split d_path() and friends into a separate file
Those parts of fs/dcache.c are pretty much self-contained.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/d_path.c b/fs/d_path.c
new file mode 100644
index 0000000..e8fce6b
--- /dev/null
+++ b/fs/d_path.c
@@ -0,0 +1,470 @@
+/* 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"
+
+static int prepend(char **buffer, int *buflen, const char *str, int namelen)
+{
+ *buflen -= namelen;
+ if (*buflen < 0)
+ return -ENAMETOOLONG;
+ *buffer -= namelen;
+ memcpy(*buffer, str, namelen);
+ return 0;
+}
+
+/**
+ * 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.
+ * The length cannot be trusted, we need to copy it byte-by-byte until
+ * the length is reached or a null byte is found. 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 that terminating NUL.
+ */
+static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
+{
+ const char *dname = smp_load_acquire(&name->name); /* ^^^ */
+ u32 dlen = READ_ONCE(name->len);
+ char *p;
+
+ *buflen -= dlen + 1;
+ if (*buflen < 0)
+ return -ENAMETOOLONG;
+ p = *buffer -= dlen + 1;
+ *p++ = '/';
+ while (dlen--) {
+ char c = *dname++;
+ if (!c)
+ break;
+ *p++ = c;
+ }
+ 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,
+ char **buffer, int *buflen)
+{
+ struct dentry *dentry;
+ struct vfsmount *vfsmnt;
+ struct mount *mnt;
+ int error = 0;
+ unsigned seq, m_seq = 0;
+ char *bptr;
+ int blen;
+
+ rcu_read_lock();
+restart_mnt:
+ read_seqbegin_or_lock(&mount_lock, &m_seq);
+ seq = 0;
+ rcu_read_lock();
+restart:
+ bptr = *buffer;
+ blen = *buflen;
+ error = 0;
+ dentry = path->dentry;
+ vfsmnt = path->mnt;
+ mnt = real_mount(vfsmnt);
+ read_seqbegin_or_lock(&rename_lock, &seq);
+ while (dentry != root->dentry || vfsmnt != root->mnt) {
+ struct dentry * parent;
+
+ if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
+ struct mount *parent = READ_ONCE(mnt->mnt_parent);
+ /* Escaped? */
+ if (dentry != vfsmnt->mnt_root) {
+ bptr = *buffer;
+ blen = *buflen;
+ error = 3;
+ break;
+ }
+ /* Global root? */
+ if (mnt != parent) {
+ dentry = READ_ONCE(mnt->mnt_mountpoint);
+ mnt = parent;
+ vfsmnt = &mnt->mnt;
+ continue;
+ }
+ if (!error)
+ error = is_mounted(vfsmnt) ? 1 : 2;
+ break;
+ }
+ parent = dentry->d_parent;
+ prefetch(parent);
+ error = prepend_name(&bptr, &blen, &dentry->d_name);
+ if (error)
+ 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 (!(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 (error >= 0 && bptr == *buffer) {
+ if (--blen < 0)
+ error = -ENAMETOOLONG;
+ else
+ *--bptr = '/';
+ }
+ *buffer = bptr;
+ *buflen = blen;
+ 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)
+{
+ char *res = buf + buflen;
+ int error;
+
+ prepend(&res, &buflen, "\0", 1);
+ error = prepend_path(path, root, &res, &buflen);
+
+ if (error < 0)
+ return ERR_PTR(error);
+ if (error > 0)
+ return NULL;
+ return res;
+}
+
+char *d_absolute_path(const struct path *path,
+ char *buf, int buflen)
+{
+ struct path root = {};
+ char *res = buf + buflen;
+ int error;
+
+ prepend(&res, &buflen, "\0", 1);
+ error = prepend_path(path, &root, &res, &buflen);
+
+ if (error > 1)
+ error = -EINVAL;
+ if (error < 0)
+ return ERR_PTR(error);
+ return res;
+}
+
+/*
+ * same as __d_path but appends "(deleted)" for unlinked files.
+ */
+static int path_with_deleted(const struct path *path,
+ const struct path *root,
+ char **buf, int *buflen)
+{
+ prepend(buf, buflen, "\0", 1);
+ if (d_unlinked(path->dentry)) {
+ int error = prepend(buf, buflen, " (deleted)", 10);
+ if (error)
+ return error;
+ }
+
+ return prepend_path(path, root, buf, buflen);
+}
+
+static int prepend_unreachable(char **buffer, int *buflen)
+{
+ return prepend(buffer, buflen, "(unreachable)", 13);
+}
+
+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)
+{
+ char *res = buf + buflen;
+ struct path root;
+ int error;
+
+ /*
+ * 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);
+ error = path_with_deleted(path, &root, &res, &buflen);
+ rcu_read_unlock();
+
+ if (error < 0)
+ res = ERR_PTR(error);
+ return res;
+}
+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)
+{
+ char *end = buffer + buflen;
+ /* these dentries are never renamed, so d_lock is not needed */
+ if (prepend(&end, &buflen, " (deleted)", 11) ||
+ prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
+ prepend(&end, &buflen, "/", 1))
+ end = ERR_PTR(-ENAMETOOLONG);
+ return end;
+}
+EXPORT_SYMBOL(simple_dname);
+
+/*
+ * Write full pathname from the root of the filesystem into the buffer.
+ */
+static char *__dentry_path(struct dentry *d, char *buf, int buflen)
+{
+ struct dentry *dentry;
+ char *end, *retval;
+ int len, seq = 0;
+ int error = 0;
+
+ if (buflen < 2)
+ goto Elong;
+
+ rcu_read_lock();
+restart:
+ dentry = d;
+ end = buf + buflen;
+ len = buflen;
+ prepend(&end, &len, "\0", 1);
+ /* Get '/' right */
+ retval = end-1;
+ *retval = '/';
+ read_seqbegin_or_lock(&rename_lock, &seq);
+ while (!IS_ROOT(dentry)) {
+ struct dentry *parent = dentry->d_parent;
+
+ prefetch(parent);
+ error = prepend_name(&end, &len, &dentry->d_name);
+ if (error)
+ break;
+
+ retval = end;
+ dentry = parent;
+ }
+ if (!(seq & 1))
+ rcu_read_unlock();
+ if (need_seqretry(&rename_lock, seq)) {
+ seq = 1;
+ goto restart;
+ }
+ done_seqretry(&rename_lock, seq);
+ if (error)
+ goto Elong;
+ return retval;
+Elong:
+ return ERR_PTR(-ENAMETOOLONG);
+}
+
+char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
+{
+ return __dentry_path(dentry, buf, buflen);
+}
+EXPORT_SYMBOL(dentry_path_raw);
+
+char *dentry_path(struct dentry *dentry, char *buf, int buflen)
+{
+ char *p = NULL;
+ char *retval;
+
+ if (d_unlinked(dentry)) {
+ p = buf + buflen;
+ if (prepend(&p, &buflen, "//deleted", 10) != 0)
+ goto Elong;
+ buflen++;
+ }
+ retval = __dentry_path(dentry, buf, buflen);
+ if (!IS_ERR(retval) && p)
+ *p = '/'; /* restore '/' overriden with '\0' */
+ return retval;
+Elong:
+ return ERR_PTR(-ENAMETOOLONG);
+}
+
+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);
+
+ error = -ENOENT;
+ if (!d_unlinked(pwd.dentry)) {
+ unsigned long len;
+ char *cwd = page + PATH_MAX;
+ int buflen = PATH_MAX;
+
+ prepend(&cwd, &buflen, "\0", 1);
+ error = prepend_path(&pwd, &root, &cwd, &buflen);
+ rcu_read_unlock();
+
+ if (error < 0)
+ goto out;
+
+ /* Unreachable from current root */
+ if (error > 0) {
+ error = prepend_unreachable(&cwd, &buflen);
+ if (error)
+ goto out;
+ }
+
+ error = -ERANGE;
+ len = PATH_MAX + page - cwd;
+ if (len <= size) {
+ error = len;
+ if (copy_to_user(buf, cwd, len))
+ error = -EFAULT;
+ }
+ } else {
+ rcu_read_unlock();
+ }
+
+out:
+ __putname(page);
+ return error;
+}