xref: /openbmc/linux/fs/devpts/inode.c (revision dd5b2498) 
1 /* -*- linux-c -*- --------------------------------------------------------- *
2  *
3  * linux/fs/devpts/inode.c
4  *
5  *  Copyright 1998-2004 H. Peter Anvin -- All Rights Reserved
6  *
7  * This file is part of the Linux kernel and is made available under
8  * the terms of the GNU General Public License, version 2, or at your
9  * option, any later version, incorporated herein by reference.
10  *
11  * ------------------------------------------------------------------------- */
12 
13 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
14 
15 #include <linux/module.h>
16 #include <linux/init.h>
17 #include <linux/fs.h>
18 #include <linux/sched.h>
19 #include <linux/namei.h>
20 #include <linux/slab.h>
21 #include <linux/mount.h>
22 #include <linux/tty.h>
23 #include <linux/mutex.h>
24 #include <linux/magic.h>
25 #include <linux/idr.h>
26 #include <linux/devpts_fs.h>
27 #include <linux/parser.h>
28 #include <linux/fsnotify.h>
29 #include <linux/seq_file.h>
30 
31 #define DEVPTS_DEFAULT_MODE 0600
32 /*
33  * ptmx is a new node in /dev/pts and will be unused in legacy (single-
34  * instance) mode. To prevent surprises in user space, set permissions of
35  * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
36  * permissions.
37  */
38 #define DEVPTS_DEFAULT_PTMX_MODE 0000
39 #define PTMX_MINOR	2
40 
41 /*
42  * sysctl support for setting limits on the number of Unix98 ptys allocated.
43  * Otherwise one can eat up all kernel memory by opening /dev/ptmx repeatedly.
44  */
45 static int pty_limit = NR_UNIX98_PTY_DEFAULT;
46 static int pty_reserve = NR_UNIX98_PTY_RESERVE;
47 static int pty_limit_min;
48 static int pty_limit_max = INT_MAX;
49 static atomic_t pty_count = ATOMIC_INIT(0);
50 
51 static struct ctl_table pty_table[] = {
52 	{
53 		.procname	= "max",
54 		.maxlen		= sizeof(int),
55 		.mode		= 0644,
56 		.data		= &pty_limit,
57 		.proc_handler	= proc_dointvec_minmax,
58 		.extra1		= &pty_limit_min,
59 		.extra2		= &pty_limit_max,
60 	}, {
61 		.procname	= "reserve",
62 		.maxlen		= sizeof(int),
63 		.mode		= 0644,
64 		.data		= &pty_reserve,
65 		.proc_handler	= proc_dointvec_minmax,
66 		.extra1		= &pty_limit_min,
67 		.extra2		= &pty_limit_max,
68 	}, {
69 		.procname	= "nr",
70 		.maxlen		= sizeof(int),
71 		.mode		= 0444,
72 		.data		= &pty_count,
73 		.proc_handler	= proc_dointvec,
74 	},
75 	{}
76 };
77 
78 static struct ctl_table pty_kern_table[] = {
79 	{
80 		.procname	= "pty",
81 		.mode		= 0555,
82 		.child		= pty_table,
83 	},
84 	{}
85 };
86 
87 static struct ctl_table pty_root_table[] = {
88 	{
89 		.procname	= "kernel",
90 		.mode		= 0555,
91 		.child		= pty_kern_table,
92 	},
93 	{}
94 };
95 
96 struct pts_mount_opts {
97 	int setuid;
98 	int setgid;
99 	kuid_t   uid;
100 	kgid_t   gid;
101 	umode_t mode;
102 	umode_t ptmxmode;
103 	int reserve;
104 	int max;
105 };
106 
107 enum {
108 	Opt_uid, Opt_gid, Opt_mode, Opt_ptmxmode, Opt_newinstance,  Opt_max,
109 	Opt_err
110 };
111 
112 static const match_table_t tokens = {
113 	{Opt_uid, "uid=%u"},
114 	{Opt_gid, "gid=%u"},
115 	{Opt_mode, "mode=%o"},
116 	{Opt_ptmxmode, "ptmxmode=%o"},
117 	{Opt_newinstance, "newinstance"},
118 	{Opt_max, "max=%d"},
119 	{Opt_err, NULL}
120 };
121 
122 struct pts_fs_info {
123 	struct ida allocated_ptys;
124 	struct pts_mount_opts mount_opts;
125 	struct super_block *sb;
126 	struct dentry *ptmx_dentry;
127 };
128 
129 static inline struct pts_fs_info *DEVPTS_SB(struct super_block *sb)
130 {
131 	return sb->s_fs_info;
132 }
133 
134 static int devpts_ptmx_path(struct path *path)
135 {
136 	struct super_block *sb;
137 	int err;
138 
139 	/* Is a devpts filesystem at "pts" in the same directory? */
140 	err = path_pts(path);
141 	if (err)
142 		return err;
143 
144 	/* Is the path the root of a devpts filesystem? */
145 	sb = path->mnt->mnt_sb;
146 	if ((sb->s_magic != DEVPTS_SUPER_MAGIC) ||
147 	    (path->mnt->mnt_root != sb->s_root))
148 		return -ENODEV;
149 
150 	return 0;
151 }
152 
153 /*
154  * Try to find a suitable devpts filesystem. We support the following
155  * scenarios:
156  * - The ptmx device node is located in the same directory as the devpts
157  *   mount where the pts device nodes are located.
158  *   This is e.g. the case when calling open on the /dev/pts/ptmx device
159  *   node when the devpts filesystem is mounted at /dev/pts.
160  * - The ptmx device node is located outside the devpts filesystem mount
161  *   where the pts device nodes are located. For example, the ptmx device
162  *   is a symlink, separate device node, or bind-mount.
163  *   A supported scenario is bind-mounting /dev/pts/ptmx to /dev/ptmx and
164  *   then calling open on /dev/ptmx. In this case a suitable pts
165  *   subdirectory can be found in the common parent directory /dev of the
166  *   devpts mount and the ptmx bind-mount, after resolving the /dev/ptmx
167  *   bind-mount.
168  *   If no suitable pts subdirectory can be found this function will fail.
169  *   This is e.g. the case when bind-mounting /dev/pts/ptmx to /ptmx.
170  */
171 struct vfsmount *devpts_mntget(struct file *filp, struct pts_fs_info *fsi)
172 {
173 	struct path path;
174 	int err = 0;
175 
176 	path = filp->f_path;
177 	path_get(&path);
178 
179 	/* Walk upward while the start point is a bind mount of
180 	 * a single file.
181 	 */
182 	while (path.mnt->mnt_root == path.dentry)
183 		if (follow_up(&path) == 0)
184 			break;
185 
186 	/* devpts_ptmx_path() finds a devpts fs or returns an error. */
187 	if ((path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) ||
188 	    (DEVPTS_SB(path.mnt->mnt_sb) != fsi))
189 		err = devpts_ptmx_path(&path);
190 	dput(path.dentry);
191 	if (!err) {
192 		if (DEVPTS_SB(path.mnt->mnt_sb) == fsi)
193 			return path.mnt;
194 
195 		err = -ENODEV;
196 	}
197 
198 	mntput(path.mnt);
199 	return ERR_PTR(err);
200 }
201 
202 struct pts_fs_info *devpts_acquire(struct file *filp)
203 {
204 	struct pts_fs_info *result;
205 	struct path path;
206 	struct super_block *sb;
207 
208 	path = filp->f_path;
209 	path_get(&path);
210 
211 	/* Has the devpts filesystem already been found? */
212 	if (path.mnt->mnt_sb->s_magic != DEVPTS_SUPER_MAGIC) {
213 		int err;
214 
215 		err = devpts_ptmx_path(&path);
216 		if (err) {
217 			result = ERR_PTR(err);
218 			goto out;
219 		}
220 	}
221 
222 	/*
223 	 * pty code needs to hold extra references in case of last /dev/tty close
224 	 */
225 	sb = path.mnt->mnt_sb;
226 	atomic_inc(&sb->s_active);
227 	result = DEVPTS_SB(sb);
228 
229 out:
230 	path_put(&path);
231 	return result;
232 }
233 
234 void devpts_release(struct pts_fs_info *fsi)
235 {
236 	deactivate_super(fsi->sb);
237 }
238 
239 #define PARSE_MOUNT	0
240 #define PARSE_REMOUNT	1
241 
242 /*
243  * parse_mount_options():
244  *	Set @opts to mount options specified in @data. If an option is not
245  *	specified in @data, set it to its default value.
246  *
247  * Note: @data may be NULL (in which case all options are set to default).
248  */
249 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
250 {
251 	char *p;
252 	kuid_t uid;
253 	kgid_t gid;
254 
255 	opts->setuid  = 0;
256 	opts->setgid  = 0;
257 	opts->uid     = GLOBAL_ROOT_UID;
258 	opts->gid     = GLOBAL_ROOT_GID;
259 	opts->mode    = DEVPTS_DEFAULT_MODE;
260 	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
261 	opts->max     = NR_UNIX98_PTY_MAX;
262 
263 	/* Only allow instances mounted from the initial mount
264 	 * namespace to tap the reserve pool of ptys.
265 	 */
266 	if (op == PARSE_MOUNT)
267 		opts->reserve =
268 			(current->nsproxy->mnt_ns == init_task.nsproxy->mnt_ns);
269 
270 	while ((p = strsep(&data, ",")) != NULL) {
271 		substring_t args[MAX_OPT_ARGS];
272 		int token;
273 		int option;
274 
275 		if (!*p)
276 			continue;
277 
278 		token = match_token(p, tokens, args);
279 		switch (token) {
280 		case Opt_uid:
281 			if (match_int(&args[0], &option))
282 				return -EINVAL;
283 			uid = make_kuid(current_user_ns(), option);
284 			if (!uid_valid(uid))
285 				return -EINVAL;
286 			opts->uid = uid;
287 			opts->setuid = 1;
288 			break;
289 		case Opt_gid:
290 			if (match_int(&args[0], &option))
291 				return -EINVAL;
292 			gid = make_kgid(current_user_ns(), option);
293 			if (!gid_valid(gid))
294 				return -EINVAL;
295 			opts->gid = gid;
296 			opts->setgid = 1;
297 			break;
298 		case Opt_mode:
299 			if (match_octal(&args[0], &option))
300 				return -EINVAL;
301 			opts->mode = option & S_IALLUGO;
302 			break;
303 		case Opt_ptmxmode:
304 			if (match_octal(&args[0], &option))
305 				return -EINVAL;
306 			opts->ptmxmode = option & S_IALLUGO;
307 			break;
308 		case Opt_newinstance:
309 			break;
310 		case Opt_max:
311 			if (match_int(&args[0], &option) ||
312 			    option < 0 || option > NR_UNIX98_PTY_MAX)
313 				return -EINVAL;
314 			opts->max = option;
315 			break;
316 		default:
317 			pr_err("called with bogus options\n");
318 			return -EINVAL;
319 		}
320 	}
321 
322 	return 0;
323 }
324 
325 static int mknod_ptmx(struct super_block *sb)
326 {
327 	int mode;
328 	int rc = -ENOMEM;
329 	struct dentry *dentry;
330 	struct inode *inode;
331 	struct dentry *root = sb->s_root;
332 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
333 	struct pts_mount_opts *opts = &fsi->mount_opts;
334 	kuid_t ptmx_uid = current_fsuid();
335 	kgid_t ptmx_gid = current_fsgid();
336 
337 	inode_lock(d_inode(root));
338 
339 	/* If we have already created ptmx node, return */
340 	if (fsi->ptmx_dentry) {
341 		rc = 0;
342 		goto out;
343 	}
344 
345 	dentry = d_alloc_name(root, "ptmx");
346 	if (!dentry) {
347 		pr_err("Unable to alloc dentry for ptmx node\n");
348 		goto out;
349 	}
350 
351 	/*
352 	 * Create a new 'ptmx' node in this mount of devpts.
353 	 */
354 	inode = new_inode(sb);
355 	if (!inode) {
356 		pr_err("Unable to alloc inode for ptmx node\n");
357 		dput(dentry);
358 		goto out;
359 	}
360 
361 	inode->i_ino = 2;
362 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
363 
364 	mode = S_IFCHR|opts->ptmxmode;
365 	init_special_inode(inode, mode, MKDEV(TTYAUX_MAJOR, 2));
366 	inode->i_uid = ptmx_uid;
367 	inode->i_gid = ptmx_gid;
368 
369 	d_add(dentry, inode);
370 
371 	fsi->ptmx_dentry = dentry;
372 	rc = 0;
373 out:
374 	inode_unlock(d_inode(root));
375 	return rc;
376 }
377 
378 static void update_ptmx_mode(struct pts_fs_info *fsi)
379 {
380 	struct inode *inode;
381 	if (fsi->ptmx_dentry) {
382 		inode = d_inode(fsi->ptmx_dentry);
383 		inode->i_mode = S_IFCHR|fsi->mount_opts.ptmxmode;
384 	}
385 }
386 
387 static int devpts_remount(struct super_block *sb, int *flags, char *data)
388 {
389 	int err;
390 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
391 	struct pts_mount_opts *opts = &fsi->mount_opts;
392 
393 	err = parse_mount_options(data, PARSE_REMOUNT, opts);
394 
395 	/*
396 	 * parse_mount_options() restores options to default values
397 	 * before parsing and may have changed ptmxmode. So, update the
398 	 * mode in the inode too. Bogus options don't fail the remount,
399 	 * so do this even on error return.
400 	 */
401 	update_ptmx_mode(fsi);
402 
403 	return err;
404 }
405 
406 static int devpts_show_options(struct seq_file *seq, struct dentry *root)
407 {
408 	struct pts_fs_info *fsi = DEVPTS_SB(root->d_sb);
409 	struct pts_mount_opts *opts = &fsi->mount_opts;
410 
411 	if (opts->setuid)
412 		seq_printf(seq, ",uid=%u",
413 			   from_kuid_munged(&init_user_ns, opts->uid));
414 	if (opts->setgid)
415 		seq_printf(seq, ",gid=%u",
416 			   from_kgid_munged(&init_user_ns, opts->gid));
417 	seq_printf(seq, ",mode=%03o", opts->mode);
418 	seq_printf(seq, ",ptmxmode=%03o", opts->ptmxmode);
419 	if (opts->max < NR_UNIX98_PTY_MAX)
420 		seq_printf(seq, ",max=%d", opts->max);
421 
422 	return 0;
423 }
424 
425 static const struct super_operations devpts_sops = {
426 	.statfs		= simple_statfs,
427 	.remount_fs	= devpts_remount,
428 	.show_options	= devpts_show_options,
429 };
430 
431 static void *new_pts_fs_info(struct super_block *sb)
432 {
433 	struct pts_fs_info *fsi;
434 
435 	fsi = kzalloc(sizeof(struct pts_fs_info), GFP_KERNEL);
436 	if (!fsi)
437 		return NULL;
438 
439 	ida_init(&fsi->allocated_ptys);
440 	fsi->mount_opts.mode = DEVPTS_DEFAULT_MODE;
441 	fsi->mount_opts.ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
442 	fsi->sb = sb;
443 
444 	return fsi;
445 }
446 
447 static int
448 devpts_fill_super(struct super_block *s, void *data, int silent)
449 {
450 	struct inode *inode;
451 	int error;
452 
453 	s->s_iflags &= ~SB_I_NODEV;
454 	s->s_blocksize = 1024;
455 	s->s_blocksize_bits = 10;
456 	s->s_magic = DEVPTS_SUPER_MAGIC;
457 	s->s_op = &devpts_sops;
458 	s->s_d_op = &simple_dentry_operations;
459 	s->s_time_gran = 1;
460 
461 	error = -ENOMEM;
462 	s->s_fs_info = new_pts_fs_info(s);
463 	if (!s->s_fs_info)
464 		goto fail;
465 
466 	error = parse_mount_options(data, PARSE_MOUNT, &DEVPTS_SB(s)->mount_opts);
467 	if (error)
468 		goto fail;
469 
470 	error = -ENOMEM;
471 	inode = new_inode(s);
472 	if (!inode)
473 		goto fail;
474 	inode->i_ino = 1;
475 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
476 	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
477 	inode->i_op = &simple_dir_inode_operations;
478 	inode->i_fop = &simple_dir_operations;
479 	set_nlink(inode, 2);
480 
481 	s->s_root = d_make_root(inode);
482 	if (!s->s_root) {
483 		pr_err("get root dentry failed\n");
484 		goto fail;
485 	}
486 
487 	error = mknod_ptmx(s);
488 	if (error)
489 		goto fail_dput;
490 
491 	return 0;
492 fail_dput:
493 	dput(s->s_root);
494 	s->s_root = NULL;
495 fail:
496 	return error;
497 }
498 
499 /*
500  * devpts_mount()
501  *
502  *     Mount a new (private) instance of devpts.  PTYs created in this
503  *     instance are independent of the PTYs in other devpts instances.
504  */
505 static struct dentry *devpts_mount(struct file_system_type *fs_type,
506 	int flags, const char *dev_name, void *data)
507 {
508 	return mount_nodev(fs_type, flags, data, devpts_fill_super);
509 }
510 
511 static void devpts_kill_sb(struct super_block *sb)
512 {
513 	struct pts_fs_info *fsi = DEVPTS_SB(sb);
514 
515 	if (fsi)
516 		ida_destroy(&fsi->allocated_ptys);
517 	kfree(fsi);
518 	kill_litter_super(sb);
519 }
520 
521 static struct file_system_type devpts_fs_type = {
522 	.name		= "devpts",
523 	.mount		= devpts_mount,
524 	.kill_sb	= devpts_kill_sb,
525 	.fs_flags	= FS_USERNS_MOUNT,
526 };
527 
528 /*
529  * The normal naming convention is simply /dev/pts/<number>; this conforms
530  * to the System V naming convention
531  */
532 
533 int devpts_new_index(struct pts_fs_info *fsi)
534 {
535 	int index = -ENOSPC;
536 
537 	if (atomic_inc_return(&pty_count) >= (pty_limit -
538 			  (fsi->mount_opts.reserve ? 0 : pty_reserve)))
539 		goto out;
540 
541 	index = ida_alloc_max(&fsi->allocated_ptys, fsi->mount_opts.max - 1,
542 			GFP_KERNEL);
543 
544 out:
545 	if (index < 0)
546 		atomic_dec(&pty_count);
547 	return index;
548 }
549 
550 void devpts_kill_index(struct pts_fs_info *fsi, int idx)
551 {
552 	ida_free(&fsi->allocated_ptys, idx);
553 	atomic_dec(&pty_count);
554 }
555 
556 /**
557  * devpts_pty_new -- create a new inode in /dev/pts/
558  * @ptmx_inode: inode of the master
559  * @device: major+minor of the node to be created
560  * @index: used as a name of the node
561  * @priv: what's given back by devpts_get_priv
562  *
563  * The created inode is returned. Remove it from /dev/pts/ by devpts_pty_kill.
564  */
565 struct dentry *devpts_pty_new(struct pts_fs_info *fsi, int index, void *priv)
566 {
567 	struct dentry *dentry;
568 	struct super_block *sb = fsi->sb;
569 	struct inode *inode;
570 	struct dentry *root;
571 	struct pts_mount_opts *opts;
572 	char s[12];
573 
574 	root = sb->s_root;
575 	opts = &fsi->mount_opts;
576 
577 	inode = new_inode(sb);
578 	if (!inode)
579 		return ERR_PTR(-ENOMEM);
580 
581 	inode->i_ino = index + 3;
582 	inode->i_uid = opts->setuid ? opts->uid : current_fsuid();
583 	inode->i_gid = opts->setgid ? opts->gid : current_fsgid();
584 	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
585 	init_special_inode(inode, S_IFCHR|opts->mode, MKDEV(UNIX98_PTY_SLAVE_MAJOR, index));
586 
587 	sprintf(s, "%d", index);
588 
589 	dentry = d_alloc_name(root, s);
590 	if (dentry) {
591 		dentry->d_fsdata = priv;
592 		d_add(dentry, inode);
593 		fsnotify_create(d_inode(root), dentry);
594 	} else {
595 		iput(inode);
596 		dentry = ERR_PTR(-ENOMEM);
597 	}
598 
599 	return dentry;
600 }
601 
602 /**
603  * devpts_get_priv -- get private data for a slave
604  * @pts_inode: inode of the slave
605  *
606  * Returns whatever was passed as priv in devpts_pty_new for a given inode.
607  */
608 void *devpts_get_priv(struct dentry *dentry)
609 {
610 	if (dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC)
611 		return NULL;
612 	return dentry->d_fsdata;
613 }
614 
615 /**
616  * devpts_pty_kill -- remove inode form /dev/pts/
617  * @inode: inode of the slave to be removed
618  *
619  * This is an inverse operation of devpts_pty_new.
620  */
621 void devpts_pty_kill(struct dentry *dentry)
622 {
623 	WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
624 
625 	dentry->d_fsdata = NULL;
626 	drop_nlink(dentry->d_inode);
627 	d_delete(dentry);
628 	dput(dentry);	/* d_alloc_name() in devpts_pty_new() */
629 }
630 
631 static int __init init_devpts_fs(void)
632 {
633 	int err = register_filesystem(&devpts_fs_type);
634 	if (!err) {
635 		register_sysctl_table(pty_root_table);
636 	}
637 	return err;
638 }
639 module_init(init_devpts_fs)
640