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.
>
> 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>
