netdev - Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive bpf_task_storage

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Message-ID: <a3eaaeb5-f14a-ef55-10c2-884dc9365f57@fb.com>
Date:   Tue, 23 Feb 2021 08:44:49 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Song Liu <songliubraving@...com>
CC:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Peter Ziljstra <peterz@...radead.org>
Subject: Re: [PATCH v4 bpf-next 2/6] bpf: prevent deadlock from recursive
 bpf_task_storage_[get|delete]

On 2/22/21 11:19 PM, Andrii Nakryiko wrote:
>>>> +       bpf_task_storage_lock();
>>>>         sdata = bpf_local_storage_update(
>>>>                 task, (struct bpf_local_storage_map *)map, value, map_flags);
>>> this should probably be container_of() instead of casting
>> bpf_task_storage.c uses casting in multiple places. How about we fix it in a
>> separate patch?
>>
> Sure, let's fix all uses in a separate patch. My point is that this
> makes an assumption (that struct bpf_map map field is always the very
> first one) which is not enforced and not documented in struct
> bpf_local_storage_map.
> 

I'd rather document it in separate patch.
Just doing container_of here and there will lead to wrong assumption
that it can be in a different place, but it's much more involved.
Consider what happens with all map_alloc callbacks... how that pointer
is hard coded into bpf prog.. then passed back into helpers...
then the logic that can read inner fields of bpf_map from the prog...