netdev - SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4 13/17] khwasan: add hooks implementation)

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Message-ID: <e3b48104-3efb-1896-0d46-792419f49a75@virtuozzo.com>
Date:   Tue, 31 Jul 2018 20:01:30 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, Gerrit Renker <gerrit@....abdn.ac.uk>,
        dccp@...r.kernel.org, Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Eric Dumazet <edumazet@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Ursula Braun <ubraun@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4 13/17]
 khwasan: add hooks implementation)


On 07/31/2018 07:04 PM, Andrey Ryabinin wrote:
>> Somewhat offtopic, but I can't understand how SLAB_TYPESAFE_BY_RCU
>> slabs can be useful without ctors or at least memset(0). Objects in
>> such slabs need to be type-stable, but I can't understand how it's
>> possible to establish type stability without a ctor... Are these bugs?
> 
> Yeah, I puzzled by this too. However, I think it's hard but possible to make it work, at least in theory.
> There must be an initializer, which consists of two parts:
> a) initilize objects fields
> b) expose object to the world (add it to list or something like that)
> 
> (a) part must somehow to be ok to race with another cpu which might already use the object.
> (b) part must must use e.g. barriers to make sure that racy users will see previously inilized fields.
> Racy users must have parring barrier of course.
> 
> But it sound fishy, and very easy to fuck up. I won't be surprised if every single one SLAB_TYPESAFE_BY_RCU user
> without ->ctor is bogus. It certainly would be better to convert those to use ->ctor.
> 
> Such caches seems used by networking subsystem in proto_register():
> 
> 		prot->slab = kmem_cache_create_usercopy(prot->name,
> 					prot->obj_size, 0,
> 					SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT |
> 					prot->slab_flags,
> 					prot->useroffset, prot->usersize,
> 					NULL);
> 
> And certain protocols specify SLAB_TYPESAFE_BY_RCU in ->slab_flags, such as:
> llc_proto, smc_proto, smc_proto6, tcp_prot, tcpv6_prot, dccp_v6_prot, dccp_v4_prot.
> 
> 
> Also nf_conntrack_cachep, kernfs_node_cache, jbd2_journal_head_cache and i915_request cache.
> 


[+CC maintainer of the relevant code.]

Guys, it seems that we have a lot of code using SLAB_TYPESAFE_BY_RCU cache without constructor.
I think it's nearly impossible to use that combination without having bugs.
It's either you don't really need the SLAB_TYPESAFE_BY_RCU, or you need to have a constructor in kmem_cache.

Could you guys, please, verify your code if it's really need SLAB_TYPSAFE or constructor?

E.g. the netlink code look extremely suspicious:

	/*
	 * Do not use kmem_cache_zalloc(), as this cache uses
	 * SLAB_TYPESAFE_BY_RCU.
	 */
	ct = kmem_cache_alloc(nf_conntrack_cachep, gfp);
	if (ct == NULL)
		goto out;

	spin_lock_init(&ct->lock);

If nf_conntrack_cachep objects really used in rcu typesafe manner, than 'ct' returned by kmem_cache_alloc might still be
in use by another cpu. So we just reinitialize spin_lock used by someone else?