Re: [PATCH 16/21] xfs: repair damaged symlinks

[Dave Chinner <david@fromorbit.com>]

On Sun, Jun 24, 2018 at 12:25:16PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Repair inconsistent symbolic link data.
.....
> +/*
> + * Symbolic Link Repair
> + * ====================
> + *
> + * There's not much we can do to repair symbolic links -- we truncate them to
> + * the first NULL byte and fix up the remote target block headers if they're
> + * incorrect.  Zero-length symlinks are turned into links to /.
> + */
> +
> +/* Blow out the whole symlink; replace contents. */
> +STATIC int
> +xfs_repair_symlink_rewrite(
> +	struct xfs_trans	**tpp,
> +	struct xfs_inode	*ip,
> +	const char		*target_path,
> +	int			pathlen)
> +{
> +	struct xfs_defer_ops	dfops;
> +	struct xfs_bmbt_irec	mval[XFS_SYMLINK_MAPS];
> +	struct xfs_ifork	*ifp;
> +	const char		*cur_chunk;
> +	struct xfs_mount	*mp = (*tpp)->t_mountp;
> +	struct xfs_buf		*bp;
> +	xfs_fsblock_t		first_block;
> +	xfs_fileoff_t		first_fsb;
> +	xfs_filblks_t		fs_blocks;
> +	xfs_daddr_t		d;
> +	int			byte_cnt;
> +	int			n;
> +	int			nmaps;
> +	int			offset;
> +	int			error = 0;
> +
> +	ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
> +
> +	/* Truncate the whole data fork if it wasn't inline. */
> +	if (!(ifp->if_flags & XFS_IFINLINE)) {
> +		error = xfs_itruncate_extents(tpp, ip, XFS_DATA_FORK, 0);
> +		if (error)
> +			goto out;
> +	}
> +
> +	/* Blow out the in-core fork and zero the on-disk fork. */
> +	xfs_idestroy_fork(ip, XFS_DATA_FORK);
> +	ip->i_d.di_format = XFS_DINODE_FMT_EXTENTS;
> +	ip->i_d.di_nextents = 0;
> +	memset(&ip->i_df, 0, sizeof(struct xfs_ifork));
> +	ip->i_df.if_flags |= XFS_IFEXTENTS;

This looks familiar - doesn't the fork zapping code do exactly this,
too? factor into a helper?

> +
> +	/* Rewrite an inline symlink. */
> +	if (pathlen <= XFS_IFORK_DSIZE(ip)) {
> +		xfs_init_local_fork(ip, XFS_DATA_FORK, target_path, pathlen);
> +
> +		i_size_write(VFS_I(ip), pathlen);
> +		ip->i_d.di_size = pathlen;
> +		ip->i_d.di_format = XFS_DINODE_FMT_LOCAL;
> +		xfs_trans_log_inode(*tpp, ip, XFS_ILOG_DDATA | XFS_ILOG_CORE);
> +		goto out;
> +
> +	}

Might make sense to separate inline vs remote into separate
functions - we tend to do that everywhere else in the symlink code.

> +
> +	/* Rewrite a remote symlink. */
> +	fs_blocks = xfs_symlink_blocks(mp, pathlen);
> +	first_fsb = 0;
> +	nmaps = XFS_SYMLINK_MAPS;
> +
> +	/* Reserve quota for new blocks. */
> +	error = xfs_trans_reserve_quota_nblks(*tpp, ip, fs_blocks, 0,
> +			XFS_QMOPT_RES_REGBLKS);
> +	if (error)
> +		goto out;
> +
> +	/* Map blocks, write symlink target. */
> +	xfs_defer_init(&dfops, &first_block);
> +
> +	error = xfs_bmapi_write(*tpp, ip, first_fsb, fs_blocks,
> +			  XFS_BMAPI_METADATA, &first_block, fs_blocks,
> +			  mval, &nmaps, &dfops);
> +	if (error)
> +		goto out_bmap_cancel;
> +
> +	ip->i_d.di_size = pathlen;
> +	i_size_write(VFS_I(ip), pathlen);
> +	xfs_trans_log_inode(*tpp, ip, XFS_ILOG_CORE);
> +
> +	cur_chunk = target_path;
> +	offset = 0;
> +	for (n = 0; n < nmaps; n++) {
> +		char	*buf;
> +
> +		d = XFS_FSB_TO_DADDR(mp, mval[n].br_startblock);
> +		byte_cnt = XFS_FSB_TO_B(mp, mval[n].br_blockcount);
> +		bp = xfs_trans_get_buf(*tpp, mp->m_ddev_targp, d,
> +				       BTOBB(byte_cnt), 0);
> +		if (!bp) {
> +			error = -ENOMEM;
> +			goto out_bmap_cancel;
> +		}
> +		bp->b_ops = &xfs_symlink_buf_ops;
> +
> +		byte_cnt = XFS_SYMLINK_BUF_SPACE(mp, byte_cnt);
> +		byte_cnt = min(byte_cnt, pathlen);
> +
> +		buf = bp->b_addr;
> +		buf += xfs_symlink_hdr_set(mp, ip->i_ino, offset,
> +					   byte_cnt, bp);
> +
> +		memcpy(buf, cur_chunk, byte_cnt);
> +
> +		cur_chunk += byte_cnt;
> +		pathlen -= byte_cnt;
> +		offset += byte_cnt;
> +
> +		xfs_trans_buf_set_type(*tpp, bp, XFS_BLFT_SYMLINK_BUF);
> +		xfs_trans_log_buf(*tpp, bp, 0, (buf + byte_cnt - 1) -
> +						(char *)bp->b_addr);
> +	}
> +	ASSERT(pathlen == 0);

This just looks like a copynpaste of main loop in xfs_symlink() -
can you factor that into a helper, please?

> +
> +	error = xfs_defer_finish(tpp, &dfops);
> +	if (error)
> +		goto out_bmap_cancel;
> +
> +	return 0;
> +
> +out_bmap_cancel:
> +	xfs_defer_cancel(&dfops);
> +out:
> +	return error;
> +}
> +
> +/* Fix everything that fails the verifiers in the remote blocks. */
> +STATIC int
> +xfs_repair_symlink_fix_remotes(
> +	struct xfs_scrub_context	*sc,
> +	loff_t				len)
> +{
> +	struct xfs_bmbt_irec		mval[XFS_SYMLINK_MAPS];
> +	struct xfs_buf			*bp;
> +	xfs_filblks_t			fsblocks;
> +	xfs_daddr_t			d;
> +	loff_t				offset;
> +	unsigned int			byte_cnt;
> +	int				n;
> +	int				nmaps = XFS_SYMLINK_MAPS;
> +	int				nr;
> +	int				error;
> +
> +	fsblocks = xfs_symlink_blocks(sc->mp, len);
> +	error = xfs_bmapi_read(sc->ip, 0, fsblocks, mval, &nmaps, 0);
> +	if (error)
> +		return error;
> +
> +	offset = 0;
> +	for (n = 0; n < nmaps; n++) {
> +		d = XFS_FSB_TO_DADDR(sc->mp, mval[n].br_startblock);
> +		byte_cnt = XFS_FSB_TO_B(sc->mp, mval[n].br_blockcount);
> +
> +		error = xfs_trans_read_buf(sc->mp, sc->tp, sc->mp->m_ddev_targp,
> +				d, BTOBB(byte_cnt), 0, &bp, NULL);
> +		if (error)
> +			return error;
> +		bp->b_ops = &xfs_symlink_buf_ops;
> +
> +		byte_cnt = XFS_SYMLINK_BUF_SPACE(sc->mp, byte_cnt);
> +		if (len < byte_cnt)
> +			byte_cnt = len;

can we make this the same as the other functions? i.e.

		byte_cnt = min(byte_cnt, len);

> +
> +		nr = xfs_symlink_hdr_set(sc->mp, sc->ip->i_ino, offset,
> +				byte_cnt, bp);
> +
> +		len -= byte_cnt;
> +		offset += byte_cnt;
> +
> +		xfs_trans_buf_set_type(sc->tp, bp, XFS_BLFT_SYMLINK_BUF);
> +		xfs_trans_log_buf(sc->tp, bp, 0, nr - 1);
> +		xfs_trans_brelse(sc->tp, bp);

xfs_trans_brelse() is a no-op here because the buffer has been
logged. It can be removed.

> +	}
> +	if (len != 0)
> +		return -EFSCORRUPTED;
> +
> +	return 0;
> +}
> +
> +/* Fix this inline symlink. */
> +STATIC int
> +xfs_repair_symlink_inline(
> +	struct xfs_scrub_context	*sc)
> +{
> +	struct xfs_inode		*ip = sc->ip;
> +	struct xfs_ifork		*ifp;
> +	loff_t				len;
> +	size_t				newlen;
> +
> +	ifp = XFS_IFORK_PTR(ip, XFS_DATA_FORK);
> +	len = i_size_read(VFS_I(ip));
> +	xfs_trans_ijoin(sc->tp, ip, 0);
> +
> +	if (ifp->if_u1.if_data) {
> +		newlen = strnlen(ifp->if_u1.if_data, XFS_IFORK_DSIZE(ip));
> +	} else {
> +		/* Zero length symlink becomes a root symlink. */
> +		ifp->if_u1.if_data = kmem_alloc(4, KM_SLEEP);
> +		snprintf(ifp->if_u1.if_data, 4, "/");
> +		newlen = 1;

helper function shared with the fork zapping code?

> +	}
> +
> +	if (len > newlen) {

shouldn't this be 'if (len != newlen) {' ?

> +		i_size_write(VFS_I(ip), newlen);
> +		ip->i_d.di_size = newlen;
> +		xfs_trans_log_inode(sc->tp, ip, XFS_ILOG_DDATA | XFS_ILOG_CORE);
> +	}
> +
> +	return 0;
> +}
> +
> +/* Repair a remote symlink. */
> +STATIC int
> +xfs_repair_symlink_remote(
> +	struct xfs_scrub_context	*sc)
> +{
> +	struct xfs_inode		*ip = sc->ip;
> +	loff_t				len;
> +	size_t				newlen;
> +	int				error = 0;
> +
> +	len = i_size_read(VFS_I(ip));
> +	xfs_trans_ijoin(sc->tp, ip, 0);
> +
> +	error = xfs_repair_symlink_fix_remotes(sc, len);
> +	if (error)
> +		return error;
> +
> +	/* Roll transaction, release buffers. */
> +	error = xfs_trans_roll_inode(&sc->tp, ip);
> +	if (error)
> +		return error;
> +
> +	/* Size set correctly? */
> +	len = i_size_read(VFS_I(ip));
> +	xfs_iunlock(ip, XFS_ILOCK_EXCL);
> +	error = xfs_readlink(ip, sc->buf);
> +	xfs_ilock(ip, XFS_ILOCK_EXCL);

Can we pass a "need_lock" flag to xfs_readlink() rather than
creating a race condition with anything that might be blocked on the
ilock waiting for repair to complete?

> +	if (error)
> +		return error;
> +
> +	/*
> +	 * Figure out the new target length.  We can't handle zero-length
> +	 * symlinks, so make sure that we don't write that out.
> +	 */
> +	newlen = strnlen(sc->buf, XFS_SYMLINK_MAXLEN);
> +	if (newlen == 0) {
> +		*((char *)sc->buf) = '/';
> +		newlen = 1;

We really need to do set the name of the repaired path for zero
length symlinks in only one place. It really seems to me that it
should done in xfs_repair_symlink_rewrite() if newlen is 0, not
here.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com