netdev - Re: [PATCH v2 bpf-next 03/17] bpf: Introduce BPF trampoline

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Message-ID: <3000B3E1-25DE-4653-B11C-AAF61492B0FF@fb.com>
Date:   Fri, 8 Nov 2019 04:06:05 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Song Liu <liu.song.a23@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "x86@...nel.org" <x86@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 03/17] bpf: Introduce BPF trampoline



> On Nov 7, 2019, at 7:11 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Thu, Nov 7, 2019 at 5:10 PM Song Liu <liu.song.a23@...il.com> wrote:
>>>>>>>>> +               goto out;
>>>>>>>>> +       tr->selector++;
>>>>>>>> 
>>>>>>>> Shall we do selector-- for unlink?
>>>>>>> 
>>>>>>> It's a bit flip. I think it would be more confusing with --
>>>>>> 
>>>>>> Right.. Maybe should use int instead of u64 for selector?
>>>>> 
>>>>> No, since int can overflow.
>>>> 
>>>> I guess it is OK to overflow, no?
>>> 
>>> overflow is not ok, since transition 0->1 should use nop->call patching
>>> whereas 1->2, 2->3 should use call->call.
>>> 
>>> In my initial implementation (one I didn't share with anyone) I had
>>> trampoline_mutex taken inside bpf_trampoline_update(). And multiple link()
>>> operation were allowed. The idea was to attach multiple progs and update
>>> trampoline once. But then I realized that I cannot do that since 'unlink +
>>> update' where only 'update' is taking lock will not guarantee success. Since
>>> other 'link' operations can race and 'update' can potentially fail in
>>> arch_prepare_bpf_trampoline() due to new things that 'link' brought in. In that
>>> version (since there several fentry/fexit progs can come in at once) I used
>>> separate 'selector' ticker to pick the side of the page. Once I realized the
>>> issue (to guarantee that unlink+update == always success) I moved mutex all the
>>> way to unlink and link and left 'selector' as-is. Just now I realized that
>>> 'selector' can be removed.  fentry_cnt + fexit_cnt can be used instead. This
>>> sum of counters will change 1 bit at a time. Am I right?
>> 
>> Yeah, I think fentry_cnt + fexit_cnt is cleaner.
> 
> ... and that didn't work.
> It's transition that matters. Either need to remember previous sum value
> or have separate selector. imo selector is cleaner, so I'm back to that.

Hmm.. is this because of the error handling path?

+	tr->progs_cnt[kind]++;
+	err = bpf_trampoline_update(prog);
+	if (err) {
+		hlist_del(&prog->aux->tramp_hlist);
+		tr->progs_cnt[kind]--;
+	}

Thanks,
Song