Re: [RFC PATCH bpf-next v3 1/2] libbpf: support selective kernel module BTF loading via bpf_object_open_opts

On 8/19/2026 5:12 PM, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [Medium] Premature loop termination in load_module_btfs when kernel contains multiple BTF objects for the same module.
> - [Low] API design prevents users from explicitly skipping all kernel module BTFs without providing a dummy name.
> --
> 
> --- Patch [1]: [RFC PATCH bpf-next v3 1/2] libbpf: support selective kernel module BTF loading via bpf_object_open_opts ---
> commit 627a7f9dbb55055d6c4d1331859084b6d2f56792
> Author: Fuyu Zhao <[email protected]>
> 
> libbpf: support selective kernel module BTF loading via bpf_object_open_opts
> 
> Add kmod_btf_names and kmod_btf_names_cnt fields to bpf_object_open_opts
> to support selective kernel module BTF loading. This allows libbpf to
> load only specified module BTFs and skip others.
> 
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index b749c01742ee0..957415341caf1 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
> 
> [ ... ]
> 
>> @@ -5851,6 +5858,121 @@ int bpf_core_add_cands(struct bpf_core_cand *local_cand,
>>  	return 0;
>>  }
> 
> [ ... ]
> 
>> +static int bpf_object__init_kmod_btfs(struct bpf_object *obj,
>> +				      const struct bpf_object_open_opts *opts)
>> +{
>> +	const char **kmod_btf_names;
>> +	size_t i, kmod_btf_names_cnt;
>> +	int err;
>> +
>> +	kmod_btf_names = OPTS_GET(opts, kmod_btf_names, NULL);
>> +	if (!kmod_btf_names)
>> +		return 0;
>> +
>> +	kmod_btf_names_cnt = OPTS_GET(opts, kmod_btf_names_cnt, 0);
>> +	if (!kmod_btf_names_cnt) {
>> +		pr_warn("kmod_btf_names_cnt must be set when kmod_btf_names is provided\n");
>> +		return -EINVAL;
>> +	}
> 
> [Severity: Low]
> If a user wants to optimize BPF object loading by skipping all module BTFs
> (for instance, when only vmlinux BTF is needed), setting kmod_btf_names to NULL
> falls back to the default behavior of loading all modules, but providing a
> valid pointer with kmod_btf_names_cnt = 0 returns -EINVAL.
> 
> Does this API design force users to allocate and pass a dummy module name just
> to skip loading module BTFs entirely?
> 

The API is not intended to explicitly skip all module BTF loading.

A NULL kmod_btf_names means no module filter is specified, so libbpf
preserves the existing behavior.

When kmod_btf_names is provided, kmod_btf_names_cnt must specify the
number of entries. A non-NULL list with a zero count is therefore treated
as invalid API usage, rather than a request to skip all module BTFs.

>> +
>> +	obj->kmod_btfs = calloc(1, sizeof(*obj->kmod_btfs));
> 
> [ ... ]
> 
>> +static bool all_needed_kmod_btfs_loaded(const struct bpf_object *obj)
>> +{
>> +	return obj->kmod_btfs &&
>> +	       obj->kmod_btfs->nr_names > 0 &&
>> +	       obj->kmod_btfs->nr_names == obj->btf_module_cnt;
>> +}
> 
> [Severity: Medium]
> If a kernel module is unloaded but its BTF object is kept alive by an open file
> descriptor, and the module is then reloaded, the kernel will contain multiple
> BTF objects with the same module name.
> 
> Because btf_module_cnt increments for every loaded BTF object, but nr_names
> tracks the number of unique requested module names, can duplicate BTF objects
> inflate btf_module_cnt?
> 
> If a user requests BTFs for multiple modules via kmod_btf_names, would this
> condition evaluate to true prematurely and skip the remaining requested modules?
> 

I don't think this can cause premature termination. `load_module_btfs()`
iterates BTFs in ascending ID order, so a reloaded module gets a higher
BTF ID and is encountered later. We stop as soon as all requested module
BTFs are loaded, before reaching such duplicate BTFs.

>> +
>>  static int load_module_btfs(struct bpf_object *obj)
>>  {
> 
> [ ... ]
> 
>> @@ -5939,6 +6066,9 @@ static int load_module_btfs(struct bpf_object *obj)
>>  			break;
>>  		}
>>  		obj->btf_module_cnt++;
>> +
>> +		if (all_needed_kmod_btfs_loaded(obj))
>> +			break;
>>  	}
>>  
>>  	if (err) {
>