dentry name snapshots

take_dentry_name_snapshot() takes a safe snapshot of dentry name;
if the name is a short one, it gets copied into caller-supplied
structure, otherwise an extra reference to external name is grabbed
(those are never modified).  In either case the pointer to stable
string is stored into the same structure.

dentry must be held by the caller of take_dentry_name_snapshot(),
but may be freely dropped afterwards - the snapshot will stay
until destroyed by release_dentry_name_snapshot().

Intended use:
	struct name_snapshot s;

	take_dentry_name_snapshot(&s, dentry);
	...
	access s.name
	...
	release_dentry_name_snapshot(&s);

Replaces fsnotify_oldname_...(), gets used in fsnotify to obtain the name
to pass down with event.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
static int do_last(struct nameidata *nd,
		   struct file *file, const struct open_flags *op,
		   int *opened)
{
	struct dentry *dir = nd->path.dentry;
	int open_flag = op->open_flag;
	bool will_truncate = (open_flag & O_TRUNC) != 0;
	bool got_write = false;
	int acc_mode = op->acc_mode;
	unsigned seq;
	struct inode *inode;
	struct path path;
	int error;

	nd->flags &= ~LOOKUP_PARENT;
	nd->flags |= op->intent;

	if (nd->last_type != LAST_NORM) {
		error = handle_dots(nd, nd->last_type);
		if (unlikely(error))
			return error;
		goto finish_open;
	}

	if (!(open_flag & O_CREAT)) {
		if (nd->last.name[nd->last.len])
			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
		/* we _can_ be in RCU mode here */
		error = lookup_fast(nd, &path, &inode, &seq);
		if (likely(error > 0))
			goto finish_lookup;

		if (error < 0)
			return error;

		BUG_ON(nd->inode != dir->d_inode);
		BUG_ON(nd->flags & LOOKUP_RCU);
	} else {
		/* create side of things */
		/*
		 * This will *only* deal with leaving RCU mode - LOOKUP_JUMPED
		 * has been cleared when we got to the last component we are
		 * about to look up
		 */
		error = complete_walk(nd);
		if (error)
			return error;

		audit_inode(nd->name, dir, LOOKUP_PARENT);
		/* trailing slashes? */
		if (unlikely(nd->last.name[nd->last.len]))
			return -EISDIR;
	}

	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
		error = mnt_want_write(nd->path.mnt);
		if (!error)
			got_write = true;
		/*
		 * do _not_ fail yet - we might not need that or fail with
		 * a different error; let lookup_open() decide; we'll be
		 * dropping this one anyway.
		 */
	}
	if (open_flag & O_CREAT)
		inode_lock(dir->d_inode);
	else
		inode_lock_shared(dir->d_inode);
	error = lookup_open(nd, &path, file, op, got_write, opened);
	if (open_flag & O_CREAT)
		inode_unlock(dir->d_inode);
	else
		inode_unlock_shared(dir->d_inode);

	if (error <= 0) {
		if (error)
			goto out;

		if ((*opened & FILE_CREATED) ||
		    !S_ISREG(file_inode(file)->i_mode))
			will_truncate = false;

		audit_inode(nd->name, file->f_path.dentry, 0);
		goto opened;
	}

	if (*opened & FILE_CREATED) {
		/* Don't check for write permission, don't truncate */
		open_flag &= ~O_TRUNC;
		will_truncate = false;
		acc_mode = 0;
		path_to_nameidata(&path, nd);
		goto finish_open_created;
	}

	/*
	 * If atomic_open() acquired write access it is dropped now due to
	 * possible mount and symlink following (this might be optimized away if
	 * necessary...)
	 */
	if (got_write) {
		mnt_drop_write(nd->path.mnt);
		got_write = false;
	}

	error = follow_managed(&path, nd);
	if (unlikely(error < 0))
		return error;

	if (unlikely(d_is_negative(path.dentry))) {
		path_to_nameidata(&path, nd);
		return -ENOENT;
	}

	/*
	 * create/update audit record if it already exists.
	 */
	audit_inode(nd->name, path.dentry, 0);

	if (unlikely((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))) {
		path_to_nameidata(&path, nd);
		return -EEXIST;
	}

	seq = 0;	/* out of RCU mode, so the value doesn't matter */
	inode = d_backing_inode(path.dentry);
finish_lookup:
	error = step_into(nd, &path, 0, inode, seq);
	if (unlikely(error))
		return error;
finish_open:
	/* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
	error = complete_walk(nd);
	if (error)
		return error;
	audit_inode(nd->name, nd->path.dentry, 0);
	error = -EISDIR;
	if ((open_flag & O_CREAT) && d_is_dir(nd->path.dentry))
		goto out;
	error = -ENOTDIR;
	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
		goto out;
	if (!d_is_reg(nd->path.dentry))
		will_truncate = false;

	if (will_truncate) {
		error = mnt_want_write(nd->path.mnt);
		if (error)
			goto out;
		got_write = true;
	}
finish_open_created:
	error = may_open(&nd->path, acc_mode, open_flag);
	if (error)
		goto out;
	BUG_ON(*opened & FILE_OPENED); /* once it's opened, it's opened */
	error = vfs_open(&nd->path, file, current_cred());
	if (error)
		goto out;
	*opened |= FILE_OPENED;
opened:
	error = open_check_o_direct(file);
	if (!error)
		error = ima_file_check(file, op->acc_mode, *opened);
	if (!error && will_truncate)
		error = handle_truncate(file);
out:
	if (unlikely(error) && (*opened & FILE_OPENED))
		fput(file);
	if (unlikely(error > 0)) {
		WARN_ON(1);
		error = -EINVAL;
	}
	if (got_write)
		mnt_drop_write(nd->path.mnt);
	return error;
}

static int do_last(struct nameidata *nd,
		   struct file *file, const struct open_flags *op,
		   int *opened)
{
	struct dentry *dir = nd->path.dentry;
	int open_flag = op->open_flag;
	bool will_truncate = (open_flag & O_TRUNC) != 0;
	bool got_write = false;
	int acc_mode = op->acc_mode;
	unsigned seq;
	struct inode *inode;
	struct path path;
	int error;

	nd->flags &= ~LOOKUP_PARENT;
	nd->flags |= op->intent;

	if (nd->last_type != LAST_NORM) {
		error = handle_dots(nd, nd->last_type);
		if (unlikely(error))
			return error;
		goto finish_open;
	}

	if (!(open_flag & O_CREAT)) {
		if (nd->last.name[nd->last.len])
			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
		/* we _can_ be in RCU mode here */
		error = lookup_fast(nd, &path, &inode, &seq);
		if (likely(error > 0))
			goto finish_lookup;

		if (error < 0)
			return error;

		BUG_ON(nd->inode != dir->d_inode);
		BUG_ON(nd->flags & LOOKUP_RCU);
	} else {
		/* create side of things */
		/*
		 * This will *only* deal with leaving RCU mode - LOOKUP_JUMPED
		 * has been cleared when we got to the last component we are
		 * about to look up
		 */
		error = complete_walk(nd);
		if (error)
			return error;

		audit_inode(nd->name, dir, LOOKUP_PARENT);
		/* trailing slashes? */
		if (unlikely(nd->last.name[nd->last.len]))
			return -EISDIR;
	}

	if (open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
		error = mnt_want_write(nd->path.mnt);
		if (!error)
			got_write = true;
		/*
		 * do _not_ fail yet - we might not need that or fail with
		 * a different error; let lookup_open() decide; we'll be
		 * dropping this one anyway.
		 */
	}
	if (open_flag & O_CREAT)
		inode_lock(dir->d_inode);
	else
		inode_lock_shared(dir->d_inode);
	error = lookup_open(nd, &path, file, op, got_write, opened);
	if (open_flag & O_CREAT)
		inode_unlock(dir->d_inode);
	else
		inode_unlock_shared(dir->d_inode);

	if (error <= 0) {
		if (error)
			goto out;

		if ((*opened & FILE_CREATED) ||
		    !S_ISREG(file_inode(file)->i_mode))
			will_truncate = false;

		audit_inode(nd->name, file->f_path.dentry, 0);
		goto opened;
	}

	if (*opened & FILE_CREATED) {
		/* Don't check for write permission, don't truncate */
		open_flag &= ~O_TRUNC;
		will_truncate = false;
		acc_mode = 0;
		path_to_nameidata(&path, nd);
		goto finish_open_created;
	}

	/*
	 * If atomic_open() acquired write access it is dropped now due to
	 * possible mount and symlink following (this might be optimized away if
	 * necessary...)
	 */
	if (got_write) {
		mnt_drop_write(nd->path.mnt);
		got_write = false;
	}

	error = follow_managed(&path, nd);
	if (unlikely(error < 0))
		return error;

	if (unlikely(d_is_negative(path.dentry))) {
		path_to_nameidata(&path, nd);
		return -ENOENT;
	}

	/*
	 * create/update audit record if it already exists.
	 */
	audit_inode(nd->name, path.dentry, 0);

	if (unlikely((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))) {
		path_to_nameidata(&path, nd);
		return -EEXIST;
	}

	seq = 0;	/* out of RCU mode, so the value doesn't matter */
	inode = d_backing_inode(path.dentry);
finish_lookup:
	error = step_into(nd, &path, 0, inode, seq);
	if (unlikely(error))
		return error;
finish_open:
	/* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
	error = complete_walk(nd);
	if (error)
		return error;
	audit_inode(nd->name, nd->path.dentry, 0);
	error = -EISDIR;
	if ((open_flag & O_CREAT) && d_is_dir(nd->path.dentry))
		goto out;
	error = -ENOTDIR;
	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
		goto out;
	if (!d_is_reg(nd->path.dentry))
		will_truncate = false;

	if (will_truncate) {
		error = mnt_want_write(nd->path.mnt);
		if (error)
			goto out;
		got_write = true;
	}
finish_open_created:
	error = may_open(&nd->path, acc_mode, open_flag);
	if (error)
		goto out;
	BUG_ON(*opened & FILE_OPENED); /* once it's opened, it's opened */
	error = vfs_open(&nd->path, file, current_cred());
	if (error)
		goto out;
	*opened |= FILE_OPENED;
opened:
	error = open_check_o_direct(file);
	if (!error)
		error = ima_file_check(file, op->acc_mode, *opened);
	if (!error && will_truncate)
		error = handle_truncate(file);
out:
	if (unlikely(error) && (*opened & FILE_OPENED))
		fput(file);
	if (unlikely(error > 0)) {
		WARN_ON(1);
		error = -EINVAL;
	}
	if (got_write)
		mnt_drop_write(nd->path.mnt);
	return error;
}
