d_path.c | Explore in Territory

/* 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"
#include "internal.h"

struct prepend_buffer { … };
#define DECLARE_BUFFER(__name, __buf, __len) …

static char *extract_string(struct prepend_buffer *p)
{ … }

static bool prepend_char(struct prepend_buffer *p, unsigned char c)
{ … }

/*
 * The source of the prepend data can be an optimistic 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)
{ … }

static bool prepend(struct prepend_buffer *p, const char *str, int namelen)
{ … }

/**
 * prepend_name - prepend a pathname in front of current buffer pointer
 * @p: prepend buffer which contains buffer pointer and allocated length
 * @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)
{ … }

static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
			  const struct path *root, struct prepend_buffer *p)
{ … }

/**
 * prepend_path - Prepend path string to a buffer
 * @path: the dentry/vfsmount to report
 * @root: root vfsmnt/dentry
 * @p: prepend buffer which contains buffer pointer and allocated 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)
{ … }

/**
 * __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)
{ … }

char *d_absolute_path(const struct path *path,
	       char *buf, int buflen)
{ … }

static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
{ … }

/**
 * 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)
{ … }
EXPORT_SYMBOL(…);

/*
 * Helper function for dentry_operations.d_dname() members
 */
char *dynamic_dname(char *buffer, int buflen, const char *fmt, ...)
{ … }

char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
{ … }

/*
 * Write full pathname from the root of the filesystem into the buffer.
 */
static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
{ … }

char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
{ … }
EXPORT_SYMBOL(…);

char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
{ … }

static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
				    struct path *pwd)
{ … }

/*
 * 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)
{ … }
linux/fs/d_path.c
