Re: [PATCH v1] NFSD: Decode NFSv4 birth time attribute

[Chuck Lever III]

> On Jul 11, 2022, at 7:36 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> 
> On Sun, 2022-07-10 at 14:46 -0400, Chuck Lever wrote:
>> NFSD has advertised support for the NFSv4 time_create attribute
>> since commit e377a3e698fb ("nfsd: Add support for the birth time
>> attribute").
>> 
>> Igor Mammedov reports that Mac OS clients attempt to set the NFSv4
>> birth time attribute via OPEN(CREATE) and SETATTR if the server
>> indicates that it supports it, but since the above commit was
>> merged, those attempts now fail.
>> 
>> Table 5 in RFC 8881 lists the time_create attribute as one that can
>> be both set and retrieved, but the above commit did not add server
>> support for clients to provide a time_create attribute. IMO that's
>> a bug in our implementation of the NFSv4 protocol, which this commit
>> addresses.
>> 
>> Whether NFSD silently ignores the new birth time or actually sets it
>> is another matter. I haven't found another filesystem service in the
>> Linux kernel that enables users or clients to modify a file's birth
>> time attribute.
>> 
>> This commit reflects my (perhaps incorrect) understanding of whether
>> Linux users can set a file's birth time. NFSD will now recognize a
>> time_create attribute but it ignores its value. It clears the
>> time_create bit in the returned attribute bitmask to indicate that
>> the value was not used.
>> 
>> Reported-by: Igor Mammedov <imammedo@xxxxxxxxxx>
>> Fixes: e377a3e698fb ("nfsd: Add support for the birth time attribute")
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>> fs/nfsd/nfs4xdr.c |    9 +++++++++
>> fs/nfsd/nfsd.h    |    3 ++-
>> 2 files changed, 11 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
>> index 61b2aae81abb..2acea7792bb2 100644
>> --- a/fs/nfsd/nfs4xdr.c
>> +++ b/fs/nfsd/nfs4xdr.c
>> @@ -470,6 +470,15 @@ nfsd4_decode_fattr4(struct nfsd4_compoundargs *argp, u32 *bmval, u32 bmlen,
>> 			return nfserr_bad_xdr;
>> 		}
>> 	}
>> +	if (bmval[1] & FATTR4_WORD1_TIME_CREATE) {
>> +		struct timespec64 ts;
>> +
>> +		/* No Linux filesystem supports setting this attribute. */
>> +		bmval[1] &= ~FATTR4_WORD1_TIME_CREATE;
>> +		status = nfsd4_decode_nfstime4(argp, &ts);
>> +		if (status)
>> +			return status;
>> +	}
>> 	if (bmval[1] & FATTR4_WORD1_TIME_MODIFY_SET) {
>> 		u32 set_it;
>> 
>> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
>> index 847b482155ae..9a8b09afc173 100644
>> --- a/fs/nfsd/nfsd.h
>> +++ b/fs/nfsd/nfsd.h
>> @@ -465,7 +465,8 @@ static inline bool nfsd_attrs_supported(u32 minorversion, const u32 *bmval)
>> 	(FATTR4_WORD0_SIZE | FATTR4_WORD0_ACL)
>> #define NFSD_WRITEABLE_ATTRS_WORD1 \
>> 	(FATTR4_WORD1_MODE | FATTR4_WORD1_OWNER | FATTR4_WORD1_OWNER_GROUP \
>> -	| FATTR4_WORD1_TIME_ACCESS_SET | FATTR4_WORD1_TIME_MODIFY_SET)
>> +	| FATTR4_WORD1_TIME_ACCESS_SET | FATTR4_WORD1_TIME_CREATE \
>> +	| FATTR4_WORD1_TIME_MODIFY_SET)
>> #ifdef CONFIG_NFSD_V4_SECURITY_LABEL
>> #define MAYBE_FATTR4_WORD2_SECURITY_LABEL \
>> 	FATTR4_WORD2_SECURITY_LABEL
>> 
>> 
> 
> RFC5661 lists time_create as being writeable, so silently ignoring it
> seems wrong.

Open for debate. The protocol does allow a SETATTR. But the
specification doesn't have much else to say about the semantics
of time_create; contrast that with mtime or ctime.


> It seems like we ought to have nfsd attempt to set the
> btime and then just return an error if it doesn't work...

The usual way the NFSv4 protocol handles this is that the
attribute's bit in the returned bitmask is cleared, so that
the rest of the COMPOUND is able to succeed. There's no
NFS4ERR status code in this case.


> but, I don't
> see a mechanism in the kernel for setting it. ATTR_BTIME doesn't exist,
> for instance.

That's what I observed: there doesn't seem to be a mechanism in
Linux for setting it. Perhaps I should have copied fsdevel.


> Still, since we can't set it, returning an error there seems more
> correct. NFS4ERR_INVAL is probably the wrong one -- maybe
> NFS4ERR_NOTSUPP ? It's a bit weird since we do support querying it, but
> not setting it. Maybe we need to propose a new NFS4ERR_ATTR_RO ?

As I said above, the protocol's way of dealing with it is to
clear the attribute's bit in the returned attribute bitmask.
"You asked me to set this attribute, but I didn't". Clients,
IMO, will be more prepared to deal with that than having
all of their OPENs fail with NFS4ERR_NOTSUPP.

IMO explicitly setting a file's birth time doesn't seem quite
kosher, and it's not a POSIX attribute anyway, so we don't
have a standard to cleave to here (at least one that I'm aware
of). I'm fine with the patch as it stands, but I'm open to
hear more opinions about this.


--
Chuck Lever