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 35680747F; Mon, 27 Jan 2025 14:34:22 +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=1737988463; cv=none; b=CpyvSnEz6AbaX9g3xSGN4l2cCFO5f7OraYUvkYlzfCFpx92lRBC8PK3uUBALcymEYgeqNaDv8phT0yDOd/JVSyZLeD4eX2HFNQOhhES9TvZm6HbQ5nMdOmgOI7PTt5k2BhAz4EFbPkZygC9YBMtX1uWweoPzJ7gPJukvZFYCs44= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737988463; c=relaxed/simple; bh=33nU/3JDqRo1KxZPmryx5Y1e1UKEkODcxfRRU9alWuw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=YEycoLGBz30LqTb9DRTyEhazsoFqjVGQVQTGSD7w6ibRukI/5fzRHkH2TKnuYXYnzp1QFI6phgJQGRaLQD0rwTF4XGZHEsFFt8UKxBSezanTPybcOjIGEvLJH4tHSUg/X2JBngKtOil9TauNgU8sZ66/C4dzPpM3rlyiL6h8bgk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Dd0Ygud8; 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="Dd0Ygud8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D6335C4CEE1; Mon, 27 Jan 2025 14:34:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737988462; bh=33nU/3JDqRo1KxZPmryx5Y1e1UKEkODcxfRRU9alWuw=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Dd0Ygud8Dnc+zF7WaNJ2j5C3lQ80gEE9BFCdejXock8eYdn3xhSN3c9G0Afp4csW9 l9zXzG+x5RdUtcfhrYJuJkNLu2+e3lALCH+n8+59GmiNwb4Bejh0SCnlYiTdBGmXnf JdPYALoBAsnDZTzORU6QJYb71GVlvMgcsH49VyNuJUuJydTmYtHmEIuDdFS7B246Ou qDTHSti2SqDp6PZJMf3jwCgb1y3kSptXwvcjvNNsasuNHD0Nc2+NcjIePnAujLAbWT ipGYFWDNRHpfaabY/mM6KYFR8lBbqoZ8dbNGk7TYp0/Z7iuvX2Ewl4wTuVsbtkUaA2 RFxUwaQ+2WDsw== Message-ID: Subject: Re: [PATCH] nfsd: validate the nfsd_serv pointer before calling svc_wake_up From: Jeff Layton To: Chuck Lever , NeilBrown Cc: Olga Kornievskaia , Dai Ngo , Tom Talpey , Salvatore Bonaccorso , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Mon, 27 Jan 2025 09:34:20 -0500 In-Reply-To: <37fb186b-ffb6-44dc-a097-ec669079c801@oracle.com> References: <> <173793694589.22054.1830112177481945773@noble.neil.brown.name> <06379c169fb0e891dae9d444ef0a06ea57e9af38.camel@kernel.org> <5923519a4c8f6bb6d5ccd2697447b313fb61bba3.camel@kernel.org> <37fb186b-ffb6-44dc-a097-ec669079c801@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 Mon, 2025-01-27 at 09:03 -0500, Chuck Lever wrote: > On 1/27/25 8:39 AM, Chuck Lever wrote: > > On 1/27/25 8:32 AM, Jeff Layton wrote: > > > On Mon, 2025-01-27 at 08:22 -0500, Chuck Lever wrote: > > > > On 1/27/25 8:07 AM, Jeff Layton wrote: > > > > > On 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=20 > > > > > > > > > > > down and > > > > > > > > > > > the nfsd_serv pointer is cleared. If nn->nfsd_serv is= NULL=20 > > > > > > > > > > > then there > > > > > > > > > > > are no threads to wake. > > > > > > > > > > >=20 > > > > > > > > > > > Ensure that the nn->nfsd_serv pointer is non-NULL bef= ore calling > > > > > > > > > > > svc_wake_up in nfsd_file_dispose_list_delayed. This i= s safe=20 > > > > > > > > > > > since the > > > > > > > > > > > svc_serv is not freed until after the filecache laund= rette is=20 > > > > > > > > > > > cancelled. > > > > > > > > > > >=20 > > > > > > > > > > > Fixes: ffb402596147 ("nfsd: Don't leave work of closi= ng files=20 > > > > > > > > > > > to a work queue") > > > > > > > > > > > Reported-by: Salvatore Bonaccorso > > > > > > > > > > > Closes: https://lore.kernel.org/linux-=20 > > > > > > > > > > > nfs/7d9f2a8aede4f7ca9935a47e1d405643220d7946.camel@ke= rnel.org/ > > > > > > > > > > > Signed-off-by: Jeff Layton > > > > > > > > > > > --- > > > > > > > > > > > This is only lightly tested, but I think it will fix = the bug that > > > > > > > > > > > Salvatore reported. > > > > > > > > > > > --- > > > > > > > > > > > =C2=A0=C2=A0 fs/nfsd/filecache.c | 11 ++++++++++- > > > > > > > > > > > =C2=A0=C2=A0 1 file changed, 10 insertions(+), 1 dele= tion(-) > > > > > > > > > > >=20 > > > > > > > > > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.= c > > > > > > > > > > > index=20 > > > > > > > > > > > e91c164b5ea21507659904690533a19ca43b1b64..fb2a4469b7a= 3c077de2dd750f43239b4af6d37b0 100644 > > > > > > > > > > > --- a/fs/nfsd/filecache.c > > > > > > > > > > > +++ b/fs/nfsd/filecache.c > > > > > > > > > > > @@ -445,11 +445,20 @@ nfsd_file_dispose_list_delayed(= struct=20 > > > > > > > > > > > list_head *dispose) > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct nfsd_file, nf_gc); > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 struct nfsd_net *nn =3D net_generic(nf->nf_net,=20 > > > > > > > > > > > nfsd_net_id); > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 struct nfsd_fcache_disposal *l =3D nn->fcache_disposal; > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 struct sv= c_serv *serv; > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 spin_lock(&l->lock); > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 list_move_tail(&nf->nf_gc, &l->freeme); > > > > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 spin_unlock(&l->lock); > > > > > > > > > > > -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 svc_wake_= up(nn->nfsd_serv); > > > > > > > > > > > + > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * T= he filecache laundrette is shut down after the > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * n= n->nfsd_serv pointer is cleared, but before the > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * s= vc_serv is freed. > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */ > > > > > > > > > > > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 serv =3D = nn->nfsd_serv; > > > > > > > > > >=20 > > > > > > > > > > I wonder if this should be READ_ONCE() to tell the comp= iler=20 > > > > > > > > > > that we > > > > > > > > > > could race with clearing nn->nfsd_serv.=C2=A0 Would the= comment=20 > > > > > > > > > > still be > > > > > > > > > > needed? > > > > > > > > > >=20 > > > > > > > > >=20 > > > > > > > > > I think we need a comment at least. The linkage between t= he=20 > > > > > > > > > laundrette > > > > > > > > > and the nfsd_serv being set to NULL is very subtle. A REA= D_ONCE() > > > > > > > > > doesn't convey that well, and is unnecessary here. > > > > > > > >=20 > > > > > > > > Why do you say "is unnecessary here" ? > > > > > > > > If the code were > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0 if (nn->nfsd_serv) > > > > > > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0 svc_wake_up(nn->nfsd_serv); > > > > > > > > that would be wrong as nn->nfds_serv could be set to NULL b= etween=20 > > > > > > > > the > > > > > > > > two. > > > > > > > > And the C compile is allowed to load the value twice becaus= e the=20 > > > > > > > > C memory > > > > > > > > model declares that would have the same effect. > > > > > > > > While I doubt it would actually change how the code is comp= iled,=20 > > > > > > > > I think > > > > > > > > we should have READ_ONCE() here (and I've been wrong before= about=20 > > > > > > > > what > > > > > > > > the compiler will actually do). > > > > > > > >=20 > > > > > > > >=20 > > > > > > >=20 > > > > > > > It's unnecessary because the outcome of either case is accept= able. > > > > > > >=20 > > > > > > > When racing with shutdown, either it's NULL and the laundrett= e won't > > > > > > > call svc_wake_up(), or it's non-NULL and it will. In the non-= NULL=20 > > > > > > > case, > > > > > > > the call to svc_wake_up() will be a no-op because the threads= are=20 > > > > > > > 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=20 > > > > > > > shutdown). I > > > > > > > don't see the need in making all of those accesses volatile. > > > > > >=20 > > > > > > One of us is confused.=C2=A0 I hope it isn't me. > > > > > >=20 > > > > >=20 > > > > > It's probably me. I think you have a much better understanding of > > > > > compiler design than I do. Still... > > > > >=20 > > > > > > The hypothetical problem I see is that the C compiler could gen= erate > > > > > > code to load the value "nn->nfsd_serv" twice.=C2=A0 The first t= ime it is=20 > > > > > > not > > > > > > NULL, the second time it is NULL. > > > > > > The first is used for the test, the second is passed to svc_wak= e_up(). > > > > > >=20 > > > > > > Unlikely though this is, it is possible and READ_ONCE() is desi= gned > > > > > > precisely to prevent this. > > > > > > To quote from include/asm-generic/rwonce.h it will > > > > > > =C2=A0=C2=A0 "Prevent the compiler from merging or refetching r= eads" > > > > > >=20 > > > > > > A "volatile" access does not add any cost (in this case).=C2=A0= What it=20 > > > > > > does > > > > > > is break any aliasing that the compile might have deduced. > > > > > > Even if the compiler thinks it has "nn->nfsd_serv" in a registe= r, it > > > > > > won't think it has the result of READ_ONCE(nn->nfsd_serv) in th= at=20 > > > > > > register. > > > > > > And if it needs the result of a previous READ_ONCE(nn->nfsd_ser= v) it > > > > > > won't decide that it can just read nn->nfsd_serv again.=C2=A0 I= t MUST keep > > > > > > the result of READ_ONCE(nn->nfsd_serv) somewhere until it is no= t=20 > > > > > > needed > > > > > > any more. > > > > >=20 > > > > > I'm mainly just considering the resulting pointer. There are two > > > > > possible outcomes to the fetch of nn->nfsd_serv. Either it's a va= lid > > > > > 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. > > > > >=20 > > > > > Maybe I should ask it this way: What bad outcome could result if = we > > > > > don't add READ_ONCE() here? > > > >=20 > > > > Neil just described it. The compiler would generate two load operat= ions, > > > > one for the test and one for the function call argument. The first = load > > > > can retrieve a non-NULL address, and the second a NULL address. > > > >=20 > > > > I agree a READ_ONCE() is necessary. > > > >=20 > > > >=20 > > >=20 > > > Now I'm confused: > > >=20 > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 struct svc_serv *serv; > > >=20 > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 [...] > > >=20 > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 /* > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * The filecache laundrette is shut down a= fter the > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * nn->nfsd_serv pointer is cleared, but b= efore the > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * svc_serv is freed. > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */ > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 serv =3D nn->nfsd_serv; > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 if (serv) > > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= svc_wake_up(serv); > > >=20 > > > This code is explicitly asking to fetch nn->nfsd_serv into the serv > > > variable, and then is testing that copy of the pointer and passing it > > > into svc_wake_up(). > > >=20 > > > How is the compiler allowed to suddenly refetch a NULL pointer into > > > serv after testing that serv is non-NULL? > >=20 > > There's nothing here that tells the compiler it is not allowed to > > optimize this into two separate fetches if it feels that is better > > code. READ_ONCE is what tells the compiler we do not want that re- > > organization /ever/. > >=20 > >=20 >=20 > Well, I think you can argue that even if the compiler does split this > code into two reads of nn->nfsd_serv, it is guaranteed that the read > value is always the same both times -- I guess that's that the comment > is arguing, yes? >=20 Exactly. That's why I didn't just do: if (nn->nfsd_serv) svc_destroy(nn->nfsd_serv); We just have to ensure that we don't pass a NULL pointer to svc_destroy() and that should be guaranteed by fetching it into an interim variable.=C2=A0 A READ_ONCE() doesn't buy us anything extra in this situation. It ensures that it doesn't use a cached version of nn->nfsd_serv when it does the fetch, but using a cached version is harmless here. Either way will still work. Plus, if this had access to a cached version of that variable in a register, it avoided a memory fetch. Given that this should almost never be NULL, adding READ_ONCE() seems like a waste. > I just wonder what will happen if that guarantee should happen to change > in the future. I think the only way this could break would be if nfsd_destroy_serv() started calling svc_destroy(&serv) before canceling the laundrette wq job. Maybe it's worth a comment in there pointing out that dependency. --=20 Jeff Layton