Re: [PATCH RFC] nfsd: don't hand out write delegations on O_WRONLY opens

NeilBrown
2023-07-31T22:11:34Z

On Tue, 01 Aug 2023, Jeff Layton wrote:
> I noticed that xfstests generic/001 was failing against linux-next nfsd.
>
> The client would request a OPEN4_SHARE_ACCESS_WRITE open, and the server
> would hand out a write delegation. The client would then try to use that
> write delegation as the source stateid in a COPY or CLONE operation, and
> the server would respond with NFS4ERR_STALE.
>
> The problem is that the struct file associated with the delegation does
> not necessarily have read permissions. It's handing out a write
> delegation on what is effectively an O_WRONLY open. RFC 8881 states:
>
> "An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its
> own, all opens."
>
> Given that the client didn't request any read permissions, and that nfsd
> didn't check for any, it seems wrong to give out a write delegation.
>
> Don't hand out a delegation if the client didn't request
> OPEN4_SHARE_ACCESS_BOTH.
>
> This fixes xfstest generic/001.
>
> Closes: https://bugzilla.linux-nfs.org/show_bug.cgi?id=412
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> fs/nfsd/nfs4state.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index ef7118ebee00..9f1c90afed72 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -5462,6 +5462,8 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> return ERR_PTR(-EAGAIN);
>
> if (open->op_share_access & NFS4_SHARE_ACCESS_WRITE) {
> + if (!(open->op_share_access & NFS4_SHARE_ACCESS_READ))
> + return ERR_PTR(-EBADF);
<bikeshed>
The actual error code returned by nfs4_set_delegation() is ignored -
only the fact of an error is relevant.
Given that, how did you choose -EBADF. nfsd doesn't use file
descriptors, and doesn't use EBADF anywhere else.
Given that you have just tested access, EACCES might be justifiable.
But I would prefer if nfs4_set_delegation() returns NULL if it could not
find or create a delegation, without bothering with giving a reason.
</bikeshed>

Reviewed-by: NeilBrown <neilb@suse.de>

NeilBrown

> nf = find_writeable_file(fp);
> dl_type = NFS4_OPEN_DELEGATE_WRITE;
> } else {
>
> ---
> base-commit: ec89391563792edd11d138a853901bce76d11f44
> change-id: 20230731-wdeleg-bbdb6b25a3c6
>
> Best regards,
> --
> Jeff Layton <jlayton@kernel.org>
>
>