Re: [PATCH 06/14] sunrpc: add helpers to count and snapshot pending cache requests

[Jeff Layton]

On Thu, 2026-03-19 at 14:07 -0400, Chuck Lever wrote:
> On 3/16/26 11:14 AM, Jeff Layton wrote:
> > Add sunrpc_cache_requests_count() and sunrpc_cache_requests_snapshot()
> > to allow callers to count and snapshot the pending upcall request list
> > without exposing struct cache_request outside of cache.c.
> > 
> > Both functions skip entries that no longer have CACHE_PENDING set.
> > 
> > The snapshot function takes a cache_get() reference on each item so the
> > caller can safely use them after the queue_lock is released.
> > 
> > These will be used by the nfsd generic netlink dumpit handler for
> > svc_export upcall requests.
> > 
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > ---
> >  include/linux/sunrpc/cache.h |  5 ++++
> >  net/sunrpc/cache.c           | 57 ++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 62 insertions(+)
> > 
> > diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
> > index c358151c23950ab48e83991c6138bb7d0e049ace..343f0fb675d761e019989e47e03645484e0aa30f 100644
> > --- a/include/linux/sunrpc/cache.h
> > +++ b/include/linux/sunrpc/cache.h
> > @@ -251,6 +251,11 @@ extern int sunrpc_cache_register_pipefs(struct dentry *parent, const char *,
> >  extern void sunrpc_cache_unregister_pipefs(struct cache_detail *);
> >  extern void sunrpc_cache_unhash(struct cache_detail *, struct cache_head *);
> >  
> > +int sunrpc_cache_requests_count(struct cache_detail *cd);
> > +int sunrpc_cache_requests_snapshot(struct cache_detail *cd,
> > +				   struct cache_head **items,
> > +				   u64 *seqnos, int max);
> > +
> >  /* Must store cache_detail in seq_file->private if using next three functions */
> >  extern void *cache_seq_start_rcu(struct seq_file *file, loff_t *pos);
> >  extern void *cache_seq_next_rcu(struct seq_file *file, void *p, loff_t *pos);
> > diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> > index 819f12add8f26562fdc6aaa200f55dec0180bfbc..2a78560edee5ca07be0fce90f87ce43a19df245f 100644
> > --- a/net/sunrpc/cache.c
> > +++ b/net/sunrpc/cache.c
> > @@ -1906,3 +1906,60 @@ void sunrpc_cache_unhash(struct cache_detail *cd, struct cache_head *h)
> >  		spin_unlock(&cd->hash_lock);
> >  }
> >  EXPORT_SYMBOL_GPL(sunrpc_cache_unhash);
> > +
> > +/**
> > + * sunrpc_cache_requests_count - count pending upcall requests
> > + * @cd: cache_detail to query
> > + *
> > + * Returns the number of requests on the cache's request list that
> > + * still have CACHE_PENDING set.
> > + */
> > +int sunrpc_cache_requests_count(struct cache_detail *cd)
> > +{
> > +	struct cache_request *crq;
> > +	int cnt = 0;
> > +
> > +	spin_lock(&cd->queue_lock);
> > +	list_for_each_entry(crq, &cd->requests, list) {
> > +		if (test_bit(CACHE_PENDING, &crq->item->flags))
> > +			cnt++;
> > +	}
> > +	spin_unlock(&cd->queue_lock);
> > +	return cnt;
> > +}
> > +EXPORT_SYMBOL_GPL(sunrpc_cache_requests_count);
> > +
> > +/**
> > + * sunrpc_cache_requests_snapshot - snapshot pending upcall requests
> > + * @cd: cache_detail to query
> > + * @items: array to fill with cache_head pointers (caller-allocated)
> > + * @seqnos: array to fill with sequence numbers (caller-allocated)
> > + * @max: size of the arrays
> > + *
> > + * Only entries with CACHE_PENDING set are included. Takes a reference
> > + * on each cache_head via cache_get(). Caller must call cache_put()
> > + * on each returned item when done.
> > + *
> > + * Returns the number of entries filled.
> > + */
> > +int sunrpc_cache_requests_snapshot(struct cache_detail *cd,
> > +				   struct cache_head **items,
> > +				   u64 *seqnos, int max)
> > +{
> > +	struct cache_request *crq;
> > +	int i = 0;
> > +
> > +	spin_lock(&cd->queue_lock);
> > +	list_for_each_entry(crq, &cd->requests, list) {
> > +		if (i >= max)
> > +			break;
> > +		if (!test_bit(CACHE_PENDING, &crq->item->flags))
> > +			continue;
> > +		items[i] = cache_get(crq->item);
> > +		seqnos[i] = crq->seqno;
> > +		i++;
> > +	}
> > +	spin_unlock(&cd->queue_lock);
> > +	return i;
> > +}
> > +EXPORT_SYMBOL_GPL(sunrpc_cache_requests_snapshot);
> > 
> 
> This API architecture introduces a TOCTOU, since as soon as the
> queue_lock is dropped, the count can immediately become stale.
> 
> The count returned by sunrpc_cache_requests_count() is used as the array
> bound. To wit:
> 
>   cnt = sunrpc_cache_requests_count(cd);
> 
>   items = kcalloc(cnt, sizeof(*items), GFP_KERNEL);
> 
>   seqnos = kcalloc(cnt, sizeof(*seqnos), GFP_KERNEL);
> 
>   cnt = sunrpc_cache_requests_snapshot(cd, items, seqnos, cnt);
> 
> 
> 
> This appears in all four dumpit handlers (ip_map, unix_gid, svc_export,
> expkey).
> 
> This isn't a memory safety issue; _snapshot() caps its output at max, so
> if the list grows between the two calls, entries are silently truncated
> rather than overflowing the buffer. If the list shrinks, the arrays are
> merely oversized.
> 
> However, the practical risk is incomplete data returned to userspace. If
> the caller is guaranteed to be re-invoked (e.g., the netlink dumpit
> callback gets called again until it returns 0), then missing items due
> to list growth between count() and snapshot() is harmless: they'll be
> picked up on the next pass.
> 
> But looking at the callers, they all do this:
> 
>   if (cb->args[0])
>       return 0;
> 
> and then set cb->args[0] after the single snapshot dump.
> 
> The dumpit is a one-shot: it snapshots once and signals completion. If
> the list grows between count() and snapshot(), the truncated entries are
> silently lost and there's no subsequent pass to pick them up, IIUC.
> 

Userland will receive a separate notify request whenever a
cache_request is queued, and that notification is only sent after the
cache_request is queued.

So while it might not receive all of the queued requests from the
kernel in the race you describe, it won't matter because select() will
return the notify descriptor again on the next pass and mountd will
pick up the remaining entries at that point.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>