Re: [PATCH 1/7] exportfs: split out the ops for layout-based block device access

["Chuck Lever" <cel@kernel.org>]

On Mon, Mar 23, 2026, at 3:07 AM, Christoph Hellwig wrote:

> diff --git a/fs/nfsd/nfs4layouts.c b/fs/nfsd/nfs4layouts.c
> index ad7af8cfcf1f..616984fe3873 100644
> --- a/fs/nfsd/nfs4layouts.c
> +++ b/fs/nfsd/nfs4layouts.c
> @@ -3,6 +3,7 @@
>   * Copyright (c) 2014 Christoph Hellwig.
>   */
>  #include <linux/blkdev.h>
> +#include <linux/exportfs_block.h>
>  #include <linux/kmod.h>
>  #include <linux/file.h>
>  #include <linux/jhash.h>
> @@ -127,6 +128,7 @@ void nfsd4_setup_layout_type(struct svc_export *exp)
>  {
>  #if defined(CONFIG_NFSD_BLOCKLAYOUT) || defined(CONFIG_NFSD_SCSILAYOUT)
>  	struct super_block *sb = exp->ex_path.mnt->mnt_sb;
> +	struct exportfs_block_ops *bops = sb->s_export_op->block_ops;
>  #endif
> 
>  	if (!(exp->ex_flags & NFSEXP_PNFS))
> @@ -136,14 +138,11 @@ void nfsd4_setup_layout_type(struct svc_export *exp)
>  	exp->ex_layout_types |= 1 << LAYOUT_FLEX_FILES;
>  #endif
>  #ifdef CONFIG_NFSD_BLOCKLAYOUT
> -	if (sb->s_export_op->get_uuid &&
> -	    sb->s_export_op->map_blocks &&
> -	    sb->s_export_op->commit_blocks)
> +	if (bops->get_uuid && bops->map_blocks && bops->commit_blocks)
>  		exp->ex_layout_types |= 1 << LAYOUT_BLOCK_VOLUME;
>  #endif
>  #ifdef CONFIG_NFSD_SCSILAYOUT
> -	if (sb->s_export_op->map_blocks &&
> -	    sb->s_export_op->commit_blocks &&
> +	if (bops->map_blocks && bops->commit_blocks &&
>  	    sb->s_bdev &&
>  	    sb->s_bdev->bd_disk->fops->pr_ops &&
>  	    sb->s_bdev->bd_disk->fops->get_unique_id)

block_ops itself is NULL for any filesystem that does not provide
block layout support (everything other than XFS today).

When an admin exports such a filesystem with pNFS enabled
(NFSEXP_PNFS), svc_export_parse() calls nfsd4_setup_layout_type(),
and bops->get_uuid dereferences a NULL pointer.

Something like the following would restore the original behavior:

    if (bops && bops->get_uuid && ...)


-- 
Chuck Lever