Subject: Re: [PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to 'bpf_obj_open_opts'.

From: Shuyi Cheng <[email-masked]>
Date: Fri, 25 Jun 2021 10:05:19 +0800

On 6/24/21 11:06 PM, Daniel Borkmann wrote:
> On 6/24/21 6:03 AM, Shuyi Cheng wrote:
>> In order to enable the older kernel to use the CO-RE feature, load the
>> vmlinux btf of the specified path.
>>
>> Learn from Andrii's comments in [0], add the custom_btf_path parameter
>> to bpf_obj_open_opts, you can directly use the skeleton's
>> <objname>_bpf__open_opts function to pass in the custom_btf_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=[email-masked]/#t 
>>
>>     [1]https://yhbt.net/lore/all/[email-masked]/ 
>>
>>
>> Signed-off-by: Shuyi Cheng <[email-masked]>
>> ---
>>   tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
>>   tools/lib/bpf/libbpf.h |  6 +++++-
>>   2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 1e04ce7..518b19f 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -509,6 +509,8 @@ struct bpf_object {
>>       void *priv;
>>       bpf_object_clear_priv_t clear_priv;
>> +    char *custom_btf_path;
>> +
> 
> nit: This should rather go to the 'Parse and load BTF vmlinux if any of 
> [...]'
> section of struct bpf_object, and for consistency, I'd keep the btf_ 
> prefix,
> like: char *btf_custom_path
> 

Thank you very much for your reply.

Agree.

>>       char path[];
>>   };
>>   #define obj_elf_valid(o)    ((o)->efile.elf)
>> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct 
>> bpf_object *obj, bool force)
>>       if (!force && !obj_needs_vmlinux_btf(obj))
>>           return 0;
>> -    obj->btf_vmlinux = libbpf_find_kernel_btf();
>> -    err = libbpf_get_error(obj->btf_vmlinux);
>> +    if (obj->custom_btf_path) {
>> +        obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
>> +        err = libbpf_get_error(obj->btf_vmlinux);
>> +        pr_debug("loading custom vmlinux BTF '%s': %d\n", 
>> obj->custom_btf_path, err);
>> +    } else {
>> +        obj->btf_vmlinux = libbpf_find_kernel_btf();
>> +        err = libbpf_get_error(obj->btf_vmlinux);
>> +    }
> 
> Couldn't we do something like (only compile-tested):
> 

Your approach is very inspiring to me. But I did it for two reasons.

1. When the developer specifies btf_custom_path, btf should only be 
loaded from btf_custom_path;
2. Now pahole supports saving vmlinux's btf in raw format, so the old 
kernel can provide btf in elf format or raw format. see [0].

	[0] 
https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/pahole.c#n1157

What do you think?
	
Regards,
Shuyi

> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index b46760b93bb4..5b88ce3e483c 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -4394,7 +4394,7 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
>    * Probe few well-known locations for vmlinux kernel image and try to 
> load BTF
>    * data out of it to use for target BTF.
>    */
> -struct btf *libbpf_find_kernel_btf(void)
> +static struct btf *__libbpf_find_kernel_btf(char *btf_custom_path)
>   {
>       struct {
>           const char *path_fmt;
> @@ -4402,6 +4402,8 @@ struct btf *libbpf_find_kernel_btf(void)
>       } locations[] = {
>           /* try canonical vmlinux BTF through sysfs first */
>           { "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
> +        /* try user defined vmlinux ELF if a path was specified */
> +        { btf_custom_path },
>           /* fall back to trying to find vmlinux ELF on disk otherwise */
>           { "/boot/vmlinux-%1$s" },
>           { "/lib/modules/%1$s/vmlinux-%1$s" },
> @@ -4419,11 +4421,11 @@ struct btf *libbpf_find_kernel_btf(void)
>       uname(&buf);
> 
>       for (i = 0; i < ARRAY_SIZE(locations); i++) {
> +        if (!locations[i].path_fmt)
> +            continue;
>           snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
> -
>           if (access(path, R_OK))
>               continue;
> -
>           if (locations[i].raw_btf)
>               btf = btf__parse_raw(path);
>           else
> @@ -4440,6 +4442,11 @@ struct btf *libbpf_find_kernel_btf(void)
>       return libbpf_err_ptr(-ESRCH);
>   }
> 
> +struct btf *libbpf_find_kernel_btf(void)
> +{
> +    return __libbpf_find_kernel_btf(NULL);
> +}
> +
>   int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn 
> visit, void *ctx)
>   {
>       int i, n, err;
> 
> And then you just call it as:
> 
>      obj->btf_vmlinux = __libbpf_find_kernel_btf(obj->btf_custom_path);
>      err = libbpf_get_error(obj->btf_vmlinux);
> 
>>       if (err) {
>>           pr_warn("Error loading vmlinux BTF: %d\n", err);
>>           obj->btf_vmlinux = NULL;