Define mknod_ptmx()

/dev/ptmx is closely tied to the devpts filesystem. An open of /dev/ptmx, allocates the next pty index and the associated device shows up in the devpts fs as /dev/pts/n. Wih multiple instancs of devpts filesystem, during an open of /dev/ptmx we would be unable to determine which instance of the devpts is being accessed. So we move the 'ptmx' node into /dev/pts and use the inode of the 'ptmx' node to identify the superblock and hence the devpts instance. This patch adds ability for the kernel to internally create the [ptmx, c, 5:2] device when mounting devpts filesystem. Since the ptmx node in devpts is new and may surprise some userspace scripts, the default permissions for the new node is 0000. These permissions can be changed either using chmod or by remounting with the new '-o ptmxmode=0666' mount option. Changelog[v5]: - [Serge Hallyn bugfix]: Letting new_inode() assign inode number to ptmx can collide with hand-assigning inode numbers to ptys. So, hand-assign specific inode number to ptmx node also. - [Serge Hallyn]: Maybe safer to grab root dentry mutex while creating ptmx node - [Bugfix with Serge Hallyn] Replace lookup_one_len() in mknod_ptmx() wih d_alloc_name() (lookup during ->get_sb() locks up system). To simplify patchset, fold the ptmx_dentry patch into this. Changelog[v4]: - Change default permissions of pts/ptmx node to 0000. - Move code for ptmxmode under #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES. Changelog[v3]: - Rename ptmx_mode to ptmxmode (for consistency with 'newinstance') Changelog[v2]: - [H. Peter Anvin] Remove mknod() system call support and create the ptmx node internally. Changelog[v1]: - Earlier version of this patch enabled creating /dev/pts/tty as well. As pointed out by Al Viro and H. Peter Anvin, that is not really necessary. Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Define mknod_ptmx()
/dev/ptmx is closely tied to the devpts filesystem. An open of /dev/ptmx, allocates the next pty index and the associated device shows up in the devpts fs as /dev/pts/n. Wih multiple instancs of devpts filesystem, during an open of /dev/ptmx we would be unable to determine which instance of the devpts is being accessed. So we move the 'ptmx' node into /dev/pts and use the inode of the 'ptmx' node to identify the superblock and hence the devpts instance. This patch adds ability for the kernel to internally create the [ptmx, c, 5:2] device when mounting devpts filesystem. Since the ptmx node in devpts is new and may surprise some userspace scripts, the default permissions for the new node is 0000. These permissions can be changed either using chmod or by remounting with the new '-o ptmxmode=0666' mount option. Changelog[v5]: - [Serge Hallyn bugfix]: Letting new_inode() assign inode number to ptmx can collide with hand-assigning inode numbers to ptys. So, hand-assign specific inode number to ptmx node also. - [Serge Hallyn]: Maybe safer to grab root dentry mutex while creating ptmx node - [Bugfix with Serge Hallyn] Replace lookup_one_len() in mknod_ptmx() wih d_alloc_name() (lookup during ->get_sb() locks up system). To simplify patchset, fold the ptmx_dentry patch into this. Changelog[v4]: - Change default permissions of pts/ptmx node to 0000. - Move code for ptmxmode under #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES. Changelog[v3]: - Rename ptmx_mode to ptmxmode (for consistency with 'newinstance') Changelog[v2]: - [H. Peter Anvin] Remove mknod() system call support and create the ptmx node internally. Changelog[v1]: - Earlier version of this patch enabled creating /dev/pts/tty as well. As pointed out by Al Viro and H. Peter Anvin, that is not really necessary. Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com> Signed-off-by: Alan Cox <alan@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Sukadev Bhattiprolu · Linus Torvalds
1 parent e4adca27bc
Showing 1 changed file with 110 additions and 5 deletions Side-by-side Diff
fs/devpts/inode.c
@@ -27,6 +27,13 @@
 #define DEVPTS_SUPER_MAGIC 0x1cd1
  
 #define DEVPTS_DEFAULT_MODE 0600
+/*
+ * ptmx is a new node in /dev/pts and will be unused in legacy (single-
+ * instance) mode. To prevent surprises in user space, set permissions of
+ * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
+ * permissions.
+ */
+#define DEVPTS_DEFAULT_PTMX_MODE 0000
 #define PTMX_MINOR	2
  
 extern int pty_limit;			/* Config limit on Unix98 ptys */
  
@@ -40,10 +47,11 @@
 	uid_t   uid;
 	gid_t   gid;
 	umode_t mode;
+	umode_t ptmxmode;
 };
  
 enum {
-	Opt_uid, Opt_gid, Opt_mode,
+	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode,
 	Opt_err
 };
  
  
@@ -51,12 +59,16 @@
 	{Opt_uid, "uid=%u"},
 	{Opt_gid, "gid=%u"},
 	{Opt_mode, "mode=%o"},
+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
+	{Opt_ptmxmode, "ptmxmode=%o"},
+#endif
 	{Opt_err, NULL}
 };
  
 struct pts_fs_info {
 	struct ida allocated_ptys;
 	struct pts_mount_opts mount_opts;
+	struct dentry *ptmx_dentry;
 };
  
 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
@@ -81,6 +93,7 @@
 	opts->uid     = 0;
 	opts->gid     = 0;
 	opts->mode    = DEVPTS_DEFAULT_MODE;
+	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
  
 	while ((p = strsep(&data, ",")) != NULL) {
 		substring_t args[MAX_OPT_ARGS];
@@ -109,6 +122,13 @@
 				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;
+#endif
 		default:
 			printk(KERN_ERR "devpts: called with bogus options\n");
 			return -EINVAL;
  
  
@@ -118,12 +138,93 @@
 	return 0;
 }
  
+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
+static int mknod_ptmx(struct super_block *sb)
+{
+	int mode;
+	int rc = -ENOMEM;
+	struct dentry *dentry;
+	struct inode *inode;
+	struct dentry *root = sb->s_root;
+	struct pts_fs_info *fsi = DEVPTS_SB(sb);
+	struct pts_mount_opts *opts = &fsi->mount_opts;
+
+	mutex_lock(&root->d_inode->i_mutex);
+
+	/* If we have already created ptmx node, return */
+	if (fsi->ptmx_dentry) {
+		rc = 0;
+		goto out;
+	}
+
+	dentry = d_alloc_name(root, "ptmx");
+	if (!dentry) {
+		printk(KERN_NOTICE "Unable to alloc dentry for ptmx node\n");
+		goto out;
+	}
+
+	/*
+	 * Create a new 'ptmx' node in this mount of devpts.
+	 */
+	inode = new_inode(sb);
+	if (!inode) {
+		printk(KERN_ERR "Unable to alloc inode for ptmx node\n");
+		dput(dentry);
+		goto out;
+	}
+
+	inode->i_ino = 2;
+	inode->i_uid = inode->i_gid = 0;
+	inode->i_blocks = 0;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
+
+	mode = S_IFCHR|opts->ptmxmode;
+	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
+
+	d_add(dentry, inode);
+
+	fsi->ptmx_dentry = dentry;
+	rc = 0;
+
+	printk(KERN_DEBUG "Created ptmx node in devpts ino %lu\n",
+			inode->i_ino);
+out:
+	mutex_unlock(&root->d_inode->i_mutex);
+	return rc;
+}
+
+static void update_ptmx_mode(struct pts_fs_info *fsi)
+{
+	struct inode *inode;
+	if (fsi->ptmx_dentry) {
+		inode = fsi->ptmx_dentry->d_inode;
+		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)
 {
+	int err;
 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
 	struct pts_mount_opts *opts = &fsi->mount_opts;
  
-	return parse_mount_options(data, opts);
+	err = parse_mount_options(data, opts);
+
+	/*
+	 * parse_mount_options() restores options to default values
+	 * before parsing and may have changed ptmxmode. So, update the
+	 * mode in the inode too. Bogus options don't fail the remount,
+	 * so do this even on error return.
+	 */
+	update_ptmx_mode(fsi);
+
+	return err;
 }
  
 static int devpts_show_options(struct seq_file *seq, struct vfsmount *vfs)
@@ -136,6 +237,9 @@
 	if (opts->setgid)
 		seq_printf(seq, ",gid=%u", opts->gid);
 	seq_printf(seq, ",mode=%03o", opts->mode);
+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
+	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
+#endif
  
 	return 0;
 }
@@ -156,6 +260,7 @@
  
 	ida_init(&fsi->allocated_ptys);
 	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
+	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
  
 	return fsi;
 }
@@ -163,7 +268,7 @@
 static int
 devpts_fill_super(struct super_block *s, void *data, int silent)
 {
-	struct inode * inode;
+	struct inode *inode;
  
 	s->s_blocksize = 1024;
 	s->s_blocksize_bits = 10;
@@ -190,7 +295,7 @@
 	s->s_root = d_alloc_root(inode);
 	if (s->s_root)
 		return 0;
-	
+
 	printk("devpts: get root dentry failed\n");
 	iput(inode);
  
@@ -211,7 +316,7 @@
 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
  
 	kfree(fsi);
-	kill_anon_super(sb);
+	kill_litter_super(sb);
 }
  
 static struct file_system_type devpts_fs_type = {
...	...	@@ -27,6 +27,13 @@
27	27	#define DEVPTS_SUPER_MAGIC 0x1cd1
28	28
29	29	#define DEVPTS_DEFAULT_MODE 0600
	30	+/*
	31	+ * ptmx is a new node in /dev/pts and will be unused in legacy (single-
	32	+ * instance) mode. To prevent surprises in user space, set permissions of
	33	+ * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
	34	+ * permissions.
	35	+ */
	36	+#define DEVPTS_DEFAULT_PTMX_MODE 0000
30	37	#define PTMX_MINOR 2
31	38
32	39	extern int pty_limit; /* Config limit on Unix98 ptys */
33	40
...	...	@@ -40,10 +47,11 @@
40	47	uid_t uid;
41	48	gid_t gid;
42	49	umode_t mode;
	50	+ umode_t ptmxmode;
43	51	};
44	52
45	53	enum {
46		- Opt_uid, Opt_gid, Opt_mode,
	54	+ Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode,
47	55	Opt_err
48	56	};
49	57
50	58
...	...	@@ -51,12 +59,16 @@
51	59	{Opt_uid, "uid=%u"},
52	60	{Opt_gid, "gid=%u"},
53	61	{Opt_mode, "mode=%o"},
	62	+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
	63	+ {Opt_ptmxmode, "ptmxmode=%o"},
	64	+#endif
54	65	{Opt_err, NULL}
55	66	};
56	67
57	68	struct pts_fs_info {
58	69	struct ida allocated_ptys;
59	70	struct pts_mount_opts mount_opts;
	71	+ struct dentry *ptmx_dentry;
60	72	};
61	73
62	74	static inline struct pts_fs_info DEVPTS_SB(struct super_block sb)
...	...	@@ -81,6 +93,7 @@
81	93	opts->uid = 0;
82	94	opts->gid = 0;
83	95	opts->mode = DEVPTS_DEFAULT_MODE;
	96	+ opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
84	97
85	98	while ((p = strsep(&data, ",")) != NULL) {
86	99	substring_t args[MAX_OPT_ARGS];
...	...	@@ -109,6 +122,13 @@
109	122	return -EINVAL;
110	123	opts->mode = option & S_IALLUGO;
111	124	break;
	125	+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
	126	+ case Opt_ptmxmode:
	127	+ if (match_octal(&args[0], &option))
	128	+ return -EINVAL;
	129	+ opts->ptmxmode = option & S_IALLUGO;
	130	+ break;
	131	+#endif
112	132	default:
113	133	printk(KERN_ERR "devpts: called with bogus options\n");
114	134	return -EINVAL;
115	135
116	136
...	...	@@ -118,12 +138,93 @@
118	138	return 0;
119	139	}
120	140
	141	+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
	142	+static int mknod_ptmx(struct super_block *sb)
	143	+{
	144	+ int mode;
	145	+ int rc = -ENOMEM;
	146	+ struct dentry *dentry;
	147	+ struct inode *inode;
	148	+ struct dentry *root = sb->s_root;
	149	+ struct pts_fs_info *fsi = DEVPTS_SB(sb);
	150	+ struct pts_mount_opts *opts = &fsi->mount_opts;
	151	+
	152	+ mutex_lock(&root->d_inode->i_mutex);
	153	+
	154	+ /* If we have already created ptmx node, return */
	155	+ if (fsi->ptmx_dentry) {
	156	+ rc = 0;
	157	+ goto out;
	158	+ }
	159	+
	160	+ dentry = d_alloc_name(root, "ptmx");
	161	+ if (!dentry) {
	162	+ printk(KERN_NOTICE "Unable to alloc dentry for ptmx node\n");
	163	+ goto out;
	164	+ }
	165	+
	166	+ /*
	167	+ * Create a new 'ptmx' node in this mount of devpts.
	168	+ */
	169	+ inode = new_inode(sb);
	170	+ if (!inode) {
	171	+ printk(KERN_ERR "Unable to alloc inode for ptmx node\n");
	172	+ dput(dentry);
	173	+ goto out;
	174	+ }
	175	+
	176	+ inode->i_ino = 2;
	177	+ inode->i_uid = inode->i_gid = 0;
	178	+ inode->i_blocks = 0;
	179	+ inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
	180	+
	181	+ mode = S_IFCHR\|opts->ptmxmode;
	182	+ init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
	183	+
	184	+ d_add(dentry, inode);
	185	+
	186	+ fsi->ptmx_dentry = dentry;
	187	+ rc = 0;
	188	+
	189	+ printk(KERN_DEBUG "Created ptmx node in devpts ino %lu\n",
	190	+ inode->i_ino);
	191	+out:
	192	+ mutex_unlock(&root->d_inode->i_mutex);
	193	+ return rc;
	194	+}
	195	+
	196	+static void update_ptmx_mode(struct pts_fs_info *fsi)
	197	+{
	198	+ struct inode *inode;
	199	+ if (fsi->ptmx_dentry) {
	200	+ inode = fsi->ptmx_dentry->d_inode;
	201	+ inode->i_mode = S_IFCHR\|fsi->mount_opts.ptmxmode;
	202	+ }
	203	+}
	204	+#else
	205	+static inline void update_ptmx_mode(struct pts_fs_info *fsi)
	206	+{
	207	+ return;
	208	+}
	209	+#endif
	210	+
121	211	static int devpts_remount(struct super_block sb, int flags, char *data)
122	212	{
	213	+ int err;
123	214	struct pts_fs_info *fsi = DEVPTS_SB(sb);
124	215	struct pts_mount_opts *opts = &fsi->mount_opts;
125	216
126		- return parse_mount_options(data, opts);
	217	+ err = parse_mount_options(data, opts);
	218	+
	219	+ /*
	220	+ * parse_mount_options() restores options to default values
	221	+ * before parsing and may have changed ptmxmode. So, update the
	222	+ * mode in the inode too. Bogus options don't fail the remount,
	223	+ * so do this even on error return.
	224	+ */
	225	+ update_ptmx_mode(fsi);
	226	+
	227	+ return err;
127	228	}
128	229
129	230	static int devpts_show_options(struct seq_file seq, struct vfsmount vfs)
...	...	@@ -136,6 +237,9 @@
136	237	if (opts->setgid)
137	238	seq_printf(seq, ",gid=%u", opts->gid);
138	239	seq_printf(seq, ",mode=%03o", opts->mode);
	240	+#ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES
	241	+ seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
	242	+#endif
139	243
140	244	return 0;
141	245	}
...	...	@@ -156,6 +260,7 @@
156	260
157	261	ida_init(&fsi->allocated_ptys);
158	262	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
	263	+ fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
159	264
160	265	return fsi;
161	266	}
...	...	@@ -163,7 +268,7 @@
163	268	static int
164	269	devpts_fill_super(struct super_block s, void data, int silent)
165	270	{
166		- struct inode * inode;
	271	+ struct inode *inode;
167	272
168	273	s->s_blocksize = 1024;
169	274	s->s_blocksize_bits = 10;
...	...	@@ -190,7 +295,7 @@
190	295	s->s_root = d_alloc_root(inode);
191	296	if (s->s_root)
192	297	return 0;
193		-
	298	+
194	299	printk("devpts: get root dentry failed\n");
195	300	iput(inode);
196	301
...	...	@@ -211,7 +316,7 @@
211	316	struct pts_fs_info *fsi = DEVPTS_SB(sb);
212	317
213	318	kfree(fsi);
214		- kill_anon_super(sb);
	319	+ kill_litter_super(sb);
215	320	}
216	321
217	322	static struct file_system_type devpts_fs_type = {