Re: [PATCH bpf-next v4 0/7] Introduce BPF_MODIFY_RET tracing progs

[KP Singh]

On 04-Mar 14:17, Alexei Starovoitov wrote:
> On Wed, Mar 04, 2020 at 08:18:46PM +0100, KP Singh wrote:
> > 
> > Here is an example of how a fmod_ret program behaves:
> > 
> > int func_to_be_attached(int a, int b)
> V> {  <--- do_fentry
> > 
> > do_fmod_ret:
> >    <update ret by calling fmod_ret>
> >    if (ret != 0)
> >         goto do_fexit;
> > 
> > original_function:
> > 
> >     <side_effects_happen_here>
> > 
> > }  <--- do_fexit
> > 
> > ALLOW_ERROR_INJECTION(func_to_be_attached, ERRNO)
> > 
> > The fmod_ret program attached to this function can be defined as:
> > 
> > SEC("fmod_ret/func_to_be_attached")
> > int BPF_PROG(func_name, int a, int b, int ret)
> > {
> >         // This will skip the original function logic.
> >         return -1;
> > }
> 
> Applied to bpf-next. Thanks.

Thanks.

> 
> I think it sets up a great base to parallelize further work.

Totally Agreed!

> 
> 1. I'm rebasing my sleepable BPF patches on top.
> It's necessary to read enviroment variables without the
> 'opportunistic copy before hand' hack I saw in your github tree

:) Sleepable BPF would indeed make it much simpler.

> to do bpf_get_env_var() helper.
> 
> 2. please continue on LSM_HOOK patches to go via security tree.
> 
> 3. we need a volunteer to generalize bpf_sk_storage to task and inode structs.

This is quite important, especially for some of the examples we had
brought up.

I can take a look at the generalization of bpf_sk_storage.

Thanks!
- KP

> This work will be super useful for all bpf tracing too.
> Sleepable progs are useful for tracing as well.