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Date:   Fri, 24 Jul 2020 18:52:20 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
        bjorn.topel@...el.com, magnus.karlsson@...el.com
Subject: Re: [PATCH v4 bpf-next 2/6] bpf: propagate poke descriptors to
 subprograms

On Fri, Jul 24, 2020 at 06:02:30PM +0200, Daniel Borkmann wrote:
> On 7/24/20 2:36 PM, Maciej Fijalkowski wrote:
> > Previously, there was no need for poke descriptors being present in
> > subprogram's bpf_prog_aux struct since tailcalls were simply not allowed
> > in them. Each subprog is JITed independently so in order to enable
> > JITing such subprograms, simply copy poke descriptors from main program
> > to subprogram's poke tab.
> > 
> > Add also subprog's aux struct to the BPF map poke_progs list by calling
> > on it map_poke_track().
> > 
> > In case of any error, call the map_poke_untrack() on subprog's aux
> > structs that have already been registered to prog array map.
> > 
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> > ---
> >   kernel/bpf/verifier.c | 41 +++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 41 insertions(+)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 9a6703bc3f36..3e931e3e2239 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -9900,9 +9900,12 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> >   {
> >   	struct bpf_prog *prog = env->prog, **func, *tmp;
> >   	int i, j, subprog_start, subprog_end = 0, len, subprog;
> > +	struct bpf_map *map_ptr;
> >   	struct bpf_insn *insn;
> >   	void *old_bpf_func;
> >   	int err, num_exentries;
> > +	int last_poke_desc = 0;
> > +	int last_subprog = 0;
> >   	if (env->subprog_cnt <= 1)
> >   		return 0;
> > @@ -9967,6 +9970,26 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> >   		func[i]->aux->btf = prog->aux->btf;
> >   		func[i]->aux->func_info = prog->aux->func_info;
> > +		for (j = 0; j < prog->aux->size_poke_tab; j++) {
> > +			int ret;
> > +
> > +			ret = bpf_jit_add_poke_descriptor(func[i],
> > +							  &prog->aux->poke_tab[j]);
> > +			if (ret < 0) {
> > +				verbose(env, "adding tail call poke descriptor failed\n");
> > +				goto out_untrack;
> > +			}
> > +			map_ptr = func[i]->aux->poke_tab[j].tail_call.map;
> > +			ret = map_ptr->ops->map_poke_track(map_ptr, func[i]->aux);
> > +			if (ret < 0) {
> > +				verbose(env, "tracking tail call prog failed\n");
> > +				goto out_untrack;
> > +			}
> > +			last_poke_desc = j;
> > +		}
> > +		last_poke_desc = 0;
> > +		last_subprog = i;
> > +
> >   		/* Use bpf_prog_F_tag to indicate functions in stack traces.
> >   		 * Long term would need debug info to populate names
> >   		 */
> > @@ -10059,7 +10082,25 @@ static int jit_subprogs(struct bpf_verifier_env *env)
> >   	prog->aux->func_cnt = env->subprog_cnt;
> >   	bpf_prog_free_unused_jited_linfo(prog);
> >   	return 0;
> > +out_untrack:
> > +	/* this loop is only for handling the case where propagating all of
> > +	 * the main prog's poke descs was not successful for a particular
> > +	 * subprog; last_poke_desc is zeroed once we walked through all
> > +	 * of the poke descs; if last_poke_desc != 0 then 'i' is valid since
> > +	 * it is pointing to the subprog that we were at when got an error
> > +	 */
> > +	while (last_poke_desc--) {
> > +		map_ptr = func[i]->aux->poke_tab[last_poke_desc].tail_call.map;
> > +		map_ptr->ops->map_poke_untrack(map_ptr, func[i]->aux);
> > +	}
> > +	last_subprog = i - 1;
> >   out_free:
> > +	for (i = last_subprog; i >= 0; i--) {
> > +		for (j = 0; j < prog->aux->size_poke_tab; j++) {
> > +			map_ptr = func[i]->aux->poke_tab[j].tail_call.map;
> > +			map_ptr->ops->map_poke_untrack(map_ptr, func[i]->aux);
> > +		}
> > +	}
> 
> After staring at this code for a while, the logic looks correct to me, but feels overly
> complicated with making sure all the corner cases do function above. I wonder, why didn't
> you consider just something like ...
> 
>    	for (i = 0; i < env->subprog_cnt; i++)
>    		if (func[i]) {
> 			for (j = 0; j < func[i]->aux->size_poke_tab; j++) {
> 				map_ptr = func[i]->aux->poke_tab[j].tail_call.map;
> 				map_ptr->ops->map_poke_untrack(map_ptr, func[i]->aux);
> 			}
>    			bpf_jit_free(func[i]);
> 		}
> 
> ... instead of last_poke_desc/last_subprog tracking and the fallthrough trick above. Am
> I missing something?

No, that's just the great example of over-engineering. If we bail out
during the poke propagation loop then on error path the size_poke_tab from
func[i]->aux as you're suggesting will let us to simply go through all of
the previously copied descs. Other out_free will be addressed as well.

So, v5.

> 
> >   	for (i = 0; i < env->subprog_cnt; i++)
> >   		if (func[i])
> >   			bpf_jit_free(func[i]);
> > 
> 

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