mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-27 11:19:37 +00:00
417 lines
10 KiB
C
417 lines
10 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#include <linux/syscalls.h>
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#include <linux/export.h>
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#include <linux/uaccess.h>
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#include <linux/fs_struct.h>
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#include <linux/fs.h>
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#include <linux/slab.h>
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#include <linux/prefetch.h>
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#include "mount.h"
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struct prepend_buffer {
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char *buf;
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int len;
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};
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#define DECLARE_BUFFER(__name, __buf, __len) \
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struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}
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static char *extract_string(struct prepend_buffer *p)
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{
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if (likely(p->len >= 0))
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return p->buf;
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return ERR_PTR(-ENAMETOOLONG);
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}
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static void prepend(struct prepend_buffer *p, const char *str, int namelen)
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{
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p->len -= namelen;
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if (likely(p->len >= 0)) {
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p->buf -= namelen;
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memcpy(p->buf, str, namelen);
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}
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}
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/**
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* prepend_name - prepend a pathname in front of current buffer pointer
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* @buffer: buffer pointer
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* @buflen: allocated length of the buffer
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* @name: name string and length qstr structure
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*
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* With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
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* make sure that either the old or the new name pointer and length are
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* fetched. However, there may be mismatch between length and pointer.
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* The length cannot be trusted, we need to copy it byte-by-byte until
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* the length is reached or a null byte is found. It also prepends "/" at
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* the beginning of the name. The sequence number check at the caller will
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* retry it again when a d_move() does happen. So any garbage in the buffer
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* due to mismatched pointer and length will be discarded.
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*
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* Load acquire is needed to make sure that we see that terminating NUL.
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*/
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static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
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{
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const char *dname = smp_load_acquire(&name->name); /* ^^^ */
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u32 dlen = READ_ONCE(name->len);
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char *s;
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p->len -= dlen + 1;
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if (unlikely(p->len < 0))
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return false;
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s = p->buf -= dlen + 1;
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*s++ = '/';
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while (dlen--) {
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char c = *dname++;
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if (!c)
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break;
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*s++ = c;
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}
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return true;
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}
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static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
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const struct path *root, struct prepend_buffer *p)
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{
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while (dentry != root->dentry || &mnt->mnt != root->mnt) {
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const struct dentry *parent = READ_ONCE(dentry->d_parent);
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if (dentry == mnt->mnt.mnt_root) {
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struct mount *m = READ_ONCE(mnt->mnt_parent);
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struct mnt_namespace *mnt_ns;
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if (likely(mnt != m)) {
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dentry = READ_ONCE(mnt->mnt_mountpoint);
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mnt = m;
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continue;
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}
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/* Global root */
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mnt_ns = READ_ONCE(mnt->mnt_ns);
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/* open-coded is_mounted() to use local mnt_ns */
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if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
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return 1; // absolute root
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else
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return 2; // detached or not attached yet
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}
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if (unlikely(dentry == parent))
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/* Escaped? */
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return 3;
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prefetch(parent);
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if (!prepend_name(p, &dentry->d_name))
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break;
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dentry = parent;
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}
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return 0;
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}
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/**
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* prepend_path - Prepend path string to a buffer
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* @path: the dentry/vfsmount to report
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* @root: root vfsmnt/dentry
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* @buffer: pointer to the end of the buffer
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* @buflen: pointer to buffer length
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*
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* The function will first try to write out the pathname without taking any
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* lock other than the RCU read lock to make sure that dentries won't go away.
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* It only checks the sequence number of the global rename_lock as any change
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* in the dentry's d_seq will be preceded by changes in the rename_lock
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* sequence number. If the sequence number had been changed, it will restart
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* the whole pathname back-tracing sequence again by taking the rename_lock.
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* In this case, there is no need to take the RCU read lock as the recursive
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* parent pointer references will keep the dentry chain alive as long as no
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* rename operation is performed.
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*/
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static int prepend_path(const struct path *path,
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const struct path *root,
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struct prepend_buffer *p)
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{
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unsigned seq, m_seq = 0;
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struct prepend_buffer b;
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int error;
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rcu_read_lock();
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restart_mnt:
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read_seqbegin_or_lock(&mount_lock, &m_seq);
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seq = 0;
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rcu_read_lock();
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restart:
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b = *p;
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read_seqbegin_or_lock(&rename_lock, &seq);
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error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
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if (!(seq & 1))
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rcu_read_unlock();
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if (need_seqretry(&rename_lock, seq)) {
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seq = 1;
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goto restart;
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}
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done_seqretry(&rename_lock, seq);
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if (!(m_seq & 1))
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rcu_read_unlock();
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if (need_seqretry(&mount_lock, m_seq)) {
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m_seq = 1;
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goto restart_mnt;
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}
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done_seqretry(&mount_lock, m_seq);
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if (unlikely(error == 3))
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b = *p;
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if (b.len == p->len)
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prepend(&b, "/", 1);
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*p = b;
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return error;
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}
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/**
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* __d_path - return the path of a dentry
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* @path: the dentry/vfsmount to report
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* @root: root vfsmnt/dentry
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* @buf: buffer to return value in
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* @buflen: buffer length
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*
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* Convert a dentry into an ASCII path name.
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*
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* Returns a pointer into the buffer or an error code if the
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* path was too long.
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*
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* "buflen" should be positive.
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*
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* If the path is not reachable from the supplied root, return %NULL.
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*/
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char *__d_path(const struct path *path,
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const struct path *root,
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char *buf, int buflen)
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{
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DECLARE_BUFFER(b, buf, buflen);
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prepend(&b, "", 1);
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if (unlikely(prepend_path(path, root, &b) > 0))
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return NULL;
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return extract_string(&b);
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}
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char *d_absolute_path(const struct path *path,
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char *buf, int buflen)
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{
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struct path root = {};
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DECLARE_BUFFER(b, buf, buflen);
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prepend(&b, "", 1);
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if (unlikely(prepend_path(path, &root, &b) > 1))
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return ERR_PTR(-EINVAL);
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return extract_string(&b);
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}
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static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
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{
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unsigned seq;
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do {
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seq = read_seqcount_begin(&fs->seq);
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*root = fs->root;
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} while (read_seqcount_retry(&fs->seq, seq));
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}
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/**
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* d_path - return the path of a dentry
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* @path: path to report
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* @buf: buffer to return value in
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* @buflen: buffer length
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*
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* Convert a dentry into an ASCII path name. If the entry has been deleted
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* the string " (deleted)" is appended. Note that this is ambiguous.
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*
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* Returns a pointer into the buffer or an error code if the path was
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* too long. Note: Callers should use the returned pointer, not the passed
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* in buffer, to use the name! The implementation often starts at an offset
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* into the buffer, and may leave 0 bytes at the start.
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*
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* "buflen" should be positive.
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*/
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char *d_path(const struct path *path, char *buf, int buflen)
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{
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DECLARE_BUFFER(b, buf, buflen);
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struct path root;
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/*
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* We have various synthetic filesystems that never get mounted. On
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* these filesystems dentries are never used for lookup purposes, and
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* thus don't need to be hashed. They also don't need a name until a
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* user wants to identify the object in /proc/pid/fd/. The little hack
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* below allows us to generate a name for these objects on demand:
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*
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* Some pseudo inodes are mountable. When they are mounted
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* path->dentry == path->mnt->mnt_root. In that case don't call d_dname
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* and instead have d_path return the mounted path.
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*/
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if (path->dentry->d_op && path->dentry->d_op->d_dname &&
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(!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root))
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return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
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rcu_read_lock();
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get_fs_root_rcu(current->fs, &root);
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if (unlikely(d_unlinked(path->dentry)))
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prepend(&b, " (deleted)", 11);
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else
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prepend(&b, "", 1);
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prepend_path(path, &root, &b);
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rcu_read_unlock();
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return extract_string(&b);
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}
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EXPORT_SYMBOL(d_path);
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/*
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* Helper function for dentry_operations.d_dname() members
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*/
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char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
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const char *fmt, ...)
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{
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va_list args;
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char temp[64];
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int sz;
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va_start(args, fmt);
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sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
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va_end(args);
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if (sz > sizeof(temp) || sz > buflen)
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return ERR_PTR(-ENAMETOOLONG);
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buffer += buflen - sz;
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return memcpy(buffer, temp, sz);
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}
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char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
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{
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DECLARE_BUFFER(b, buffer, buflen);
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/* these dentries are never renamed, so d_lock is not needed */
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prepend(&b, " (deleted)", 11);
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prepend(&b, dentry->d_name.name, dentry->d_name.len);
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prepend(&b, "/", 1);
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return extract_string(&b);
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}
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/*
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* Write full pathname from the root of the filesystem into the buffer.
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*/
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static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
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{
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const struct dentry *dentry;
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struct prepend_buffer b;
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int seq = 0;
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rcu_read_lock();
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restart:
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dentry = d;
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b = *p;
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read_seqbegin_or_lock(&rename_lock, &seq);
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while (!IS_ROOT(dentry)) {
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const struct dentry *parent = dentry->d_parent;
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prefetch(parent);
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if (!prepend_name(&b, &dentry->d_name))
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break;
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dentry = parent;
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}
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if (!(seq & 1))
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rcu_read_unlock();
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if (need_seqretry(&rename_lock, seq)) {
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seq = 1;
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goto restart;
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}
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done_seqretry(&rename_lock, seq);
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if (b.len == p->len)
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prepend(&b, "/", 1);
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return extract_string(&b);
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}
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char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
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{
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DECLARE_BUFFER(b, buf, buflen);
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prepend(&b, "", 1);
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return __dentry_path(dentry, &b);
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}
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EXPORT_SYMBOL(dentry_path_raw);
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char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
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{
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DECLARE_BUFFER(b, buf, buflen);
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if (unlikely(d_unlinked(dentry)))
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prepend(&b, "//deleted", 10);
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else
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prepend(&b, "", 1);
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return __dentry_path(dentry, &b);
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}
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static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
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struct path *pwd)
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{
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unsigned seq;
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do {
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seq = read_seqcount_begin(&fs->seq);
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*root = fs->root;
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*pwd = fs->pwd;
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} while (read_seqcount_retry(&fs->seq, seq));
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}
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/*
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* NOTE! The user-level library version returns a
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* character pointer. The kernel system call just
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* returns the length of the buffer filled (which
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* includes the ending '\0' character), or a negative
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* error value. So libc would do something like
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*
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* char *getcwd(char * buf, size_t size)
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* {
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* int retval;
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*
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* retval = sys_getcwd(buf, size);
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* if (retval >= 0)
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* return buf;
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* errno = -retval;
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* return NULL;
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* }
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*/
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SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
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{
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int error;
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struct path pwd, root;
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char *page = __getname();
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if (!page)
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return -ENOMEM;
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rcu_read_lock();
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get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
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if (unlikely(d_unlinked(pwd.dentry))) {
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rcu_read_unlock();
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error = -ENOENT;
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} else {
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unsigned len;
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DECLARE_BUFFER(b, page, PATH_MAX);
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prepend(&b, "", 1);
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if (unlikely(prepend_path(&pwd, &root, &b) > 0))
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prepend(&b, "(unreachable)", 13);
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rcu_read_unlock();
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len = PATH_MAX - b.len;
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if (unlikely(len > PATH_MAX))
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error = -ENAMETOOLONG;
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else if (unlikely(len > size))
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error = -ERANGE;
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else if (copy_to_user(buf, b.buf, len))
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error = -EFAULT;
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else
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error = len;
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}
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__putname(page);
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return error;
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}
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