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    SubjectSLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4 13/17] khwasan: add hooks implementation)
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    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?

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