On Thu, 2022-07-07 at 12:55 -0400, Jeff Layton wrote:

On Thu, 2022-07-07 at 11:58 -0400, Chuck Lever wrote:

quoted

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@oracle.com>
---
 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?

quoted

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 [off-list ref]