1// SPDX-License-Identifier: GPL-2.0
2#include <linux/anon_inodes.h>
3#include <linux/file.h>
4#include <linux/fs.h>
5#include <linux/magic.h>
6#include <linux/mount.h>
7#include <linux/pid.h>
8#include <linux/pidfs.h>
9#include <linux/pid_namespace.h>
10#include <linux/poll.h>
11#include <linux/proc_fs.h>
12#include <linux/proc_ns.h>
13#include <linux/pseudo_fs.h>
14#include <linux/seq_file.h>
15#include <uapi/linux/pidfd.h>
16
17#include "internal.h"
18
19#ifdef CONFIG_PROC_FS
20/**
21 * pidfd_show_fdinfo - print information about a pidfd
22 * @m: proc fdinfo file
23 * @f: file referencing a pidfd
24 *
25 * Pid:
26 * This function will print the pid that a given pidfd refers to in the
27 * pid namespace of the procfs instance.
28 * If the pid namespace of the process is not a descendant of the pid
29 * namespace of the procfs instance 0 will be shown as its pid. This is
30 * similar to calling getppid() on a process whose parent is outside of
31 * its pid namespace.
32 *
33 * NSpid:
34 * If pid namespaces are supported then this function will also print
35 * the pid of a given pidfd refers to for all descendant pid namespaces
36 * starting from the current pid namespace of the instance, i.e. the
37 * Pid field and the first entry in the NSpid field will be identical.
38 * If the pid namespace of the process is not a descendant of the pid
39 * namespace of the procfs instance 0 will be shown as its first NSpid
40 * entry and no others will be shown.
41 * Note that this differs from the Pid and NSpid fields in
42 * /proc/<pid>/status where Pid and NSpid are always shown relative to
43 * the pid namespace of the procfs instance. The difference becomes
44 * obvious when sending around a pidfd between pid namespaces from a
45 * different branch of the tree, i.e. where no ancestral relation is
46 * present between the pid namespaces:
47 * - create two new pid namespaces ns1 and ns2 in the initial pid
48 * namespace (also take care to create new mount namespaces in the
49 * new pid namespace and mount procfs)
50 * - create a process with a pidfd in ns1
51 * - send pidfd from ns1 to ns2
52 * - read /proc/self/fdinfo/<pidfd> and observe that both Pid and NSpid
53 * have exactly one entry, which is 0
54 */
55static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
56{
57 struct pid *pid = pidfd_pid(file: f);
58 struct pid_namespace *ns;
59 pid_t nr = -1;
60
61 if (likely(pid_has_task(pid, PIDTYPE_PID))) {
62 ns = proc_pid_ns(sb: file_inode(f: m->file)->i_sb);
63 nr = pid_nr_ns(pid, ns);
64 }
65
66 seq_put_decimal_ll(m, delimiter: "Pid:\t", num: nr);
67
68#ifdef CONFIG_PID_NS
69 seq_put_decimal_ll(m, delimiter: "\nNSpid:\t", num: nr);
70 if (nr > 0) {
71 int i;
72
73 /* If nr is non-zero it means that 'pid' is valid and that
74 * ns, i.e. the pid namespace associated with the procfs
75 * instance, is in the pid namespace hierarchy of pid.
76 * Start at one below the already printed level.
77 */
78 for (i = ns->level + 1; i <= pid->level; i++)
79 seq_put_decimal_ll(m, delimiter: "\t", num: pid->numbers[i].nr);
80 }
81#endif
82 seq_putc(m, c: '\n');
83}
84#endif
85
86/*
87 * Poll support for process exit notification.
88 */
89static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
90{
91 struct pid *pid = pidfd_pid(file);
92 bool thread = file->f_flags & PIDFD_THREAD;
93 struct task_struct *task;
94 __poll_t poll_flags = 0;
95
96 poll_wait(filp: file, wait_address: &pid->wait_pidfd, p: pts);
97 /*
98 * Depending on PIDFD_THREAD, inform pollers when the thread
99 * or the whole thread-group exits.
100 */
101 guard(rcu)();
102 task = pid_task(pid, PIDTYPE_PID);
103 if (!task)
104 poll_flags = EPOLLIN | EPOLLRDNORM | EPOLLHUP;
105 else if (task->exit_state && (thread || thread_group_empty(p: task)))
106 poll_flags = EPOLLIN | EPOLLRDNORM;
107
108 return poll_flags;
109}
110
111static const struct file_operations pidfs_file_operations = {
112 .poll = pidfd_poll,
113#ifdef CONFIG_PROC_FS
114 .show_fdinfo = pidfd_show_fdinfo,
115#endif
116};
117
118struct pid *pidfd_pid(const struct file *file)
119{
120 if (file->f_op != &pidfs_file_operations)
121 return ERR_PTR(error: -EBADF);
122 return file_inode(f: file)->i_private;
123}
124
125static struct vfsmount *pidfs_mnt __ro_after_init;
126
127#if BITS_PER_LONG == 32
128/*
129 * Provide a fallback mechanism for 32-bit systems so processes remain
130 * reliably comparable by inode number even on those systems.
131 */
132static DEFINE_IDA(pidfd_inum_ida);
133
134static int pidfs_inum(struct pid *pid, unsigned long *ino)
135{
136 int ret;
137
138 ret = ida_alloc_range(&pidfd_inum_ida, RESERVED_PIDS + 1,
139 UINT_MAX, GFP_ATOMIC);
140 if (ret < 0)
141 return -ENOSPC;
142
143 *ino = ret;
144 return 0;
145}
146
147static inline void pidfs_free_inum(unsigned long ino)
148{
149 if (ino > 0)
150 ida_free(&pidfd_inum_ida, ino);
151}
152#else
153static inline int pidfs_inum(struct pid *pid, unsigned long *ino)
154{
155 *ino = pid->ino;
156 return 0;
157}
158#define pidfs_free_inum(ino) ((void)(ino))
159#endif
160
161/*
162 * The vfs falls back to simple_setattr() if i_op->setattr() isn't
163 * implemented. Let's reject it completely until we have a clean
164 * permission concept for pidfds.
165 */
166static int pidfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
167 struct iattr *attr)
168{
169 return -EOPNOTSUPP;
170}
171
172static int pidfs_getattr(struct mnt_idmap *idmap, const struct path *path,
173 struct kstat *stat, u32 request_mask,
174 unsigned int query_flags)
175{
176 struct inode *inode = d_inode(dentry: path->dentry);
177
178 generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
179 return 0;
180}
181
182static const struct inode_operations pidfs_inode_operations = {
183 .getattr = pidfs_getattr,
184 .setattr = pidfs_setattr,
185};
186
187static void pidfs_evict_inode(struct inode *inode)
188{
189 struct pid *pid = inode->i_private;
190
191 clear_inode(inode);
192 put_pid(pid);
193 pidfs_free_inum(inode->i_ino);
194}
195
196static const struct super_operations pidfs_sops = {
197 .drop_inode = generic_delete_inode,
198 .evict_inode = pidfs_evict_inode,
199 .statfs = simple_statfs,
200};
201
202static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
203{
204 struct inode *inode = d_inode(dentry);
205 struct pid *pid = inode->i_private;
206
207 return dynamic_dname(buffer, buflen, "pidfd:[%llu]", pid->ino);
208}
209
210static const struct dentry_operations pidfs_dentry_operations = {
211 .d_delete = always_delete_dentry,
212 .d_dname = pidfs_dname,
213 .d_prune = stashed_dentry_prune,
214};
215
216static int pidfs_init_inode(struct inode *inode, void *data)
217{
218 inode->i_private = data;
219 inode->i_flags |= S_PRIVATE;
220 inode->i_mode |= S_IRWXU;
221 inode->i_op = &pidfs_inode_operations;
222 inode->i_fop = &pidfs_file_operations;
223 /*
224 * Inode numbering for pidfs start at RESERVED_PIDS + 1. This
225 * avoids collisions with the root inode which is 1 for pseudo
226 * filesystems.
227 */
228 return pidfs_inum(pid: data, ino: &inode->i_ino);
229}
230
231static void pidfs_put_data(void *data)
232{
233 struct pid *pid = data;
234 put_pid(pid);
235}
236
237static const struct stashed_operations pidfs_stashed_ops = {
238 .init_inode = pidfs_init_inode,
239 .put_data = pidfs_put_data,
240};
241
242static int pidfs_init_fs_context(struct fs_context *fc)
243{
244 struct pseudo_fs_context *ctx;
245
246 ctx = init_pseudo(fc, PID_FS_MAGIC);
247 if (!ctx)
248 return -ENOMEM;
249
250 ctx->ops = &pidfs_sops;
251 ctx->dops = &pidfs_dentry_operations;
252 fc->s_fs_info = (void *)&pidfs_stashed_ops;
253 return 0;
254}
255
256static struct file_system_type pidfs_type = {
257 .name = "pidfs",
258 .init_fs_context = pidfs_init_fs_context,
259 .kill_sb = kill_anon_super,
260};
261
262struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
263{
264
265 struct file *pidfd_file;
266 struct path path;
267 int ret;
268
269 ret = path_from_stashed(stashed: &pid->stashed, mnt: pidfs_mnt, data: get_pid(pid), path: &path);
270 if (ret < 0)
271 return ERR_PTR(error: ret);
272
273 pidfd_file = dentry_open(path: &path, flags, current_cred());
274 path_put(&path);
275 return pidfd_file;
276}
277
278void __init pidfs_init(void)
279{
280 pidfs_mnt = kern_mount(&pidfs_type);
281 if (IS_ERR(ptr: pidfs_mnt))
282 panic(fmt: "Failed to mount pidfs pseudo filesystem");
283}
284

source code of linux/fs/pidfs.c