From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CC72A38D685; Thu, 19 Mar 2026 18:22:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773944548; cv=none; b=DMWtt15W4o5dHoJoHWKxCJFNB9RnYOgEPRkUHDjo2ssTwF6f640wl8eInd83lLhT07PA/AYXIMORroh4XvMz8JlLsIs+N2TNHACqjGczlMqzFi54hMrsfgq5dBjiczYpPAFMymmdEIdQXJxHMSK4LH0RVkYr2ccZaBcqdYfaF84= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773944548; c=relaxed/simple; bh=Xc/xBoq/UjF0S/fSdiqUyiKLDJSNLSpMj78/1XWgJYI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ClFoDlC7tTpGi7hyzWf+z/JJaZPQhz0omzKLheQw6eIr7SCHR/vtAdjdDIwsh4ShJSvXjy1JJw4q+viyyhOBLCXmqF/8/UIcAVgcmiUYfp1h/bJsyN7PzgpL5YJxpoWkKaCkgHIex7EkfzbO3lAaYhkMbB3GG7yLRhq/iu6MceI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SdaJKv0k; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SdaJKv0k" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 89A9DC19424; Thu, 19 Mar 2026 18:22:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773944548; bh=Xc/xBoq/UjF0S/fSdiqUyiKLDJSNLSpMj78/1XWgJYI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=SdaJKv0kJ4Nxclf8wb6Hahoalxxg2n5XMKnxyWFuLaCHczhxDBgmqpbVywdRlp/2d 8rsrvYftvaEshcGeMxMET2sMX+nDdPF5bIR1/Am5Y8HlanEqm6fNkxxzYuY65ETp0Q UkKA6K4rbxRak/xY855GyFhJxBC0Dnp3xlB/4Ixaly5GQ9JZFILjORvHR0kO1iYwO1 +xLVNSmjXO3adS/y3N7sQNYzyM0WmBHObexq+HZG/iZy+qmFVjRcbA9088Qe1Uls9B TRNzq+wvCBa4a/JH1nrSNIkZhOhqL8hKdjEdMGMlO43XzDo5Ce902zPwVDLeGhfzeq G2OLGKKVLFeHg== Message-ID: <579b0239abbbb0b95d619e6b400bb919fedab60d.camel@kernel.org> Subject: Re: [PATCH 06/14] sunrpc: add helpers to count and snapshot pending cache requests From: Jeff Layton To: Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey Cc: Trond Myklebust , Anna Schumaker , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Thu, 19 Mar 2026 14:22:25 -0400 In-Reply-To: <0908b7ac-658f-491b-89be-f5a1d97e991e@oracle.com> References: <20260316-exportd-netlink-v1-0-6125dc62b955@kernel.org> <20260316-exportd-netlink-v1-6-6125dc62b955@kernel.org> <0908b7ac-658f-491b-89be-f5a1d97e991e@oracle.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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. > >=20 > > Both functions skip entries that no longer have CACHE_PENDING set. > >=20 > > The snapshot function takes a cache_get() reference on each item so the > > caller can safely use them after the queue_lock is released. > >=20 > > These will be used by the nfsd generic netlink dumpit handler for > > svc_export upcall requests. > >=20 > > Signed-off-by: Jeff Layton > > --- > > include/linux/sunrpc/cache.h | 5 ++++ > > net/sunrpc/cache.c | 57 ++++++++++++++++++++++++++++++++++++= ++++++++ > > 2 files changed, 62 insertions(+) > >=20 > > diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.= h > > index c358151c23950ab48e83991c6138bb7d0e049ace..343f0fb675d761e019989e4= 7e03645484e0aa30f 100644 > > --- a/include/linux/sunrpc/cache.h > > +++ b/include/linux/sunrpc/cache.h > > @@ -251,6 +251,11 @@ extern int sunrpc_cache_register_pipefs(struct den= try *parent, const char *, > > extern void sunrpc_cache_unregister_pipefs(struct cache_detail *); > > extern void sunrpc_cache_unhash(struct cache_detail *, struct cache_he= ad *); > > =20 > > +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 fu= nctions */ > > 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..2a78560edee5ca07be0fce9= 0f87ce43a19df245f 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 =3D 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 =3D 0; > > + > > + spin_lock(&cd->queue_lock); > > + list_for_each_entry(crq, &cd->requests, list) { > > + if (i >=3D max) > > + break; > > + if (!test_bit(CACHE_PENDING, &crq->item->flags)) > > + continue; > > + items[i] =3D cache_get(crq->item); > > + seqnos[i] =3D crq->seqno; > > + i++; > > + } > > + spin_unlock(&cd->queue_lock); > > + return i; > > +} > > +EXPORT_SYMBOL_GPL(sunrpc_cache_requests_snapshot); > >=20 >=20 > This API architecture introduces a TOCTOU, since as soon as the > queue_lock is dropped, the count can immediately become stale. >=20 > The count returned by sunrpc_cache_requests_count() is used as the array > bound. To wit: >=20 > cnt =3D sunrpc_cache_requests_count(cd); >=20 > items =3D kcalloc(cnt, sizeof(*items), GFP_KERNEL); >=20 > seqnos =3D kcalloc(cnt, sizeof(*seqnos), GFP_KERNEL); >=20 > cnt =3D sunrpc_cache_requests_snapshot(cd, items, seqnos, cnt); >=20 >=20 >=20 > This appears in all four dumpit handlers (ip_map, unix_gid, svc_export, > expkey). >=20 > 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. >=20 > 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. >=20 > But looking at the callers, they all do this: >=20 > if (cb->args[0]) > return 0; >=20 > and then set cb->args[0] after the single snapshot dump. >=20 > 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. >=20 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. --=20 Jeff Layton