linux-stable/fs/pidfs.c
Christian Brauner 9d9539db86 pidfs: remove config option
As Linus suggested this enables pidfs unconditionally. A key property to
retain is the ability to compare pidfds by inode number (cf. [1]).
That's extremely helpful just as comparing namespace file descriptors by
inode number is. They are used in a variety of scenarios where they need
to be compared, e.g., when receiving a pidfd via SO_PEERPIDFD from a
socket to trivially authenticate a the sender and various other
use-cases.

For 64bit systems this is pretty trivial to do. For 32bit it's slightly
more annoying as we discussed but we simply add a dumb ida based
allocator that gets used on 32bit. This gives the same guarantees about
inode numbers on 64bit without any overflow risk. Practically, we'll
never run into overflow issues because we're constrained by the number
of processes that can exist on 32bit and by the number of open files
that can exist on a 32bit system. On 64bit none of this matters and
things are very simple.

If 32bit also needs the uniqueness guarantee they can simply parse the
contents of /proc/<pid>/fd/<nr>. The uniqueness guarantees have a
variety of use-cases. One of the most obvious ones is that they will
make pidfiles (or "pidfdfiles", I guess) reliable as the unique
identifier can be placed into there that won't be reycled. Also a
frequent request.

Note, I took the chance and simplified path_from_stashed() even further.
Instead of passing the inode number explicitly to path_from_stashed() we
let the filesystem handle that internally. So path_from_stashed() ends
up even simpler than it is now. This is also a good solution allowing
the cleanup code to be clean and consistent between 32bit and 64bit. The
cleanup path in prepare_anon_dentry() is also switched around so we put
the inode before the dentry allocation. This means we only have to call
the cleanup handler for the filesystem's inode data once and can rely
->evict_inode() otherwise.

Aside from having to have a bit of extra code for 32bit it actually ends
up a nice cleanup for path_from_stashed() imho.

Tested on both 32 and 64bit including error injection.

Link: https://github.com/systemd/systemd/pull/31713 [1]
Link: https://lore.kernel.org/r/20240312-dingo-sehnlich-b3ecc35c6de7@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2024-03-13 12:53:53 -07:00

283 lines
7.4 KiB
C

// SPDX-License-Identifier: GPL-2.0
#include <linux/anon_inodes.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/magic.h>
#include <linux/mount.h>
#include <linux/pid.h>
#include <linux/pidfs.h>
#include <linux/pid_namespace.h>
#include <linux/poll.h>
#include <linux/proc_fs.h>
#include <linux/proc_ns.h>
#include <linux/pseudo_fs.h>
#include <linux/seq_file.h>
#include <uapi/linux/pidfd.h>
#include "internal.h"
#ifdef CONFIG_PROC_FS
/**
* pidfd_show_fdinfo - print information about a pidfd
* @m: proc fdinfo file
* @f: file referencing a pidfd
*
* Pid:
* This function will print the pid that a given pidfd refers to in the
* pid namespace of the procfs instance.
* If the pid namespace of the process is not a descendant of the pid
* namespace of the procfs instance 0 will be shown as its pid. This is
* similar to calling getppid() on a process whose parent is outside of
* its pid namespace.
*
* NSpid:
* If pid namespaces are supported then this function will also print
* the pid of a given pidfd refers to for all descendant pid namespaces
* starting from the current pid namespace of the instance, i.e. the
* Pid field and the first entry in the NSpid field will be identical.
* If the pid namespace of the process is not a descendant of the pid
* namespace of the procfs instance 0 will be shown as its first NSpid
* entry and no others will be shown.
* Note that this differs from the Pid and NSpid fields in
* /proc/<pid>/status where Pid and NSpid are always shown relative to
* the pid namespace of the procfs instance. The difference becomes
* obvious when sending around a pidfd between pid namespaces from a
* different branch of the tree, i.e. where no ancestral relation is
* present between the pid namespaces:
* - create two new pid namespaces ns1 and ns2 in the initial pid
* namespace (also take care to create new mount namespaces in the
* new pid namespace and mount procfs)
* - create a process with a pidfd in ns1
* - send pidfd from ns1 to ns2
* - read /proc/self/fdinfo/<pidfd> and observe that both Pid and NSpid
* have exactly one entry, which is 0
*/
static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
{
struct pid *pid = pidfd_pid(f);
struct pid_namespace *ns;
pid_t nr = -1;
if (likely(pid_has_task(pid, PIDTYPE_PID))) {
ns = proc_pid_ns(file_inode(m->file)->i_sb);
nr = pid_nr_ns(pid, ns);
}
seq_put_decimal_ll(m, "Pid:\t", nr);
#ifdef CONFIG_PID_NS
seq_put_decimal_ll(m, "\nNSpid:\t", nr);
if (nr > 0) {
int i;
/* If nr is non-zero it means that 'pid' is valid and that
* ns, i.e. the pid namespace associated with the procfs
* instance, is in the pid namespace hierarchy of pid.
* Start at one below the already printed level.
*/
for (i = ns->level + 1; i <= pid->level; i++)
seq_put_decimal_ll(m, "\t", pid->numbers[i].nr);
}
#endif
seq_putc(m, '\n');
}
#endif
/*
* Poll support for process exit notification.
*/
static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
{
struct pid *pid = pidfd_pid(file);
bool thread = file->f_flags & PIDFD_THREAD;
struct task_struct *task;
__poll_t poll_flags = 0;
poll_wait(file, &pid->wait_pidfd, pts);
/*
* Depending on PIDFD_THREAD, inform pollers when the thread
* or the whole thread-group exits.
*/
guard(rcu)();
task = pid_task(pid, PIDTYPE_PID);
if (!task)
poll_flags = EPOLLIN | EPOLLRDNORM | EPOLLHUP;
else if (task->exit_state && (thread || thread_group_empty(task)))
poll_flags = EPOLLIN | EPOLLRDNORM;
return poll_flags;
}
static const struct file_operations pidfs_file_operations = {
.poll = pidfd_poll,
#ifdef CONFIG_PROC_FS
.show_fdinfo = pidfd_show_fdinfo,
#endif
};
struct pid *pidfd_pid(const struct file *file)
{
if (file->f_op != &pidfs_file_operations)
return ERR_PTR(-EBADF);
return file_inode(file)->i_private;
}
static struct vfsmount *pidfs_mnt __ro_after_init;
#if BITS_PER_LONG == 32
/*
* Provide a fallback mechanism for 32-bit systems so processes remain
* reliably comparable by inode number even on those systems.
*/
static DEFINE_IDA(pidfd_inum_ida);
static int pidfs_inum(struct pid *pid, unsigned long *ino)
{
int ret;
ret = ida_alloc_range(&pidfd_inum_ida, RESERVED_PIDS + 1,
UINT_MAX, GFP_ATOMIC);
if (ret < 0)
return -ENOSPC;
*ino = ret;
return 0;
}
static inline void pidfs_free_inum(unsigned long ino)
{
if (ino > 0)
ida_free(&pidfd_inum_ida, ino);
}
#else
static inline int pidfs_inum(struct pid *pid, unsigned long *ino)
{
*ino = pid->ino;
return 0;
}
#define pidfs_free_inum(ino) ((void)(ino))
#endif
/*
* The vfs falls back to simple_setattr() if i_op->setattr() isn't
* implemented. Let's reject it completely until we have a clean
* permission concept for pidfds.
*/
static int pidfs_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
struct iattr *attr)
{
return -EOPNOTSUPP;
}
static int pidfs_getattr(struct mnt_idmap *idmap, const struct path *path,
struct kstat *stat, u32 request_mask,
unsigned int query_flags)
{
struct inode *inode = d_inode(path->dentry);
generic_fillattr(&nop_mnt_idmap, request_mask, inode, stat);
return 0;
}
static const struct inode_operations pidfs_inode_operations = {
.getattr = pidfs_getattr,
.setattr = pidfs_setattr,
};
static void pidfs_evict_inode(struct inode *inode)
{
struct pid *pid = inode->i_private;
clear_inode(inode);
put_pid(pid);
pidfs_free_inum(inode->i_ino);
}
static const struct super_operations pidfs_sops = {
.drop_inode = generic_delete_inode,
.evict_inode = pidfs_evict_inode,
.statfs = simple_statfs,
};
static char *pidfs_dname(struct dentry *dentry, char *buffer, int buflen)
{
struct inode *inode = d_inode(dentry);
struct pid *pid = inode->i_private;
return dynamic_dname(buffer, buflen, "pidfd:[%llu]", pid->ino);
}
static const struct dentry_operations pidfs_dentry_operations = {
.d_delete = always_delete_dentry,
.d_dname = pidfs_dname,
.d_prune = stashed_dentry_prune,
};
static int pidfs_init_inode(struct inode *inode, void *data)
{
inode->i_private = data;
inode->i_flags |= S_PRIVATE;
inode->i_mode |= S_IRWXU;
inode->i_op = &pidfs_inode_operations;
inode->i_fop = &pidfs_file_operations;
/*
* Inode numbering for pidfs start at RESERVED_PIDS + 1. This
* avoids collisions with the root inode which is 1 for pseudo
* filesystems.
*/
return pidfs_inum(data, &inode->i_ino);
}
static void pidfs_put_data(void *data)
{
struct pid *pid = data;
put_pid(pid);
}
static const struct stashed_operations pidfs_stashed_ops = {
.init_inode = pidfs_init_inode,
.put_data = pidfs_put_data,
};
static int pidfs_init_fs_context(struct fs_context *fc)
{
struct pseudo_fs_context *ctx;
ctx = init_pseudo(fc, PID_FS_MAGIC);
if (!ctx)
return -ENOMEM;
ctx->ops = &pidfs_sops;
ctx->dops = &pidfs_dentry_operations;
fc->s_fs_info = (void *)&pidfs_stashed_ops;
return 0;
}
static struct file_system_type pidfs_type = {
.name = "pidfs",
.init_fs_context = pidfs_init_fs_context,
.kill_sb = kill_anon_super,
};
struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
{
struct file *pidfd_file;
struct path path;
int ret;
ret = path_from_stashed(&pid->stashed, pidfs_mnt, get_pid(pid), &path);
if (ret < 0)
return ERR_PTR(ret);
pidfd_file = dentry_open(&path, flags, current_cred());
path_put(&path);
return pidfd_file;
}
void __init pidfs_init(void)
{
pidfs_mnt = kern_mount(&pidfs_type);
if (IS_ERR(pidfs_mnt))
panic("Failed to mount pidfs pseudo filesystem");
}