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 CD3E614D28C; Mon, 27 Jan 2025 13:07:07 +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=1737983227; cv=none; b=Oafskox+WiN+N1GQplekU2ShbGoFarYPIqgmRHK2/Ih1kpS5B9v5GtPyZrHgNjRz6aUCQruQvCWlej9eO7LQOUBIbVuaiLKU9ORt4IqGs1N8Mnprq+c3hYYIPnZX9TOKHI70Yl9Kj6aFgIUZKl9gv83/xZMQEiR9ZwJ2HLsbIXg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737983227; c=relaxed/simple; bh=V037Z1yq+jT36J7FvQUUTfwu75qWyw4/lbNF5NQkAMk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=iIgxc28ckE5hkwe5Beu7S1t90xu9l27ZFm+tB9moVG9YovFXQpMIfNym8sWqjnZA/8HdkB5eOBqUerrY205VWh2O6lqj9cB87cOKHYxYOmr2D3iebuD3qlHSDa95pGqYlrwt/+m/P7QGlacSc74M20Phfa/ch2+zDMPvIvOLK2o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=XXyFeCSe; 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="XXyFeCSe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7AE10C4CED2; Mon, 27 Jan 2025 13:07:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737983227; bh=V037Z1yq+jT36J7FvQUUTfwu75qWyw4/lbNF5NQkAMk=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=XXyFeCSeZENpI2imqaEoEJ/nLG4u1fdnNSPv9u7mDTv1ZUCp/2nvLY7pcX4Qw4sJe wJEYhyEk3r7gxkmhKP1BBYv4wrlBFRbg1T0wb/crRHffr3rntmKPODrfs5PM2KRap6 ptu/oWWGQoEQWGBXJtqjP6oWvhSw4b5h3Y1vJ7jvvptpgwi9/13374Px8+AF1iGxl6 XADIFSZy1Ya8SF19XfW/2M7+sjWBuzrQznK0tBYF4sK7uOUL4HB/MWEFcE1tDhzFpW iR13Vo+EnmnO1G7xn55ZrOd+wIp53ZqZP/FFJ1Zz5C2yoaRAHetjm7V7Jx1yb0un/h XQVRHoPPfy+tw== Message-ID: <06379c169fb0e891dae9d444ef0a06ea57e9af38.camel@kernel.org> Subject: Re: [PATCH] nfsd: validate the nfsd_serv pointer before calling svc_wake_up From: Jeff Layton To: NeilBrown Cc: Chuck Lever , Olga Kornievskaia , Dai Ngo , Tom Talpey , Salvatore Bonaccorso , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 27 Jan 2025 08:07:05 -0500 In-Reply-To: <173793694589.22054.1830112177481945773@noble.neil.brown.name> References: <>, <173793694589.22054.1830112177481945773@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Mon, 2025-01-27 at 11:15 +1100, NeilBrown wrote: > On Mon, 27 Jan 2025, Jeff Layton wrote: > > On Mon, 2025-01-27 at 08:53 +1100, NeilBrown wrote: > > > On Sun, 26 Jan 2025, Jeff Layton wrote: > > > > On Sun, 2025-01-26 at 13:39 +1100, NeilBrown wrote: > > > > > On Sun, 26 Jan 2025, Jeff Layton wrote: > > > > > > nfsd_file_dispose_list_delayed can be called from the filecache > > > > > > laundrette, which is shut down after the nfsd threads are shut = down and > > > > > > the nfsd_serv pointer is cleared. If nn->nfsd_serv is NULL then= there > > > > > > are no threads to wake. > > > > > >=20 > > > > > > Ensure that the nn->nfsd_serv pointer is non-NULL before callin= g > > > > > > svc_wake_up in nfsd_file_dispose_list_delayed. This is safe sin= ce the > > > > > > svc_serv is not freed until after the filecache laundrette is c= ancelled. > > > > > >=20 > > > > > > Fixes: ffb402596147 ("nfsd: Don't leave work of closing files t= o a work queue") > > > > > > Reported-by: Salvatore Bonaccorso > > > > > > Closes: https://lore.kernel.org/linux-nfs/7d9f2a8aede4f7ca9935a= 47e1d405643220d7946.camel@kernel.org/ > > > > > > Signed-off-by: Jeff Layton > > > > > > --- > > > > > > This is only lightly tested, but I think it will fix the bug th= at > > > > > > Salvatore reported. > > > > > > --- > > > > > > fs/nfsd/filecache.c | 11 ++++++++++- > > > > > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > > >=20 > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c > > > > > > index e91c164b5ea21507659904690533a19ca43b1b64..fb2a4469b7a3c07= 7de2dd750f43239b4af6d37b0 100644 > > > > > > --- a/fs/nfsd/filecache.c > > > > > > +++ b/fs/nfsd/filecache.c > > > > > > @@ -445,11 +445,20 @@ nfsd_file_dispose_list_delayed(struct lis= t_head *dispose) > > > > > > struct nfsd_file, nf_gc); > > > > > > struct nfsd_net *nn =3D net_generic(nf->nf_net, nfsd_net_id)= ; > > > > > > struct nfsd_fcache_disposal *l =3D nn->fcache_disposal; > > > > > > + struct svc_serv *serv; > > > > > > =20 > > > > > > spin_lock(&l->lock); > > > > > > list_move_tail(&nf->nf_gc, &l->freeme); > > > > > > spin_unlock(&l->lock); > > > > > > - svc_wake_up(nn->nfsd_serv); > > > > > > + > > > > > > + /* > > > > > > + * The filecache laundrette is shut down after the > > > > > > + * nn->nfsd_serv pointer is cleared, but before the > > > > > > + * svc_serv is freed. > > > > > > + */ > > > > > > + serv =3D nn->nfsd_serv; > > > > >=20 > > > > > I wonder if this should be READ_ONCE() to tell the compiler that = we > > > > > could race with clearing nn->nfsd_serv. Would the comment still = be > > > > > needed? > > > > >=20 > > > >=20 > > > > I think we need a comment at least. The linkage between the laundre= tte > > > > and the nfsd_serv being set to NULL is very subtle. A READ_ONCE() > > > > doesn't convey that well, and is unnecessary here. > > >=20 > > > Why do you say "is unnecessary here" ? > > > If the code were > > > if (nn->nfsd_serv) > > > svc_wake_up(nn->nfsd_serv); > > > that would be wrong as nn->nfds_serv could be set to NULL between the > > > two. > > > And the C compile is allowed to load the value twice because the C me= mory > > > model declares that would have the same effect. > > > While I doubt it would actually change how the code is compiled, I th= ink > > > we should have READ_ONCE() here (and I've been wrong before about wha= t > > > the compiler will actually do). > > >=20 > > >=20 > >=20 > > It's unnecessary because the outcome of either case is acceptable. > >=20 > > When racing with shutdown, either it's NULL and the laundrette won't > > call svc_wake_up(), or it's non-NULL and it will. In the non-NULL case, > > the call to svc_wake_up() will be a no-op because the threads are shut > > down. > >=20 > > The vastly common case in this code is that this pointer will be non- > > NULL, because the server is running (i.e. not racing with shutdown). I > > don't see the need in making all of those accesses volatile. >=20 > One of us is confused. I hope it isn't me. >=20 It's probably me. I think you have a much better understanding of compiler design than I do. Still... > The hypothetical problem I see is that the C compiler could generate > code to load the value "nn->nfsd_serv" twice. The first time it is not > NULL, the second time it is NULL. > The first is used for the test, the second is passed to svc_wake_up(). >=20 > Unlikely though this is, it is possible and READ_ONCE() is designed > precisely to prevent this. > To quote from include/asm-generic/rwonce.h it will > "Prevent the compiler from merging or refetching reads" >=20 > A "volatile" access does not add any cost (in this case). What it does > is break any aliasing that the compile might have deduced. > Even if the compiler thinks it has "nn->nfsd_serv" in a register, it > won't think it has the result of READ_ONCE(nn->nfsd_serv) in that registe= r. > And if it needs the result of a previous READ_ONCE(nn->nfsd_serv) it > won't decide that it can just read nn->nfsd_serv again. It MUST keep > the result of READ_ONCE(nn->nfsd_serv) somewhere until it is not needed > any more. I'm mainly just considering the resulting pointer. There are two possible outcomes to the fetch of nn->nfsd_serv. Either it's a valid pointer that points to the svc_serv, or it's NULL. The resulting code can handle either case, so it doesn't seem like adding READ_ONCE() will create any material difference here. Maybe I should ask it this way: What bad outcome could result if we don't add READ_ONCE() here? --=20 Jeff Layton