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SubjectRe: [PATCH bpf-next v4 1/3] libbpf: Introduce 'btf_custom_path' to 'bpf_obj_open_opts'
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On 7/16/21 12:51 PM, Andrii Nakryiko wrote:
> On Tue, Jul 13, 2021 at 5:43 AM Shuyi Cheng
> <chengshuyi@linux.alibaba.com> wrote:
>>
>> btf_custom_path allows developers to load custom BTF, and subsequent
>> CO-RE will use custom BTF for relocation.
>>
>> Learn from Andrii's comments in [0], add the btf_custom_path parameter
>> to bpf_obj_open_opts, you can directly use the skeleton's
>> <objname>_bpf__open_opts function to pass in the btf_custom_path
>> parameter.
>>
>> Prior to this, there was also a developer who provided a patch with
>> similar functions. It is a pity that the follow-up did not continue to
>> advance. See [1].
>>
>> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>>
>> Signed-off-by: Shuyi Cheng <chengshuyi@linux.alibaba.com>
>> ---
>> tools/lib/bpf/libbpf.c | 36 ++++++++++++++++++++++++++++++------
>> tools/lib/bpf/libbpf.h | 9 ++++++++-
>> 2 files changed, 38 insertions(+), 7 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 1e04ce7..6e11a7b 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -498,6 +498,13 @@ struct bpf_object {
>> * it at load time.
>> */
>> struct btf *btf_vmlinux;
>> + /* Path to the custom BTF to be used for BPF CO-RE relocations.
>> + * This custom BTF completely replaces the use of vmlinux BTF
>> + * for the purpose of CO-RE relocations.
>> + * NOTE: any other BPF feature (e.g., fentry/fexit programs,
>> + * struct_ops, etc) will need actual kernel BTF at /sys/kernel/btf/vmlinux.
>> + */
>
> this comment completely duplicates the one from bpf_object_open_opts,
> I'll remove or shorten it
>
>> + char *btf_custom_path;
>> /* vmlinux BTF override for CO-RE relocations */
>> struct btf *btf_vmlinux_override;
>> /* Lazily initialized kernel module BTFs */
>> @@ -2645,10 +2652,6 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>> struct bpf_program *prog;
>> int i;
>>
>> - /* CO-RE relocations need kernel BTF */
>> - if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
>> - return true;
>> -
>> /* Support for typed ksyms needs kernel BTF */
>> for (i = 0; i < obj->nr_extern; i++) {
>> const struct extern_desc *ext;
>> @@ -2665,6 +2668,13 @@ static bool obj_needs_vmlinux_btf(const struct bpf_object *obj)
>> return true;
>> }
>>
>> + /* CO-RE relocations need kernel BTF, only when btf_custom_path
>> + * is not specified
>> + */
>> + if (obj->btf_ext && obj->btf_ext->core_relo_info.len
>> + && !obj->btf_custom_path)
>> + return true;
>
> not sure why you moved it, I'll move it back to minimize code churn

You're right. 👍

regards,
Shuyi
>
>> +
>> return false;
>> }
>>
>
> [...]
>

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