On Wed, 2023-01-04 at 11:27 +1100, NeilBrown wrote:

If a NFSv4 OPEN reply reports that the file was successfully opened
but
the subsequent GETATTR fails, Linux-NFS will attempt a stand-alone
GETATTR request.  If that also fails, handling of the reply is
aborted
with error -EAGAIN and the open is attempted again from the start.

This leaves the server with an active state (because the OPEN
operation
succeeded) which the client doesn't know about.  If the open-owner
(local user) did not have the file already open, this has minimal
consequences for the client and only causes the server to spend
resources on an open state that will never be used or explicitly
closed.

If the open-owner DID already have the file open, then it will hold a
reference to the open-state for that file, but the seq-id in the
state-id will now be out-of-sync with the server.  The server will
have
incremented the seq-id, but the client will not have noticed.  So
when
the client next attempts to access the file using that state (READ,
WRITE, SETATTR), the attempt will fail with NFS4ERR_OLD_STATEID.

The Linux-client assumes this error is due to a race and simply
retries
on the basis that the local state-id information should have been
updated by another thread.  This basis is invalid in this case and
the
result is an infinite loop attempting IO and getting OLD_STATEID.

This has been observed with a NetApp Filer as the server (ONTAP 9.8
p5,
using NFSv4.0).  The client is creating, writing, and unlinking a
particular file from multiple clients (.bash_history).  If a new OPEN
from one client races with a REMOVE from another client while the
first
client already has the file open, the Filer can report success for
the
OPEN op, but NFS4ERR_STALE for the ACCESS or GETATTR ops in the OPEN
request.  This gets the seq-id out-of-sync and a subsequent write to
the
other open on the first client causes the infinite loop to occur.

The reason that the client returns -EAGAIN is that it needs to find
the
inode so it can find the associated state to update the seq-id, but
the
inode lookup requires the file-id which is provided in the GETATTR
reply.  Without the file-id normal inode lookup cannot be used.

This patch changes the lookup so that when the file-id is not
available
the list of states owned by the open-owner is examined to find the
state
with the correct state-id (ignoring the seq-id part of the state-id).
If this is found it is used just as when a normal inode lookup finds
an
inode.  If it isn't found, -EAGAIN is returned as before.

This bug can be demonstrated by modifying the Linux NFS server as
follows:

1/ The second time a file is opened, unlink it.  This simulates
   a race with another client, without needing to have a race:

    --- a/fs/nfsd/nfs4proc.c
    +++ b/fs/nfsd/nfs4proc.c
    @@ -594,6 +594,12 @@ nfsd4_open(struct svc_rqst *rqstp, struct
nfsd4_compound_state *cstate,
        if (reclaim && !status)
                nn->somebody_reclaimed = true;
     out:
    +   if (!status && open->op_stateid.si_generation > 1) {
    +           printk("Opening gen %d\n", (int)open-

quoted

op_stateid.si_generation);

    +           vfs_unlink(mnt_user_ns(resfh->fh_export-

quoted

ex_path.mnt),

    +                      resfh->fh_dentry->d_parent->d_inode,
    +                      resfh->fh_dentry, NULL);
    +   }
        if (open->op_filp) {
                fput(open->op_filp);
                open->op_filp = NULL;

2/ When a GETATTR op is attempted on an unlinked file, return ESTALE

    @@ -852,6 +858,11 @@ nfsd4_getattr(struct svc_rqst *rqstp, struct
nfsd4_compound_state *cstate,
        if (status)
                return status;

    +   if (cstate->current_fh.fh_dentry->d_inode->i_nlink == 0) {
    +           printk("Return Estale for unlinked file\n");
    +           return nfserr_stale;
    +   }
    +
        if (getattr->ga_bmval[1] & NFSD_WRITEONLY_ATTRS_WORD1)
                return nfserr_inval;

Then mount the filesystem and

  Thread 1            Thread 2
  open a file
                      open the same file (will fail)
  write to that file

I use this shell fragment, using 'sleep' for synchronisation.
The use of /bin/echo ensures the write is flushed when /bin/echo
closes
the fd on exit.

    (
        /bin/echo hello
        sleep 3
        /bin/echo there
    ) > /import/A/afile &
    sleep 3
    cat /import/A/afile

Probably when the OPEN succeeds, the GETATTR fails, and we don't
already
have the state open, we should explicitly close the state.  Leaving
it
open could cause problems if, for example, the server revoked it and
signalled the client that there was a revoked state.  The client
would
not be able to find the state that needed to be relinquished.  I
haven't
attempted to implement this.

If the server starts to reply NFS4ERR_STALE to GETATTR requests, why do
we care about stateid values? Just mark the inode as stale and drop it
on the floor.

If the server tries to declare the state as revoked, then it is clearly
borken, so let NetApp fix their own bug.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com