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

dai.ngo
2023-08-02T21:26:55Z

On 8/2/23 2:13 PM, Jeff Layton wrote:
> On Wed, 2023-08-02 at 20:57 +0000, Chuck Lever III wrote:
>>> On Aug 2, 2023, at 4:48 PM, Jeff Layton <jlayton@kernel.org> wrote:
>>>
>>> On Wed, 2023-08-02 at 13:15 -0700, dai.ngo@oracle.com wrote:
>>>> On 8/2/23 11:15 AM, Jeff Layton wrote:
>>>>> On Wed, 2023-08-02 at 09:29 -0700, dai.ngo@oracle.com wrote:
>>>>>> On 8/1/23 6:33 AM, 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
>>>>>> not sure why the client opens the source file of a COPY operation with
>>>>>> OPEN4_SHARE_ACCESS_WRITE?
>>>>>>
>>>>> It doesn't. The original open is to write the data for the file being
>>>>> copied. It then opens the file again for READ, but since it has a write
>>>>> delegation, it doesn't need to talk to the server at all -- it can just
>>>>> use that stateid for later operations.
>>>>>
>>>>>>> or CLONE operation, and
>>>>>>> the server would respond with NFS4ERR_STALE.
>>>>>> If the server does not allow client to use write delegation for the
>>>>>> READ, should the correct error return be NFS4ERR_OPENMODE?
>>>>>>
>>>>> The server must allow the client to use a write delegation for read
>>>>> operations. It's required by the spec, AFAIU.
>>>>>
>>>>> The error in this case was just bogus. The vfs copy routine would return
>>>>> -EBADF since the file didn't have FMODE_READ, and the nfs server would
>>>>> translate that into NFS4ERR_STALE.
>>>>>
>>>>> Probably there is a better v4 error code that we could translate EBADF
>>>>> to, but with this patch it shouldn't be a problem any longer.
>>>>>
>>>>>>> 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]);
>>>>>>> dl_type = NFS4_OPEN_DELEGATE_WRITE;
>>>>>>> - } else {
>>>>>> Does this mean OPEN4_SHARE_ACCESS_WRITE do not get a write delegation?
>>>>>> It does not seem right.
>>>>>>
>>>>>> -Dai
>>>>>>
>>>>> Why? Per RFC 8881:
>>>>>
>>>>> "An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its
>>>>> own, all opens."
>>>>>
>>>>> All opens. That includes read opens.
>>>>>
>>>>> An OPEN4_SHARE_ACCESS_WRITE open will succeed on a file to which the
>>>>> user has no read permissions. Therefore, we can't grant a write
>>>>> delegation since can't guarantee that the user is allowed to do that.
>>>> If the server grants the write delegation on an OPEN with
>>>> OPEN4_SHARE_ACCESS_WRITE on the file with WR-only access mode then
>>>> why can't the server checks and denies the subsequent READ?
>>>>
>>>> Per RFC 8881, section 9.1.2:
>>>>
>>>> For delegation stateids, the access mode is based on the type of
>>>> delegation.
>>>>
>>>> When a READ, WRITE, or SETATTR (that specifies the size attribute)
>>>> operation is done, the operation is subject to checking against the
>>>> access mode to verify that the operation is appropriate given the
>>>> stateid with which the operation is associated.
>>>>
>>>> In the case of WRITE-type operations (i.e., WRITEs and SETATTRs that
>>>> set size), the server MUST verify that the access mode allows writing
>>>> and MUST return an NFS4ERR_OPENMODE error if it does not. In the case
>>>> of READ, the server may perform the corresponding check on the access
>>>> mode, or it may choose to allow READ on OPENs for OPEN4_SHARE_ACCESS_WRITE,
>>>> to accommodate clients whose WRITE implementation may unavoidably do
>>>> reads (e.g., due to buffer cache constraints). However, even if READs
>>>> are allowed in these circumstances, the server MUST still check for
>>>> locks that conflict with the READ (e.g., another OPEN specified
>>>> OPEN4_SHARE_DENY_READ or OPEN4_SHARE_DENY_BOTH). Note that a server
>>>> that does enforce the access mode check on READs need not explicitly
>>>> check for conflicting share reservations since the existence of OPEN
>>>> for OPEN4_SHARE_ACCESS_READ guarantees that no conflicting share
>>>> reservation can exist.
>>>>
>>>> FWIW, The Solaris server grants write delegation on OPEN with
>>>> OPEN4_SHARE_ACCESS_WRITE on file with access mode either RW or
>>>> WR-only. Maybe this is a bug? or the spec is not clear?
>>>>
>>> I don't think that's necessarily a bug.
>>>
>>> It's not that the spec demands that we only hand out delegations on BOTH
>>> opens. This is more of a quirk of the Linux implementation. Linux'
>>> write delegations require an open O_RDWR file descriptor because we may
>>> be called upon to do a read on its behalf.
>>>
>>> Technically, we could probably just have it check for
>>> OPEN4_SHARE_ACCESS_WRITE, but in the case where READ isn't also set,
>>> then you're unlikely to get a delegation. Either the O_RDWR descriptor
>>> will be NULL, or there are other, conflicting opens already present.
>>>
>>> Solaris may have a completely different design that doesn't require
>>> this. I haven't looked at its code to know.
>> I'm comfortable for now with not handing out write delegations for
>> SHARE_ACCESS_WRITE opens. I prefer that to permission checking on
>> every READ operation.
>>
>> If we find that it's a significant performance issue, we can revisit.
>>
>>
> Yeah. The thing to remember here is that delegations are optional. We
> can always say no.
>
> One thing we could consider to allow this is trying to open O_RDWR
> first, and then only fall back to doing an O_WRONLY open if that fails.
> I'm not sure how that would work out in practice though.
>
> One thing that'd be interesting to know with Solaris (and maybe OnTap)
> is whether they will give you a write delegation for an O_WRONLY open
> when you don't have any read permissions on the file.

Yes, the Solaris server does hand out the write delegation for a
O_WRONLY open on a file with write only permission for the owner.

-Dai

>
> If they do, then is the client expected to do permissions enforcement
> for the cached open and reject local openers for read? I guess I ought
> to be looking at the Linux client code for this...
>
>>>> It'd would be interesting to know how ONTAP server behaves in
>>>> this scenario.
>>>>
>>> Indeed. Most likely it behaves more like Solaris does, but it'd nice to
>>> know.
>>>
>>>>>
>>>>>>> + }
>>>>>>> +
>>>>>>> + /*
>>>>>>> + * 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>
>> --
>> Chuck Lever
>>
>>