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

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Date:   Thu, 7 Nov 2019 16:09:43 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     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 Thu, Nov 07, 2019 at 11:16:20PM +0000, Song Liu wrote:
> 
> 
> > On Nov 7, 2019, at 3:09 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > 
> > On Thu, Nov 07, 2019 at 11:07:21PM +0000, Song Liu wrote:
> >> 
> >> 
> >>> 
> >>> 
> >>>>> +
> >>>>> +static int bpf_trampoline_update(struct bpf_prog *prog)
> >>>> 
> >>>> Seems argument "prog" is not used at all? 
> >>> 
> >>> like one below ? ;)
> >> e... I was really dumb... sorry..
> >> 
> >> Maybe we should just pass the tr in? 
> > 
> > that would be imbalanced.
> 
> Hmm.. what do you mean by imbalanced?

I take it back. Yeah. It can be tr.

> 
> > 
> >>> 
> >>>>> +{
> >>>>> +	struct bpf_trampoline *tr = prog->aux->trampoline;
> >>>>> +	void *old_image = tr->image + ((tr->selector + 1) & 1) * PAGE_SIZE/2;
> >>>>> +	void *new_image = tr->image + (tr->selector & 1) * PAGE_SIZE/2;
> >>>>> +	if (err)
> >>>>> +		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?