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Subject: Re: [PATCH bpf-next v10 06/10] bpf,landlock: Add a new map type:
 inode
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
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From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mickael.salaun@ssi.gouv.fr>
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Date:   Wed, 31 Jul 2019 21:11:10 +0200
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On 31/07/2019 20:58, Alexei Starovoitov wrote:
> On Wed, Jul 31, 2019 at 11:46 AM Micka=C3=ABl Sala=C3=BCn
> <mickael.salaun@ssi.gouv.fr> wrote:
>>>> +    for (i =3D 0; i < htab->n_buckets; i++) {
>>>> +            head =3D select_bucket(htab, i);
>>>> +            hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
>>>> +                    landlock_inode_remove_map(*((struct inode **)l->k=
ey), map);
>>>> +            }
>>>> +    }
>>>> +    htab_map_free(map);
>>>> +}
>>>
>>> user space can delete the map.
>>> that will trigger inode_htab_map_free() which will call
>>> landlock_inode_remove_map().
>>> which will simply itereate the list and delete from the list.
>>
>> landlock_inode_remove_map() removes the reference to the map (being
>> freed) from the inode (with an RCU lock).
>
> I'm going to ignore everything else for now and focus only on this bit,
> since it's fundamental issue to address before this discussion can
> go any further.
> rcu_lock is not a spin_lock. I'm pretty sure you know this.
> But you're arguing that it's somehow protecting from the race
> I mentioned above?
>

I was just clarifying your comment to avoid misunderstanding about what
is being removed.

As said in the full response, there is currently a race but, if I add a
bpf_map_inc() call when the map is referenced by inode->security, then I
don't see how a race could occur because such added map could only be
freed in a security_inode_free() (as long as it retains a reference to
this inode).


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