Re: [PATCH 1/2] bpf: add a bpf_override_function helper

[Josef Bacik]

On Fri, Nov 10, 2017 at 10:34:59AM +0100, Ingo Molnar wrote:
> 
> * Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
> 
> > @@ -551,6 +578,10 @@ static const struct bpf_func_proto *kprobe_prog_func_proto(enum bpf_func_id func
> >  		return &bpf_get_stackid_proto;
> >  	case BPF_FUNC_perf_event_read_value:
> >  		return &bpf_perf_event_read_value_proto;
> > +	case BPF_FUNC_override_return:
> > +		pr_warn_ratelimited("%s[%d] is installing a program with bpf_override_return helper that may cause unexpected behavior!",
> > +				    current->comm, task_pid_nr(current));
> > +		return &bpf_override_return_proto;
> 
> So if this new functionality is used we'll always print this into the syslog?
> 
> The warning is also a bit passive aggressive about informing the user: what 
> unexpected behavior can happen, what is the worst case?
> 

It's modeled after the other warnings bpf will spit out, but with this feature
you are skipping a function and instead returning some arbitrary value, so
anything could go wrong if you mess something up.  For instance I screwed up my
initial test case and made every IO submitted return an error instead of just on
the one file system I was attempting to test, so all sorts of hilarity ensued.
Thanks,

Josef