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

Martin KaFai Lau
2025-01-07T23:25:16Z

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.