Re: [PATCH bpf-next v3 4/5] libbpf: fix error when st-prefix_ops and ops from differ btf

D. Wythe
2025-01-08T13:45:31Z

On Tue, Jan 07, 2025 at 03:24:51PM -0800, Martin KaFai Lau wrote:
> On 12/22/24 6:10 PM, D. Wythe wrote:
> >On Thu, Dec 19, 2024 at 02:43:30PM -0800, Martin KaFai Lau wrote:
> >>On 12/17/24 6:44 PM, D. Wythe wrote:
> >>>Here are four possible case:
> >>>
> >>>+--------+-------------+-------------+---------------------------------+
> >>>| | st_opx_xxx | xxx | |
> >>>+--------+-------------+-------------+---------------------------------+
> >>>| case 0 | btf_vmlinux | bft_vmlinux | be used and reg only in vmlinux |
> >>>+--------+-------------+-------------+---------------------------------+
> >>>| case 1 | btf_vmlinux | bpf_mod | INVALID |
> >>>+--------+-------------+-------------+---------------------------------+
> >>>| case 2 | btf_mod | btf_vmlinux | reg in mod but be used both in |
> >>>| | | | vmlinux and mod. |
> >>>+--------+-------------+-------------+---------------------------------+
> >>>| case 3 | btf_mod | btf_mod | be used and reg only in mod |
> >>>+--------+-------------+-------------+---------------------------------+
> >>>
> >>>At present, cases 0, 1, and 3 can be correctly identified, because
> >>>st_ops_xxx is searched from the same btf with xxx. In order to
> >>>handle case 2 correctly without affecting other cases, we cannot simply
> >>>change the search method for st_ops_xxx from find_btf_by_prefix_kind()
> >>>to find_ksym_btf_id(), because in this way, case 1 will not be
> >>>recognized anymore.
> >>> snprintf(tname, sizeof(tname), "%.*s",
> >>>@@ -1020,17 +1021,25 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
> >>> }
> >>> kern_type = btf__type_by_id(btf, kern_type_id);
> >>>+ ret = snprintf(stname, sizeof(stname), "%s%s", STRUCT_OPS_VALUE_PREFIX, tname);
> >>
> >>How about always look for "struct bpf_struct_ops_smc_ops" first,
> >>figure out the btf, and then look for "struct smc_ops", would it
> >>work?
> >
> >I think this might not work, as the core issue lies in the fact that
> >bpf_struct_ops_smc_ops and smc_ops are located on different btf.
> >Searching for one fisrt cannot lead to the inference of the other.
>
> Take a look at btf_find_by_name_kind(btf, 1 /* from base_btf */,
> ...) and also btf_type_by_id(). It starts searching from the
> btf->base_btf which should be the btf_vmlinux here and should have
> the "struct smc_ops". Please try.

Got it, I will try it, thanks for your suggestion.

D. Wythe