Re: [PATCH] NFS: Fix mode bits and nlink count for v4 referral dirs

[Chuck Lever]

> On Oct 15, 2020, at 8:06 AM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote:
> 
> On Thu, 2020-10-15 at 00:39 +0530, Ashish Sangwan wrote:
>> On Wed, Oct 14, 2020 at 11:47 PM Trond Myklebust
>> <trondmy@xxxxxxxxxxxxxxx> wrote:
>>> On Tue, 2020-10-06 at 08:14 -0700, Ashish Sangwan wrote:
>>>> Request for mode bits and nlink count in the nfs4_get_referral
>>>> call
>>>> and if server returns them use them instead of hard coded values.
>>>> 
>>>> CC: stable@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Ashish Sangwan <ashishsangwan2@xxxxxxxxx>
>>>> ---
>>>> fs/nfs/nfs4proc.c | 20 +++++++++++++++++---
>>>> 1 file changed, 17 insertions(+), 3 deletions(-)
>>>> 
>>>> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
>>>> index 6e95c85fe395..efec05c5f535 100644
>>>> --- a/fs/nfs/nfs4proc.c
>>>> +++ b/fs/nfs/nfs4proc.c
>>>> @@ -266,7 +266,9 @@ const u32 nfs4_fs_locations_bitmap[3] = {
>>>>      | FATTR4_WORD0_FSID
>>>>      | FATTR4_WORD0_FILEID
>>>>      | FATTR4_WORD0_FS_LOCATIONS,
>>>> -     FATTR4_WORD1_OWNER
>>>> +     FATTR4_WORD1_MODE
>>>> +     | FATTR4_WORD1_NUMLINKS
>>>> +     | FATTR4_WORD1_OWNER
>>>>      | FATTR4_WORD1_OWNER_GROUP
>>>>      | FATTR4_WORD1_RAWDEV
>>>>      | FATTR4_WORD1_SPACE_USED
>>>> @@ -7594,16 +7596,28 @@ nfs4_listxattr_nfs4_user(struct inode
>>>> *inode,
>>>> char *list, size_t list_len)
>>>>  */
>>>> static void nfs_fixup_referral_attributes(struct nfs_fattr
>>>> *fattr)
>>>> {
>>>> +     bool fix_mode = true, fix_nlink = true;
>>>> +
>>>>      if (!(((fattr->valid & NFS_ATTR_FATTR_MOUNTED_ON_FILEID) ||
>>>>             (fattr->valid & NFS_ATTR_FATTR_FILEID)) &&
>>>>            (fattr->valid & NFS_ATTR_FATTR_FSID) &&
>>>>            (fattr->valid & NFS_ATTR_FATTR_V4_LOCATIONS)))
>>>>              return;
>>>> 
>>>> +     if (fattr->valid & NFS_ATTR_FATTR_MODE)
>>>> +             fix_mode = false;
>>>> +     if (fattr->valid & NFS_ATTR_FATTR_NLINK)
>>>> +             fix_nlink = false;
>>>>      fattr->valid |= NFS_ATTR_FATTR_TYPE | NFS_ATTR_FATTR_MODE |
>>>>              NFS_ATTR_FATTR_NLINK | NFS_ATTR_FATTR_V4_REFERRAL;
>>>> -     fattr->mode = S_IFDIR | S_IRUGO | S_IXUGO;
>>>> -     fattr->nlink = 2;
>>>> +
>>>> +     if (fix_mode)
>>>> +             fattr->mode = S_IFDIR | S_IRUGO | S_IXUGO;
>>>> +     else
>>>> +             fattr->mode |= S_IFDIR;
>>>> +
>>>> +     if (fix_nlink)
>>>> +             fattr->nlink = 2;
>>>> }
>>>> 
>>>> static int _nfs4_proc_fs_locations(struct rpc_clnt *client,
>>>> struct
>>>> inode *dir,
>>> 
>>> NACK to this patch. The whole point is that if the server has a
>>> referral, then it is not going to give us any attributes other than
>>> the
>>> ones we're already asking for because it may not even have a real
>>> directory. The client is required to fake up an inode, hence the
>>> existing code.
>> 
>> Hi Trond, thanks for reviewing the patch!
>> Sorry but I didn't understand the reason to NACK it. Could you please
>> elaborate your concern?
>> These are the current attributes we request from the server on a
>> referral:
>> FATTR4_WORD0_CHANGE
>>> FATTR4_WORD0_SIZE
>>> FATTR4_WORD0_FSID
>>> FATTR4_WORD0_FILEID
>>> FATTR4_WORD0_FS_LOCATIONS,
>> FATTR4_WORD1_OWNER
>>> FATTR4_WORD1_OWNER_GROUP
>>> FATTR4_WORD1_RAWDEV
>>> FATTR4_WORD1_SPACE_USED
>>> FATTR4_WORD1_TIME_ACCESS
>>> FATTR4_WORD1_TIME_METADATA
>>> FATTR4_WORD1_TIME_MODIFY
>>> FATTR4_WORD1_MOUNTED_ON_FILEID,
>> 
>> So you are suggesting that it's ok to ask for SIZE, OWNER, OWNER
>> GROUP, SPACE USED, TIMESTAMPs etc but not ok to ask for mode bits and
>> numlinks?
> 
> No. We shouldn't be asking for any of that information for a referral
> because the server isn't supposed to return any values for it.
> 
> Chuck and Anna, what's the deal with commit c05cefcc7241? That appears
> to have changed the original code to speculatively assume that the
> server will violate RFC5661 Section 11.3.1 and/or RFC7530 Section
> 8.3.1.

The commit is an attempt to address the many complaints we've had
about the ugly appearance of referral anchors. The strange "special"
default values made the client appear to be broken, and was confusing
to some. I consider this to be a UX issue: the information displayed
in this case is not meant to be factual, but rather to prevent the
user concluding that something is wrong.

I'm not attached to this particular solution, though. Does it make
sense to perform the referral mount before returning "ls" results
so that the target server has a chance to supply reasonable
attribute values for the mounted-on directory object? Just spit
balling here.


> Specifically, the paragraph that says:
> 
> "
>   Other attributes SHOULD NOT be made available for absent file
>   systems, even when it is possible to provide them.  The server should
>   not assume that more information is always better and should avoid
>   gratuitously providing additional information."
> 
> So why is the client asking for them?

This paragraph (and it's most modern incarnation in RFC 8881 Section
11.4.1) describes server behavior. The current client behavior is
spec-compliant because there is no explicit prohibition in the spec
language against a client requesting additional attributes in this
case.

Either the server can clear those bitmap flags on the GETATTR reply
and not supply those attributes, and clients must be prepared for
that.

Or, it's also possible to read this paragraph to mean that the
server can provide those attributes and the values should not
reflect attributes for the absent file system, but rather something
else (eg, server-manufactured defaults, or the attributes from the
object on the source server).

And since this is a SHOULD NOT rather than a MUST NOT, servers are
still free to return information about the absent file system.
Clients are not guaranteed this will be the case, however.

I don't think c05cefcc7241 makes any assumption about whether the
server is lying about the extra attributes. Perhaps the server has
no better values for these attributes than the client's defaults
were.


--
Chuck Lever