NFSD/blocklayout: Fix minlength check in proc_layoutget

The extent returned by the file system may have a smaller offset than
the segment offset requested by the client. In this case, the minimum
segment length must be checked against the requested range. Otherwise,
the client may not be able to continue the read/write operation.

Fixes: 8650b8a ("nfsd: pNFS block layout driver")
Signed-off-by: Sergey Bashirov <sergeybashirov@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: sbashiro

fs/nfsd/blocklayout.c

@@ -23,6 +23,7 @@ nfsd4_block_proc_layoutget(struct svc_rqst *rqstp, struct inode *inode,
 {
 	struct nfsd4_layout_seg *seg = &args->lg_seg;
 	struct super_block *sb = inode->i_sb;
+	u64 length;
 	u32 block_size = i_blocksize(inode);
 	struct pnfs_block_extent *bex;
 	struct iomap iomap;
@@ -56,7 +57,8 @@ nfsd4_block_proc_layoutget(struct svc_rqst *rqstp, struct inode *inode,
 		goto out_error;
 	}
 
-	if (iomap.length < args->lg_minlength) {
+	length = iomap.offset + iomap.length - seg->offset;
+	if (length < args->lg_minlength) {
 		dprintk("pnfsd: extent smaller than minlength\n");
 		goto out_layoutunavailable;
 	}