Re: [PATCH v2] nfsd: don't hand out write delegations on O_WRONLY opens

NeilBrown
2023-08-01T22:26:27Z

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.
>
> Only hand out a write delegation if we have a O_RDWR descriptor
> available. If it fails to find an appropriate write descriptor, go
> ahead and try for a read delegation if NFS4_SHARE_ACCESS_READ was
> requested.
>
> This fixes xfstest generic/001.
>
> Closes: https://bugzilla.linux-nfs.org/show_bug.cgi?id=412
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> Changes in v2:
> - Rework the logic when finding struct file for the delegation. The
> earlier patch might still have attached a O_WRONLY file to the deleg
> in some cases, and could still have handed out a write delegation on
> an O_WRONLY OPEN request in some cases.
> ---
> fs/nfsd/nfs4state.c | 29 ++++++++++++++++++-----------
> 1 file changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index ef7118ebee00..e79d82fd05e7 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -5449,7 +5449,7 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> struct nfs4_file *fp = stp->st_stid.sc_file;
> struct nfs4_clnt_odstate *odstate = stp->st_clnt_odstate;
> struct nfs4_delegation *dp;
> - struct nfsd_file *nf;
> + struct nfsd_file *nf = NULL;
> struct file_lock *fl;
> u32 dl_type;
>
> @@ -5461,21 +5461,28 @@ nfs4_set_delegation(struct nfsd4_open *open, struct nfs4_ol_stateid *stp,
> if (fp->fi_had_conflict)
> return ERR_PTR(-EAGAIN);
>
> - if (open->op_share_access & NFS4_SHARE_ACCESS_WRITE) {
> - nf = find_writeable_file(fp);
> + /*
> + * Try for a write delegation first. We need an O_RDWR file
> + * since a write delegation allows the client to perform any open
> + * from its cache.
> + */
> + if ((open->op_share_access & NFS4_SHARE_ACCESS_BOTH) == NFS4_SHARE_ACCESS_BOTH) {
> + nf = nfsd_file_get(fp->fi_fds[O_RDWR]);

This doesn't take fp->fi_lock before accessing ->fi_fds[], while the
find_readable_file() call below does. This inconsistency suggests a
bug?

Maybe the provided API is awkward. Should we have
find_suitable_file() and find_suitable_file_locked()
that gets passed an nfs4_file and an O_MODE?
It tries the given mode, then O_RDWR

NeilBrown

> dl_type = NFS4_OPEN_DELEGATE_WRITE;
> - } else {
> + }
> +
> + /*
> + * If the file is being opened O_RDONLY or we couldn't get a O_RDWR
> + * file for some reason, then try for a read deleg instead.
> + */
> + if (!nf && (open->op_share_access & NFS4_SHARE_ACCESS_READ)) {
> nf = find_readable_file(fp);
> dl_type = NFS4_OPEN_DELEGATE_READ;
> }
> - if (!nf) {
> - /*
> - * We probably could attempt another open and get a read
> - * delegation, but for now, don't bother until the
> - * client actually sends us one.
> - */
> +
> + if (!nf)
> return ERR_PTR(-EAGAIN);
> - }
> +
> spin_lock(&state_lock);
> spin_lock(&fp->fi_lock);
> if (nfs4_delegation_exists(clp, fp))
>
> ---
> base-commit: a734662572708cf062e974f659ae50c24fc1ad17
> change-id: 20230731-wdeleg-bbdb6b25a3c6
>
> Best regards,
> --
> Jeff Layton <jlayton@kernel.org>
>
>