Re: [PATCH v3 2/2] nfsd: do not allow exporting of special kernel filesystems

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

On Wed, Jan 28, 2026, at 6:16 AM, Amir Goldstein wrote:
> pidfs and nsfs recently gained support for encode/decode of file handles
> via name_to_handle_at(2)/opan_by_handle_at(2).

s/opan/open

One more below:


> These special kernel filesystems have custom ->open() and ->permission()
> export methods, which nfsd does not respect and it was never meant to be
> used for exporting those filesystems by nfsd.
>
> Therefore, do not allow nfsd to export filesystems with custom ->open()
> or ->permission() methods.
>
> Fixes: b3caba8f7a34a ("pidfs: implement file handle support")
> Fixes: 5222470b2fbb3 ("nsfs: support file handles")
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/nfsd/export.c         | 8 +++++---
>  include/linux/exportfs.h | 9 +++++++++
>  2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index 2a1499f2ad196..09fe268fe2c76 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -427,7 +427,8 @@ static int check_export(const struct path *path, 
> int *flags, unsigned char *uuid
>  	 *       either a device number (so FS_REQUIRES_DEV needed)
>  	 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
>  	 * 2:  We must be able to find an inode from a filehandle.
> -	 *       This means that s_export_op must be set.
> +	 *       This means that s_export_op must be set and comply with
> +	 *       the requirements for remote filesystem export.
>  	 * 3: We must not currently be on an idmapped mount.
>  	 */
>  	if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
> @@ -437,8 +438,9 @@ static int check_export(const struct path *path, 
> int *flags, unsigned char *uuid
>  		return -EINVAL;
>  	}
> 
> -	if (!exportfs_can_decode_fh(inode->i_sb->s_export_op)) {
> -		dprintk("exp_export: export of invalid fs type.\n");
> +	if (!exportfs_may_export(inode->i_sb->s_export_op)) {
> +		dprintk("exp_export: export of invalid fs type (%s).\n",
> +			inode->i_sb->s_type->name);
>  		return -EINVAL;
>  	}
> 
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index fafd22ed4c648..bf3dee2ad5f97 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -340,6 +340,15 @@ static inline bool exportfs_can_decode_fh(const 
> struct export_operations *nop)
>  	return nop && nop->fh_to_dentry;
>  }
> 
> +static inline bool exportfs_may_export(const struct export_operations *nop)
> +{
> +	/*
> +	 * Do not allow nfs export for filesystems with custom ->open() and
> +	 * ->permission() ops, which nfsd does not respect (e.g. pidfs, nsfs).
> +	 */

The comment says "with custom ->open() and ->permission() ops" but the
code blocks export if either one is set. The commit message correctly
says "or" - should the comment be updated to match?


> +	return exportfs_can_decode_fh(nop) && !nop->open && !nop->permission;
> +}
> +
>  static inline bool exportfs_can_encode_fh(const struct export_operations *nop,
>  					  int fh_flags)
>  {
> -- 
> 2.52.0

-- 
Chuck Lever