On Thu, 2022-07-07 at 12:55 -0400, Jeff Layton wrote:
> On Thu, 2022-07-07 at 11:58 -0400, Chuck Lever wrote:
> > The documenting comment for struct nf_file states:
> >
> > /*
> > * A representation of a file that has been opened by knfsd. These are hashed
> > * in the hashtable by inode pointer value. Note that this object doesn't
> > * hold a reference to the inode by itself, so the nf_inode pointer should
> > * never be dereferenced, only used for comparison.
> > */
> >
> > However, nfsd_file_mark_find_or_create() does dereference the pointer stored
> > in this field.
> >
> > Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > ---
> > fs/nfsd/filecache.c | 3 +++
> > fs/nfsd/filecache.h | 4 +---
> > 2 files changed, 4 insertions(+), 3 deletions(-)
> >
> > Hi Jeff-
> >
> > I'm still testing this one, but I'm wondering what you think of it.
> > I did hit a KASAN splat that might be related, but it's not 100%
> > reproducible.
> >
>
> My first thought is "what the hell was I thinking, tracking an inode
> field without holding a reference to it?"
>
> But now that I look, the nf_inode value only gets dereferenced in one
> place -- nfs4_show_superblock, and I think that's a bug. The comments
> over struct nfsd_file say:
>
> "Note that this object doesn't hold a reference to the inode by itself,
> so the nf_inode pointer should never be dereferenced, only used for
> comparison."
>
> We should probably annotate nf_inode better. __attribute__((noderef))
> maybe? It would also be good to make nfs4_show_superblock use a
> different way to get the sb.
>
> In any case, this is unlikely to fix anything unless the crash happened
> in nfs4_show_superblock.
>
>
One other spot. We also dereference it in nfsd_file_mark_find_or_create,
but I think that specific instance is OK. We know that we still hold a
reference to the inode at that point since it comes from fhp->fh_dentry,
so we shouldn't need to worry about it disappearing out from under us.
What did the crash look like?
> >
> > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > index 9cb2d590c036..7b43bb427a53 100644
> > --- a/fs/nfsd/filecache.c
> > +++ b/fs/nfsd/filecache.c
> > @@ -180,6 +180,7 @@ nfsd_file_alloc(struct inode *inode, unsigned int may, unsigned int hashval,
> > nf->nf_cred = get_current_cred();
> > nf->nf_net = net;
> > nf->nf_flags = 0;
> > + ihold(inode);
> > nf->nf_inode = inode;
> > nf->nf_hashval = hashval;
> > refcount_set(&nf->nf_ref, 1);
> > @@ -210,6 +211,7 @@ nfsd_file_free(struct nfsd_file *nf)
> > fput(nf->nf_file);
> > flush = true;
> > }
> > + iput(nf->nf_inode);
> > call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
> > return flush;
> > }
> > @@ -940,6 +942,7 @@ nfsd_do_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > if (nf == NULL)
> > goto open_file;
> > spin_unlock(&nfsd_file_hashtbl[hashval].nfb_lock);
> > + iput(new->nf_inode);
> > nfsd_file_slab_free(&new->nf_rcu);
> >
> > wait_for_construction:
> > diff --git a/fs/nfsd/filecache.h b/fs/nfsd/filecache.h
> > index 1da0c79a5580..01fbf6e88cce 100644
> > --- a/fs/nfsd/filecache.h
> > +++ b/fs/nfsd/filecache.h
> > @@ -24,9 +24,7 @@ struct nfsd_file_mark {
> >
> > /*
> > * A representation of a file that has been opened by knfsd. These are hashed
> > - * in the hashtable by inode pointer value. Note that this object doesn't
> > - * hold a reference to the inode by itself, so the nf_inode pointer should
> > - * never be dereferenced, only used for comparison.
> > + * in the hashtable by inode pointer value.
> > */
> > struct nfsd_file {
> > struct hlist_node nf_node;
> >
> >
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
