/* 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);
                        struct mnt_namespace *mnt_ns;

                        /* 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;
                        }
                        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))
                                error = 1;      // absolute root
                        else
                                error = 2;      // detached or not attached yet
                        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;
}

/*
 * 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;
}