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

On Thu, Nov 7, 2019 at 4:10 PM Alexei Starovoitov
[off-list ref] wrote:

On Thu, Nov 07, 2019 at 11:16:20PM +0000, Song Liu wrote:

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On Nov 7, 2019, at 3:09 PM, Alexei Starovoitov [off-list ref] wrote:

On Thu, Nov 07, 2019 at 11:07:21PM +0000, Song Liu wrote:

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+
+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.

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+{
+       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?

Yeah, I think fentry_cnt + fexit_cnt is cleaner.

Thanks,
Song