linux/fs/d_path.c
Linus Torvalds b0cfcdd9b9 d_path: make 'prepend()' fill up the buffer exactly on overflow
Instead of just marking the buffer as having overflowed, fill it up as
much as we can.  That will allow the overflow case to then return
whatever truncated result if it wants to.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2021-09-02 10:07:29 -07:00

451 lines
12 KiB
C

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