount).
+ * specified in @data, set it to its default value.
*
* Note: @data may be NULL (in which case all options are set to default).
*/
opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
opts->max = NR_UNIX98_PTY_MAX;
- /* newinstance makes sense only on initial mount */
+ /* Only allow instances mounted from the initial mount
+ * namespace to tap the reserve pool of ptys.
+ */
if (op == PARSE_MOUNT)
- opts->newinstance = 0;
+ opts->reserve =
+ (current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns);
while ((p = strsep(&data, ",")) != NULL) {
substring_t args[MAX_OPT_ARGS];
return -EINVAL;
opts->mode = option & S_IALLUGO;
break;
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
case Opt_ptmxmode:
if (match_octal(&args[0], &option))
return -EINVAL;
opts->ptmxmode = option & S_IALLUGO;
break;
case Opt_newinstance:
- /* newinstance makes sense only on initial mount */
- if (op == PARSE_MOUNT)
- opts->newinstance = 1;
break;
case Opt_max:
if (match_int(&args[0], &option) ||
return -EINVAL;
opts->max = option;
break;
-#endif
default:
pr_err("called with bogus options\n");
return -EINVAL;
return 0;
}
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
static int mknod_ptmx(struct super_block *sb)
{
int mode;
inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
}
}
-#else
-static inline void update_ptmx_mode(struct pts_fs_info *fsi)
-{
- return;
-}
-#endif
static int devpts_remount(struct super_block *sb, int *flags, char *data)
{
seq_printf(seq, ",gid=%u",
from_kgid_munged(&init_user_ns, opts->gid));
seq_printf(seq, ",mode=%03o", opts->mode);
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
if (opts->max < NR_UNIX98_PTY_MAX)
seq_printf(seq, ",max=%d", opts->max);
-#endif
return 0;
}
return -ENOMEM;
}
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
-static int compare_init_pts_sb(struct super_block *s, void *p)
-{
- if (devpts_mnt)
- return devpts_mnt->mnt_sb == s;
- return 0;
-}
-
/*
* devpts_mount()
*
- * If the '-o newinstance' mount option was specified, mount a new
- * (private) instance of devpts. PTYs created in this instance are
- * independent of the PTYs in other devpts instances.
- *
- * If the '-o newinstance' option was not specified, mount/remount the
- * initial kernel mount of devpts. This type of mount gives the
- * legacy, single-instance semantics.
- *
- * The 'newinstance' option is needed to support multiple namespace
- * semantics in devpts while preserving backward compatibility of the
- * current 'single-namespace' semantics. i.e all mounts of devpts
- * without the 'newinstance' mount option should bind to the initial
- * kernel mount, like mount_single().
- *
- * Mounts with 'newinstance' option create a new, private namespace.
- *
- * NOTE:
- *
- * For single-mount semantics, devpts cannot use mount_single(),
- * because mount_single()/sget() find and use the super-block from
- * the most recent mount of devpts. But that recent mount may be a
- * 'newinstance' mount and mount_single() would pick the newinstance
- * super-block instead of the initial super-block.
+ * Mount a new (private) instance of devpts. PTYs created in this
+ * instance are independent of the PTYs in other devpts instances.
*/
static struct dentry *devpts_mount(struct file_system_type *fs_type,
int flags, const char *dev_name, void *data)
if (error)
return ERR_PTR(error);
- /* Require newinstance for all user namespace mounts to ensure
- * the mount options are not changed.
- */
- if ((current_user_ns() != &init_user_ns) && !opts.newinstance)
- return ERR_PTR(-EINVAL);
-
- if (opts.newinstance)
- s = sget(fs_type, NULL, set_anon_super, flags, NULL);
- else
- s = sget(fs_type, compare_init_pts_sb, set_anon_super, flags,
- NULL);
-
+ s = sget(fs_type, NULL, set_anon_super, flags, NULL);
if (IS_ERR(s))
return ERR_CAST(s);
return ERR_PTR(error);
}
-#else
-/*
- * This supports only the legacy single-instance semantics (no
- * multiple-instance semantics)
- */
-static struct dentry *devpts_mount(struct file_system_type *fs_type, int flags,
- const char *dev_name, void *data)
-{
- return mount_single(fs_type, flags, data, devpts_fill_super);
-}
-#endif
-
static void devpts_kill_sb(struct super_block *sb)
{
struct pts_fs_info *fsi = DEVPTS_SB(sb);
.name = "devpts",
.mount = devpts_mount,
.kill_sb = devpts_kill_sb,
-#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
.fs_flags = FS_USERNS_MOUNT | FS_USERNS_DEV_MOUNT,
-#endif
};
/*
int index;
int ida_ret;
- if (!fsi)
- return -ENODEV;
-
retry:
if (!ida_pre_get(&fsi->allocated_ptys, GFP_KERNEL))
return -ENOMEM;
mutex_lock(&allocated_ptys_lock);
- if (pty_count >= pty_limit -
- (fsi->mount_opts.newinstance ? pty_reserve : 0)) {
+ if (pty_count >= (pty_limit -
+ (fsi->mount_opts.reserve ? 0 : pty_reserve))) {
mutex_unlock(&allocated_ptys_lock);
return -ENOSPC;
}
mutex_unlock(&allocated_ptys_lock);
}
-/*
- * pty code needs to hold extra references in case of last /dev/tty close
- */
-struct pts_fs_info *devpts_get_ref(struct inode *ptmx_inode, struct file *file)
-{
- struct super_block *sb;
- struct pts_fs_info *fsi;
-
- sb = pts_sb_from_inode(ptmx_inode);
- if (!sb)
- return NULL;
- fsi = DEVPTS_SB(sb);
- if (!fsi)
- return NULL;
-
- atomic_inc(&sb->s_active);
- return fsi;
-}
-
-void devpts_put_ref(struct pts_fs_info *fsi)
-{
- deactivate_super(fsi->sb);
-}
-
/**
* devpts_pty_new -- create a new inode in /dev/pts/
* @ptmx_inode: inode of the master
struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
{
struct dentry *dentry;
- struct super_block *sb;
+ struct super_block *sb = fsi->sb;
struct inode *inode;
struct dentry *root;
struct pts_mount_opts *opts;
char s[12];
- if (!fsi)
- return ERR_PTR(-ENODEV);
-
- sb = fsi->sb;
root = sb->s_root;
opts = &fsi->mount_opts;
static int __init init_devpts_fs(void)
{
int err = register_filesystem(&devpts_fs_type);
- struct ctl_table_header *table;
-
if (!err) {
- struct vfsmount *mnt;
-
- table = register_sysctl_table(pty_root_table);
- mnt = kern_mount(&devpts_fs_type);
- if (IS_ERR(mnt)) {
- err = PTR_ERR(mnt);
- unregister_filesystem(&devpts_fs_type);
- unregister_sysctl_table(table);
- } else {
- devpts_mnt = mnt;
- }
+ register_sysctl_table(pty_root_table);
}
return err;
}
}
}
+static int path_parent_directory(struct path *path)
+{
+ struct dentry *old = path->dentry;
+ /* rare case of legitimate dget_parent()... */
+ path->dentry = dget_parent(path->dentry);
+ dput(old);
+ if (unlikely(!path_connected(path)))
+ return -ENOENT;
+ return 0;
+}
+
static int follow_dotdot(struct nameidata *nd)
{
while(1) {
- struct dentry *old = nd->path.dentry;
-
if (nd->path.dentry == nd->root.dentry &&
nd->path.mnt == nd->root.mnt) {
break;
}
if (nd->path.dentry != nd->path.mnt->mnt_root) {
- /* rare case of legitimate dget_parent()... */
- nd->path.dentry = dget_parent(nd->path.dentry);
- dput(old);
- if (unlikely(!path_connected(&nd->path)))
- return -ENOENT;
+ int ret = path_parent_directory(&nd->path);
+ if (ret)
+ return ret;
break;
}
if (!follow_up(&nd->path))
}
EXPORT_SYMBOL(lookup_one_len_unlocked);
+#ifdef CONFIG_UNIX98_PTYS
+int path_pts(struct path *path)
+{
+ /* Find something mounted on "pts" in the same directory as
+ * the input path.
+ */
+ struct dentry *child, *parent;
+ struct qstr this;
+ int ret;
+
+ ret = path_parent_directory(path);
+ if (ret)
+ return ret;
+
+ parent = path->dentry;
+ this.name = "pts";
+ this.len = 3;
+ child = d_hash_and_lookup(parent, &this);
+ if (!child)
+ return -ENOENT;
+
+ path->dentry = child;
+ dput(parent);
+ follow_mount(path);
+ return 0;
+}
+#endif
+
int user_path_at_empty(int dfd, const char __user *name, unsigned flags,
struct path *path, int *empty)
{
#include <linux/errno.h>
-struct pts_fs_info;
-
#ifdef CONFIG_UNIX98_PTYS
-/* Look up a pts fs info and get a ref to it */
-struct pts_fs_info *devpts_get_ref(struct inode *, struct file *);
-void devpts_put_ref(struct pts_fs_info *);
+struct pts_fs_info;
+
+struct pts_fs_info *devpts_acquire(struct file *);
+void devpts_release(struct pts_fs_info *);
int devpts_new_index(struct pts_fs_info *);
void devpts_kill_index(struct pts_fs_info *, int);
#define LOOKUP_ROOT 0x2000
#define LOOKUP_EMPTY 0x4000
+extern int path_pts(struct path *path);
+
extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
static inline int user_path_at(int dfd, const char __user *name, unsigned flags,
projects / cascardo / linux.git / commitdiff