Re: [RFC PATCH] fs: Plumb case sensitivity bits into statx

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-09-25 at 11:11 -0400, Chuck Lever wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> Both the NFSv3 and NFSv4 protocols enable NFS clients to query NFS
> servers about the case sensitivity and case preservation behaviors
> of shared file systems. Today, the Linux NFSD implementation
> unconditionally returns "the export is case sensitive and case
> preserving".
> 
> However, a few Linux in-tree file system types appear to have some
> ability to handle case-folded filenames. Some of our users would
> like to exploit that functionality from their non-POSIX NFS clients.
> 
> Enable upper layers such as NFSD to retrieve case sensitivity
> information from file systems by adding a statx API for this
> purpose. Introduce a sample producer and a sample consumer for this
> information.
> 
> If this mechanism seems sensible, a future patch might add a similar
> field to the user-space-visible statx structure. User-space file
> servers already use a variety of APIs to acquire this information.
> 
> Suggested-by: Jeff Layton <jlayton@kernel.org>
> Cc: Volker Lendecke <Volker.Lendecke@sernet.de>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/fat/file.c             |  5 +++++
>  fs/nfsd/nfs3proc.c        | 35 +++++++++++++++++++++++++++--------
>  include/linux/stat.h      |  1 +
>  include/uapi/linux/stat.h | 15 +++++++++++++++
>  4 files changed, 48 insertions(+), 8 deletions(-)
> 
> I'm certain this RFC patch has a number of problems, but it should
> serve as a discussion point.
> 
> 
> diff --git a/fs/fat/file.c b/fs/fat/file.c
> index 4fc49a614fb8..8572e36d8f27 100644
> --- a/fs/fat/file.c
> +++ b/fs/fat/file.c
> @@ -413,6 +413,11 @@ int fat_getattr(struct mnt_idmap *idmap, const struct path *path,
>  		stat->result_mask |= STATX_BTIME;
>  		stat->btime = MSDOS_I(inode)->i_crtime;
>  	}
> +	if (request_mask & STATX_CASE_INFO) {
> +		stat->result_mask |= STATX_CASE_INFO;
> +		/* STATX_CASE_PRESERVING is cleared */
> +		stat->case_info = statx_case_ascii;
> +	}
>  
>  	return 0;
>  }
> diff --git a/fs/nfsd/nfs3proc.c b/fs/nfsd/nfs3proc.c
> index b6d03e1ef5f7..b319d1c4385c 100644
> --- a/fs/nfsd/nfs3proc.c
> +++ b/fs/nfsd/nfs3proc.c
> @@ -697,6 +697,31 @@ nfsd3_proc_fsinfo(struct svc_rqst *rqstp)
>  	return rpc_success;
>  }
>  
> +static __be32
> +nfsd3_proc_case(struct svc_fh *fhp, struct nfsd3_pathconfres *resp)
> +{
> +	struct path p = {
> +		.mnt		= fhp->fh_export->ex_path.mnt,
> +		.dentry		= fhp->fh_dentry,
> +	};
> +	u32 request_mask = STATX_CASE_INFO;
> +	struct kstat stat;
> +	__be32 nfserr;
> +
> +	nfserr = nfserrno(vfs_getattr(&p, &stat, request_mask,
> +				      AT_STATX_SYNC_AS_STAT));
> +	if (nfserr != nfs_ok)
> +		return nfserr;
> +	if (!(stat.result_mask & STATX_CASE_INFO))
> +		return nfs_ok;
> +
> +	resp->p_case_insensitive =
> +		stat.case_info & STATX_CASE_FOLDING_TYPE ? 0 : 1;
> +	resp->p_case_preserving =
> +		stat.case_info & STATX_CASE_PRESERVING ? 1 : 0;
> +	return nfs_ok;
> +}
> +
>  /*
>   * Get pathconf info for the specified file
>   */
> @@ -722,17 +747,11 @@ nfsd3_proc_pathconf(struct svc_rqst *rqstp)
>  	if (resp->status == nfs_ok) {
>  		struct super_block *sb = argp->fh.fh_dentry->d_sb;
>  
> -		/* Note that we don't care for remote fs's here */
> -		switch (sb->s_magic) {
> -		case EXT2_SUPER_MAGIC:
> +		if (sb->s_magic == EXT2_SUPER_MAGIC) {
>  			resp->p_link_max = EXT2_LINK_MAX;
>  			resp->p_name_max = EXT2_NAME_LEN;
> -			break;
> -		case MSDOS_SUPER_MAGIC:
> -			resp->p_case_insensitive = 1;
> -			resp->p_case_preserving  = 0;
> -			break;
>  		}
> +		resp->status = nfsd3_proc_case(&argp->fh, resp);
>  	}
>  
>  	fh_put(&argp->fh);
> diff --git a/include/linux/stat.h b/include/linux/stat.h
> index e3d00e7bb26d..abb47cbb233a 100644
> --- a/include/linux/stat.h
> +++ b/include/linux/stat.h
> @@ -59,6 +59,7 @@ struct kstat {
>  	u32		atomic_write_unit_max;
>  	u32		atomic_write_unit_max_opt;
>  	u32		atomic_write_segments_max;
> +	u32		case_info;
>  };
>  
>  /* These definitions are internal to the kernel for now. Mainly used by nfsd. */
> diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> index 1686861aae20..e929b30d64b6 100644
> --- a/include/uapi/linux/stat.h
> +++ b/include/uapi/linux/stat.h
> @@ -219,6 +219,7 @@ struct statx {
>  #define STATX_SUBVOL		0x00008000U	/* Want/got stx_subvol */
>  #define STATX_WRITE_ATOMIC	0x00010000U	/* Want/got atomic_write_* fields */
>  #define STATX_DIO_READ_ALIGN	0x00020000U	/* Want/got dio read alignment info */
> +#define STATX_CASE_INFO		0x00040000U	/* Want/got case folding info */
> 
> 

Do you intend to expose this new attribute to userland? If not, then it
should probably snuggle up next to STATX_CHANGE_COOKIE. If so, then you
need to claim a field for it in struct statx, and populate it.

I'll also note that you're not using a lot of the bits in this field,
and struct statx has a 16-bit spare field next to stx_mode. You may
want to rework this to use a 16 bit field and expose it to userland
there.

>  
>  #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
>  
> @@ -257,4 +258,18 @@ struct statx {
>  #define STATX_ATTR_WRITE_ATOMIC		0x00400000 /* File supports atomic write operations */
>  
>  
> +/*
> + * File system support for case folding is available via a bitmap.
> + */
> +#define STATX_CASE_PRESERVING		0x80000000 /* File name case is preserved */
> +
> +/* Values stored in the low-order byte of .case_info */
> +enum {
> +	statx_case_sensitive = 0,
> +	statx_case_ascii,
> +	statx_case_utf8,
> +	statx_case_utf16,
> +};

These all need _very_ clear definitions, especially if you're exposing
this to userland. The devil is in the details when it comes to
casefolding and there is not a lot of consistency across filesystems.
The callers need to know what they can expect when they see that these
bits are set.

> +#define STATX_CASE_FOLDING_TYPE		0x000000ff
> +
>  #endif /* _UAPI_LINUX_STAT_H */
 
-- 
Jeff Layton <jlayton@kernel.org>