Re: [PATCH] nfsd: remove unsafe BUG_ON from set_change_info

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2023-07-20 at 15:37 +0000, Chuck Lever III wrote:
> 
> > On Jul 20, 2023, at 11:33 AM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Thu, 2023-07-20 at 15:15 +0000, Chuck Lever III wrote:
> > > 
> > > > On Jul 20, 2023, at 10:59 AM, Jeff Layton <jlayton@kernel.org> wrote:
> > > > 
> > > > At one time, nfsd would scrape inode information directly out of struct
> > > > inode in order to populate the change_info4. At that time, the BUG_ON in
> > > > set_change_info made some sense, since having it unset meant a coding
> > > > error.
> > > > 
> > > > More recently, it calls vfs_getattr to get this information, which can
> > > > fail. If that fails, fh_pre_saved can end up not being set. While this
> > > > situation is unfortunate, we don't need to crash the box.
> > > 
> > > I'm always happy to get rid of a BUG_ON(). But I'm not sure even
> > > a warning is necessary in this case. It's not likely that it's
> > > a software bug or something that the server administrator can
> > > do something about.
> > > 
> > > Can you elaborate on why the vfs_getattr() might fail? Eg, how
> > > was it failing in 2223560 ?
> > > 
> > 
> > I'm fine with dropping the WARN_ON. You are correct that there is
> > probably little the admin can do about it.
> > 
> > vfs_getattr can fail for all sorts of reasons. It really depends on the
> > underlying filesystem. In 2223560, I don't know for sure, but just prior
> > to the oops, there were these messages in the log:
> > 
> > [51935.482019] XFS (vda3): Filesystem has been shut down due to log error (0x2). 
> > [51935.482020] XFS (vda3): Please unmount the filesystem and rectify the problem(s). 
> > [51935.482550] vda3: writeback error on inode 25320400, offset 2097152, sector 58684120 
> > 
> > My assumption was that the fs being shut down caused some VFS operations
> > to start returning errors (including getattr) and that is why
> > fh_pre_saved ultimately didn't get set.
> 
> I'm wondering if the operation should just fail in this case
> rather than return a cobbled-up changeinfo4. Maybe for another
> day.
> 
> 
> > > > Move set_change_info to nfs4proc.c since all of the callers are there.
> > > > Revise the condition for setting "atomic" to also check for
> > > > fh_pre_saved. Drop the BUG_ON and make it a WARN_ON, and just have it
> > > > zero out both change_attr4s when this occurs.
> > > > 
> > > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=2223560
> > > > Reported-by: Boyang Xue <bxue@redhat.com>
> > > 
> > > checkpatch now wants
> > > 
> > > Reported-by:
> > > Closes:
> > > 
> > > in that order.
> > > 
> > 
> > 
> > Mmmmkay. So I assume the URL should go in the Closes: field then?
> 
> Yes, a bug link goes in the Closes: field.
> 
> 
> > I'll take out the WARN_ON_ONCE and resend, once others have had a chance
> > to comment.
> 
> Don't miss the other comments below.
> 
> 
> > Thanks!
> > 
> > > 
> > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > > ---
> > > > fs/nfsd/nfs4proc.c | 30 ++++++++++++++++++++++++++++++
> > > > fs/nfsd/xdr4.h     | 11 -----------
> > > > 2 files changed, 30 insertions(+), 11 deletions(-)
> > > > 
> > > > diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> > > > index d8e7a533f9d2..e6f406f27821 100644
> > > > --- a/fs/nfsd/nfs4proc.c
> > > > +++ b/fs/nfsd/nfs4proc.c
> > > > @@ -380,6 +380,36 @@ nfsd4_create_file(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > > > return status;
> > > > }
> > > > 
> > > > +/**
> > > > + * set_change_info - set up the change_info4 for a reply
> > > > + * @cinfo: pointer to nfsd4_change_info to be populated
> > > > + * @fhp: pointer to svc_fh to use as source
> > > > + *
> > > > + * Many operations in NFSv4 require change_info4 in the reply. This function
> > > > + * populates that from the info that we (should!) have already collected. In
> > > > + * the event that we didn't get any pre-attrs, throw a warning and just
> > > > + * zero out both change_attr4 fields.
> > > > + */
> > > > +static void
> > > > +set_change_info(struct nfsd4_change_info *cinfo, struct svc_fh *fhp)
> > > > +{
> > > > + cinfo->atomic = (u32)(fhp->fh_pre_saved && fhp->fh_post_saved && !fhp->fh_no_atomic_attr);
> > > > +
> > > > + /*
> > > > + * In the event that we couldn't fetch attributes from the
> > > > + * server for some reason, just zero out the before and after
> > > 
> > > "From the server"? Is it only likely to fail if the exported
> > > filesystem is an NFS mount? Or did you mean "from the filesystem" ?
> > > 

Yep, will fix.

> > > 
> > > > + * change values, after throwing a warning.
> > > > + */
> > > > + if (WARN_ON_ONCE(!fhp->fh_pre_saved)) {
> > > 
> > > Maybe you should clear ->atomic as well in this case.
> > > 
> > > 

It should already be clear since fh_pre_saved isn't set.

> > > > + cinfo->before_change = 0;
> > > > + cinfo->after_change = 0;
> > > > + return;
> > > > + }
> > > > +
> > > > + cinfo->before_change = fhp->fh_pre_change;
> > > > + cinfo->after_change = fhp->fh_post_change;
> > > > +}
> > > > +
> > > > static __be32
> > > > do_open_lookup(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate, struct nfsd4_open *open, struct svc_fh **resfh)
> > > > {
> > > > diff --git a/fs/nfsd/xdr4.h b/fs/nfsd/xdr4.h
> > > > index b2931fdf53be..9e67f63c5f4d 100644
> > > > --- a/fs/nfsd/xdr4.h
> > > > +++ b/fs/nfsd/xdr4.h
> > > > @@ -775,17 +775,6 @@ void warn_on_nonidempotent_op(struct nfsd4_op *op);
> > > > 
> > > > #define NFS4_SVC_XDRSIZE sizeof(struct nfsd4_compoundargs)
> > > > 
> > > > -static inline void
> > > > -set_change_info(struct nfsd4_change_info *cinfo, struct svc_fh *fhp)
> > > > -{
> > > > - BUG_ON(!fhp->fh_pre_saved);
> > > > - cinfo->atomic = (u32)(fhp->fh_post_saved && !fhp->fh_no_atomic_attr);
> > > > -
> > > > - cinfo->before_change = fhp->fh_pre_change;
> > > > - cinfo->after_change = fhp->fh_post_change;
> > > > -}
> > > > -
> > > > -
> > > > bool nfsd4_mach_creds_match(struct nfs4_client *cl, struct svc_rqst *rqstp);
> > > > bool nfs4svc_decode_compoundargs(struct svc_rqst *rqstp, struct xdr_stream *xdr);
> > > > bool nfs4svc_encode_compoundres(struct svc_rqst *rqstp, struct xdr_stream *xdr);
> > > > 
> > > > ---
> > > > base-commit: 070f391ca4d48e1750ee6108eb44f751a9e9448e
> > > > change-id: 20230720-bz2223560-9c4690a8217b
> > > > 
> > > > Best regards,
> > > > -- 
> > > > Jeff Layton <jlayton@kernel.org>
> > > > 
> > > 
> > > --
> > > Chuck Lever
> > > 
> > > 
> > 
> > -- 
> > Jeff Layton <jlayton@kernel.org>
> 
> 
> --
> Chuck Lever
> 
> 

-- 
Jeff Layton <jlayton@kernel.org>