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@xxxxxxxxx> 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?
