Re: [RFC bpf-next 05/15] bpftool: Add ability to dump LOC_PARAM, LOC_PROTO and LOCSEC

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2025-10-23 at 09:38 +0100, Alan Maguire wrote:
> On 23/10/2025 01:57, Eduard Zingerman wrote:
> > On Wed, 2025-10-08 at 18:35 +0100, Alan Maguire wrote:
> > 
> > [...]
> > 
> > > @@ -420,6 +423,98 @@ static int dump_btf_type(const struct btf *btf, __u32 id,
> > >  		}
> > >  		break;
> > >  	}
> > > +	case BTF_KIND_LOC_PARAM: {
> > > +		const struct btf_loc_param *p = btf_loc_param(t);
> > > +		__s32 sz = (__s32)t->size;
> > > +
> > > +		if (btf_kflag(t)) {
> > > +			__u64 uval = btf_loc_param_value(t);
> > > +			__s64 sval = (__s64)uval;
> > > +
> > > +			if (json_output) {
> > > +				jsonw_int_field(w, "size", sz);
> > > +				if (sz >= 0)
> > > +					jsonw_uint_field(w, "value", uval);
> > > +				else
> > > +					jsonw_int_field(w, "value", sval);
> > > +			} else {
> > > +				if (sz >= 0)
> > > +					printf(" size=%d value=%llu", sz, uval);
> > > +				else
> > > +					printf(" size=%d value=%lld", sz, sval);
> > > +			}
> > > +		} else {
> > > +			if (json_output) {
> > > +				jsonw_int_field(w, "size", sz);
> > > +				jsonw_uint_field(w, "reg", p->reg);
> > > +				jsonw_uint_field(w, "flags", p->flags);
> > > +				jsonw_int_field(w, "offset", p->offset);
> > > +			} else {
> > > +				printf(" size=%d reg=%u flags=0x%x offset=%d",
> > > +				       sz, p->reg, p->flags, p->offset);
> > 
> > Did you consider printing this in a more user readable form?
> > E.g. `*(u64 *)(rbp - 8)`?
> > 
> 
> That's a good idea. However currently we use register numbers we get
> from DWARF, so would it be more confusing to see something like
> 
> 	*(u64 *)(reg1 -8)
> 
> Not sure (we could translate reg# -> regname but I'm not sure where the
> right place to host such a translation might be).

We can start with hosting the table in bpftool.
Additionally, bpftool can check which architecture the ELF file
containing BPF is built for, and decide whether to apply the
translation.

For x86 you can just grab the table from here:
https://github.com/eddyz87/inline-address-printer/blob/master/main.c#L107

> > > +			}
> > > +		}
> > > +		break;
> > > +	}
> > 
> > [...]
> > 
> >