// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright (C) Neil Brown 2002
 * Copyright (C) Christoph Hellwig 2007
 *
 * This file contains the code mapping from inodes to NFS file handles,
 * and for mapping back from file handles to dentries.
 *
 * For details on why we do all the strange and hairy things in here
 * take a look at Documentation/filesystems/nfs/exporting.rst.
 */
#include <linux/exportfs.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/module.h>
#include <linux/mount.h>
#include <linux/namei.h>
#include <linux/sched.h>
#include <linux/cred.h>

#define dprintk(fmt, args...) do{}while(0)


static int get_name(const struct path *path, char *name, struct dentry *child);


static int exportfs_get_name(struct vfsmount *mnt, struct dentry *dir,
                char *name, struct dentry *child)
{
        const struct export_operations *nop = dir->d_sb->s_export_op;
        struct path path = {.mnt = mnt, .dentry = dir};

        if (nop->get_name)
                return nop->get_name(dir, name, child);
        else
                return get_name(&path, name, child);
}

/*
 * Check if the dentry or any of it's aliases is acceptable.
 */
static struct dentry *
find_acceptable_alias(struct dentry *result,
                int (*acceptable)(void *context, struct dentry *dentry),
                void *context)
{
        struct dentry *dentry, *toput = NULL;
        struct inode *inode;

        if (acceptable(context, result))
                return result;

        inode = result->d_inode;
        spin_lock(&inode->i_lock);
        hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
                dget(dentry);
                spin_unlock(&inode->i_lock);
                if (toput)
                        dput(toput);
                if (dentry != result && acceptable(context, dentry)) {
                        dput(result);
                        return dentry;
                }
                spin_lock(&inode->i_lock);
                toput = dentry;
        }
        spin_unlock(&inode->i_lock);

        if (toput)
                dput(toput);
        return NULL;
}

static bool dentry_connected(struct dentry *dentry)
{
        dget(dentry);
        while (dentry->d_flags & DCACHE_DISCONNECTED) {
                struct dentry *parent = dget_parent(dentry);

                dput(dentry);
                if (dentry == parent) {
                        dput(parent);
                        return false;
                }
                dentry = parent;
        }
        dput(dentry);
        return true;
}

static void clear_disconnected(struct dentry *dentry)
{
        dget(dentry);
        while (dentry->d_flags & DCACHE_DISCONNECTED) {
                struct dentry *parent = dget_parent(dentry);

                WARN_ON_ONCE(IS_ROOT(dentry));

                spin_lock(&dentry->d_lock);
                dentry->d_flags &= ~DCACHE_DISCONNECTED;
                spin_unlock(&dentry->d_lock);

                dput(dentry);
                dentry = parent;
        }
        dput(dentry);
}

/*
 * Reconnect a directory dentry with its parent.
 *
 * This can return a dentry, or NULL, or an error.
 *
 * In the first case the returned dentry is the parent of the given
 * dentry, and may itself need to be reconnected to its parent.
 *
 * In the NULL case, a concurrent VFS operation has either renamed or
 * removed this directory.  The concurrent operation has reconnected our
 * dentry, so we no longer need to.
 */
static struct dentry *reconnect_one(struct vfsmount *mnt,
                struct dentry *dentry, char *nbuf)
{
        struct dentry *parent;
        struct dentry *tmp;
        int err;

        parent = ERR_PTR(-EACCES);
        inode_lock(dentry->d_inode);
        if (mnt->mnt_sb->s_export_op->get_parent)
                parent = mnt->mnt_sb->s_export_op->get_parent(dentry);
        inode_unlock(dentry->d_inode);

        if (IS_ERR(parent)) {
                dprintk("%s: get_parent of %ld failed, err %d\n",
                        __func__, dentry->d_inode->i_ino, PTR_ERR(parent));
                return parent;
        }

        dprintk("%s: find name of %lu in %lu\n", __func__,
                dentry->d_inode->i_ino, parent->d_inode->i_ino);
        err = exportfs_get_name(mnt, parent, nbuf, dentry);
        if (err == -ENOENT)
                goto out_reconnected;
        if (err)
                goto out_err;
        dprintk("%s: found name: %s\n", __func__, nbuf);
        tmp = lookup_one_len_unlocked(nbuf, parent, strlen(nbuf));
        if (IS_ERR(tmp)) {
                dprintk("%s: lookup failed: %d\n", __func__, PTR_ERR(tmp));
                err = PTR_ERR(tmp);
                goto out_err;
        }
        if (tmp != dentry) {
                /*
                 * Somebody has renamed it since exportfs_get_name();
                 * great, since it could've only been renamed if it
                 * got looked up and thus connected, and it would
                 * remain connected afterwards.  We are done.
                 */
                dput(tmp);
                goto out_reconnected;
        }
        dput(tmp);
        if (IS_ROOT(dentry)) {
                err = -ESTALE;
                goto out_err;
        }
        return parent;

out_err:
        dput(parent);
        return ERR_PTR(err);
out_reconnected:
        dput(parent);
        /*
         * Someone must have renamed our entry into another parent, in
         * which case it has been reconnected by the rename.
         *
         * Or someone removed it entirely, in which case filehandle
         * lookup will succeed but the directory is now IS_DEAD and
         * subsequent operations on it will fail.
         *
         * Alternatively, maybe there was no race at all, and the
         * filesystem is just corrupt and gave us a parent that doesn't
         * actually contain any entry pointing to this inode.  So,
         * double check that this worked and return -ESTALE if not:
         */
        if (!dentry_connected(dentry))
                return ERR_PTR(-ESTALE);
        return NULL;
}

/*
 * Make sure target_dir is fully connected to the dentry tree.
 *
 * On successful return, DCACHE_DISCONNECTED will be cleared on
 * target_dir, and target_dir->d_parent->...->d_parent will reach the
 * root of the filesystem.
 *
 * Whenever DCACHE_DISCONNECTED is unset, target_dir is fully connected.
 * But the converse is not true: target_dir may have DCACHE_DISCONNECTED
 * set but already be connected.  In that case we'll verify the
 * connection to root and then clear the flag.
 *
 * Note that target_dir could be removed by a concurrent operation.  In
 * that case reconnect_path may still succeed with target_dir fully
 * connected, but further operations using the filehandle will fail when
 * necessary (due to S_DEAD being set on the directory).
 */
static int
reconnect_path(struct vfsmount *mnt, struct dentry *target_dir, char *nbuf)
{
        struct dentry *dentry, *parent;

        dentry = dget(target_dir);

        while (dentry->d_flags & DCACHE_DISCONNECTED) {
                BUG_ON(dentry == mnt->mnt_sb->s_root);

                if (IS_ROOT(dentry))
                        parent = reconnect_one(mnt, dentry, nbuf);
                else
                        parent = dget_parent(dentry);

                if (!parent)
                        break;
                dput(dentry);
                if (IS_ERR(parent))
                        return PTR_ERR(parent);
                dentry = parent;
        }
        dput(dentry);
        clear_disconnected(target_dir);
        return 0;
}

struct getdents_callback {
        struct dir_context ctx;
        char *name;             /* name that was found. It already points to a
                                   buffer NAME_MAX+1 is size */
        u64 ino;                /* the inum we are looking for */
        int found;              /* inode matched? */
        int sequence;           /* sequence counter */
};

/*
 * A rather strange filldir function to capture
 * the name matching the specified inode number.
 */
static int filldir_one(struct dir_context *ctx, const char *name, int len,
                        loff_t pos, u64 ino, unsigned int d_type)
{
        struct getdents_callback *buf =
                container_of(ctx, struct getdents_callback, ctx);
        int result = 0;

        buf->sequence++;
        if (buf->ino == ino && len <= NAME_MAX) {
                memcpy(buf->name, name, len);
                buf->name[len] = '\0';
                buf->found = 1;
                result = -1;
        }
        return result;
}

/**
 * get_name - default export_operations->get_name function
 * @path:   the directory in which to find a name
 * @name:   a pointer to a %NAME_MAX+1 char buffer to store the name
 * @child:  the dentry for the child directory.
 *
 * calls readdir on the parent until it finds an entry with
 * the same inode number as the child, and returns that.
 */
static int get_name(const struct path *path, char *name, struct dentry *child)
{
        const struct cred *cred = current_cred();
        struct inode *dir = path->dentry->d_inode;
        int error;
        struct file *file;
        struct kstat stat;
        struct path child_path = {
                .mnt = path->mnt,
                .dentry = child,
        };
        struct getdents_callback buffer = {
                .ctx.actor = filldir_one,
                .name = name,
        };

        error = -ENOTDIR;
        if (!dir || !S_ISDIR(dir->i_mode))
                goto out;
        error = -EINVAL;
        if (!dir->i_fop)
                goto out;
        /*
         * inode->i_ino is unsigned long, kstat->ino is u64, so the
         * former would be insufficient on 32-bit hosts when the
         * filesystem supports 64-bit inode numbers.  So we need to
         * actually call ->getattr, not just read i_ino:
         */
        error = vfs_getattr_nosec(&child_path, &stat,
                                  STATX_INO, AT_STATX_SYNC_AS_STAT);
        if (error)
                return error;
        buffer.ino = stat.ino;
        /*
         * Open the directory ...
         */
        file = dentry_open(path, O_RDONLY, cred);
        error = PTR_ERR(file);
        if (IS_ERR(file))
                goto out;

        error = -EINVAL;
        if (!file->f_op->iterate && !file->f_op->iterate_shared)
                goto out_close;

        buffer.sequence = 0;
        while (1) {
                int old_seq = buffer.sequence;

                error = iterate_dir(file, &buffer.ctx);
                if (buffer.found) {
                        error = 0;
                        break;
                }

                if (error < 0)
                        break;

                error = -ENOENT;
                if (old_seq == buffer.sequence)
                        break;
        }

out_close:
        fput(file);
out:
        return error;
}

/**
 * export_encode_fh - default export_operations->encode_fh function
 * @inode:   the object to encode
 * @fid:     where to store the file handle fragment
 * @max_len: maximum length to store there
 * @parent:  parent directory inode, if wanted
 *
 * This default encode_fh function assumes that the 32 inode number
 * is suitable for locating an inode, and that the generation number
 * can be used to check that it is still valid.  It places them in the
 * filehandle fragment where export_decode_fh expects to find them.
 */
static int export_encode_fh(struct inode *inode, struct fid *fid,
                int *max_len, struct inode *parent)
{
        int len = *max_len;
        int type = FILEID_INO32_GEN;

        if (parent && (len < 4)) {
                *max_len = 4;
                return FILEID_INVALID;
        } else if (len < 2) {
                *max_len = 2;
                return FILEID_INVALID;
        }

        len = 2;
        fid->i32.ino = inode->i_ino;
        fid->i32.gen = inode->i_generation;
        if (parent) {
                fid->i32.parent_ino = parent->i_ino;
                fid->i32.parent_gen = parent->i_generation;
                len = 4;
                type = FILEID_INO32_GEN_PARENT;
        }
        *max_len = len;
        return type;
}

int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
                             int *max_len, struct inode *parent)
{
        const struct export_operations *nop = inode->i_sb->s_export_op;

        if (nop && nop->encode_fh)
                return nop->encode_fh(inode, fid->raw, max_len, parent);

        return export_encode_fh(inode, fid, max_len, parent);
}
EXPORT_SYMBOL_GPL(exportfs_encode_inode_fh);

int exportfs_encode_fh(struct dentry *dentry, struct fid *fid, int *max_len,
                int connectable)
{
        int error;
        struct dentry *p = NULL;
        struct inode *inode = dentry->d_inode, *parent = NULL;

        if (connectable && !S_ISDIR(inode->i_mode)) {
                p = dget_parent(dentry);
                /*
                 * note that while p might've ceased to be our parent already,
                 * it's still pinned by and still positive.
                 */
                parent = p->d_inode;
        }

        error = exportfs_encode_inode_fh(inode, fid, max_len, parent);
        dput(p);

        return error;
}
EXPORT_SYMBOL_GPL(exportfs_encode_fh);

struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
                int fh_len, int fileid_type,
                int (*acceptable)(void *, struct dentry *), void *context)
{
        const struct export_operations *nop = mnt->mnt_sb->s_export_op;
        struct dentry *result, *alias;
        char nbuf[NAME_MAX+1];
        int err;

        /*
         * Try to get any dentry for the given file handle from the filesystem.
         */
        if (!nop || !nop->fh_to_dentry)
                return ERR_PTR(-ESTALE);
        result = nop->fh_to_dentry(mnt->mnt_sb, fid, fh_len, fileid_type);
        if (PTR_ERR(result) == -ENOMEM)
                return ERR_CAST(result);
        if (IS_ERR_OR_NULL(result))
                return ERR_PTR(-ESTALE);

        /*
         * If no acceptance criteria was specified by caller, a disconnected
         * dentry is also accepatable. Callers may use this mode to query if
         * file handle is stale or to get a reference to an inode without
         * risking the high overhead caused by directory reconnect.
         */
        if (!acceptable)
                return result;

        if (d_is_dir(result)) {
                /*
                 * This request is for a directory.
                 *
                 * On the positive side there is only one dentry for each
                 * directory inode.  On the negative side this implies that we
                 * to ensure our dentry is connected all the way up to the
                 * filesystem root.
                 */
                if (result->d_flags & DCACHE_DISCONNECTED) {
                        err = reconnect_path(mnt, result, nbuf);
                        if (err)
                                goto err_result;
                }

                if (!acceptable(context, result)) {
                        err = -EACCES;
                        goto err_result;
                }

                return result;
        } else {
                /*
                 * It's not a directory.  Life is a little more complicated.
                 */
                struct dentry *target_dir, *nresult;

                /*
                 * See if either the dentry we just got from the filesystem
                 * or any alias for it is acceptable.  This is always true
                 * if this filesystem is exported without the subtreecheck
                 * option.  If the filesystem is exported with the subtree
                 * check option there's a fair chance we need to look at
                 * the parent directory in the file handle and make sure
                 * it's connected to the filesystem root.
                 */
                alias = find_acceptable_alias(result, acceptable, context);
                if (alias)
                        return alias;

                /*
                 * Try to extract a dentry for the parent directory from the
                 * file handle.  If this fails we'll have to give up.
                 */
                err = -ESTALE;
                if (!nop->fh_to_parent)
                        goto err_result;

                target_dir = nop->fh_to_parent(mnt->mnt_sb, fid,
                                fh_len, fileid_type);
                if (!target_dir)
                        goto err_result;
                err = PTR_ERR(target_dir);
                if (IS_ERR(target_dir))
                        goto err_result;

                /*
                 * And as usual we need to make sure the parent directory is
                 * connected to the filesystem root.  The VFS really doesn't
                 * like disconnected directories..
                 */
                err = reconnect_path(mnt, target_dir, nbuf);
                if (err) {
                        dput(target_dir);
                        goto err_result;
                }

                /*
                 * Now that we've got both a well-connected parent and a
                 * dentry for the inode we're after, make sure that our
                 * inode is actually connected to the parent.
                 */
                err = exportfs_get_name(mnt, target_dir, nbuf, result);
                if (err) {
                        dput(target_dir);
                        goto err_result;
                }

                inode_lock(target_dir->d_inode);
                nresult = lookup_one_len(nbuf, target_dir, strlen(nbuf));
                if (!IS_ERR(nresult)) {
                        if (unlikely(nresult->d_inode != result->d_inode)) {
                                dput(nresult);
                                nresult = ERR_PTR(-ESTALE);
                        }
                }
                inode_unlock(target_dir->d_inode);
                /*
                 * At this point we are done with the parent, but it's pinned
                 * by the child dentry anyway.
                 */
                dput(target_dir);

                if (IS_ERR(nresult)) {
                        err = PTR_ERR(nresult);
                        goto err_result;
                }
                dput(result);
                result = nresult;

                /*
                 * And finally make sure the dentry is actually acceptable
                 * to NFSD.
                 */
                alias = find_acceptable_alias(result, acceptable, context);
                if (!alias) {
                        err = -EACCES;
                        goto err_result;
                }

                return alias;
        }

 err_result:
        dput(result);
        if (err != -ENOMEM)
                err = -ESTALE;
        return ERR_PTR(err);
}
EXPORT_SYMBOL_GPL(exportfs_decode_fh);

MODULE_LICENSE("GPL");