netdev - Re: [RFC PATCH] bpf: Fix potential call bpf_link

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Date:   Mon, 21 Sep 2020 21:27:43 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     Alexei Starovoitov <ast@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH] bpf: Fix potential call bpf_link_free() in atomic
 context

On 9/21/20 8:31 PM, Andrii Nakryiko wrote:
> On Mon, Sep 21, 2020 at 10:29 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
>>
>> On Thu, Sep 17, 2020 at 12:46 AM Muchun Song <songmuchun@...edance.com> wrote:
>>>
>>> The in_atomic macro cannot always detect atomic context. In particular,
>>> it cannot know about held spinlocks in non-preemptible kernels. Although,
>>> there is no user call bpf_link_put() with holding spinlock now. Be the
>>> safe side, we can avoid this in the feature.
>>>
>>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>>> ---
>>
>> This change seems unnecessary (or at least premature), as if we ever
>> get a use case that does bpf_link_put() from under held spinlock, we
>> should see a warning about that (and in that case I bet code can be
>> rewritten to not hold spinlock during bpf_link_put()). But on the
>> other hand it makes bpf_link_put() to follow the pattern of
>> bpf_map_put(), which always defers the work, so I'm ok with this. As
>> Song mentioned, this is not called from a performance-critical hot
>> path, so doesn't matter all that much.
>>
>> Acked-by: Andrii Nakryiko <andriin@...com>

Agree, SGTM.

> btw, you probably need to resubmit this patch as a non-RFC one for it
> to be applied?..

Given first time BPF contributor & it has already several ACKs, I took it
into bpf-next, thanks!