Re: [PATCH v4 6/9] xfs: clamp the pNFS layout range to the maximum file size

["Pankaj Raghav (Samsung)" <pankaj.raghav@linux.dev>]

On Tue, Oct 20, 2026 at 12:08:49PM +0900, Daejun Park wrote:
> xfs_fs_map_blocks() checks the range of a layout with
> 
> 	if (offset > limit)
> 		goto out_unlock;
> 	if (offset > limit - length)
> 		length = limit - offset;
> 
> limit - length is computed in u64, so a length larger than the limit
> wraps and the length is not clamped. nfsd passes the LAYOUTGET range
> through, and RFC 8881 lets a client ask for NFS4_UINT64_MAX bytes, for
> the rest of the file. offset + length then wraps to offset - 1, end_fsb
> rounds back to offset_fsb, and xfs_bmapi_read() maps nothing. For a read
> layout, xfs_bmbt_to_iomap() then gets an uninitialized imap: with a
> zeroed stack it logs "Access to block zero", marks the data fork sick
> and fails with -EFSCORRUPTED, and otherwise the mapping is whatever was
> on the stack and can reach the client. For a write layout,
> xfs_iomap_write_direct() is called with a count of zero, which trips an
> ASSERT with XFS_DEBUG. An offset of 2^63 or more, which nfsd accepts
> too, becomes negative in the loff_t argument and passes the first check.
> 
> Reject a negative offset and an offset at the limit, and compare the
> length with what is left up to the limit, which cannot wrap once the
> offset is below it.
> 
> With pynfs as the client, a READ LAYOUTGET for NFS4_UINT64_MAX bytes at
> offset 0 got NFS4ERR_IO, as did an RW one on an empty file, which also
> made nfsd warn
> about the non-standard errno -63, and a READ LAYOUTGET at offset 2^63
> got NFS4ERR_IO as well. With this patch and the previous one, the one at
> 2^63 gets NFS4ERR_INVAL, the RW one gets NFS4ERR_NOSPC after nfsd has
> allocated one extent after the other until fewer than
> XFS_MAX_BMBT_EXTLEN blocks were free, which left less than 8 GiB of the
> 32 GiB free, and the server logs nothing. The READ one gets NFS4ERR_INVAL
> as well: nfsd, which maps a layout one extent at a time since
> commit cc6c40e09d7b ("NFSD/blocklayout: Support multiple extents per
> LAYOUTGET"), asks again from the maximum file size on. An RW one on a
> file whose first 64 KiB are allocated, with a loga_maxcount that has
> room for one extent, is granted 0+65536.
> 
> This depends on the previous patch. Without it, an RW LAYOUTGET to the
> end of the file, whose length is now clamped instead of wrapping, asks
> xfs_iomap_write_direct() for every block up to the maximum file size,
> which wraps the block reservation and shuts the filesystem down where it
> got NFS4ERR_IO before.
> 
> Fixes: 527851124d10 ("xfs: implement pNFS export operations")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daejun Park <daejun7.park@samsung.com>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> ---

Looks good,
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
-- 
Pankaj