RE: [PATCH v3 bpf-next] libbpf: add bpf object kern_version attribute setter

[John Fastabend <john.fastabend@gmail.com>]

Rafael David Tinoco wrote:
> Unfortunately some distros don't have their kernel version defined
> accurately in <linux/version.h> due to different long term support
> reasons.
> 
> It is important to have a way to override the bpf kern_version
> attribute during runtime: some old kernels might still check for
> kern_version attribute during bpf_prog_load().
> 
> Signed-off-by: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>
> ---
>  tools/lib/bpf/libbpf.c   | 10 ++++++++++
>  tools/lib/bpf/libbpf.h   |  1 +
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 12 insertions(+)
> 

Hi Andrii and Rafael,

Did you consider making kernel version an attribute of the load
API, bpf_prog_load_xattr()? This feels slightly more natural
to me, to tell the API the kernel you need at load time.

Although, I don't use the skeleton pieces so maybe it would be
awkward for that usage.

Sorry, missed v1,v2 so didn't reply sooner.

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 058b643cbcb1..3ac3d8dced7f 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -8269,6 +8269,16 @@ int bpf_object__btf_fd(const struct bpf_object *obj)
>  	return obj->btf ? btf__fd(obj->btf) : -1;
>  }
>  
> +int bpf_object__set_kversion(struct bpf_object *obj, __u32 kern_version)
> +{
> +	if (obj->loaded)
> +		return -EINVAL;
> +
> +	obj->kern_version = kern_version;
> +
> +	return 0;
> +}
> +

Having a test to read uname and feed it into libbpf using
above to be sure we don't break this in the future would be
nice.

>  int bpf_object__set_priv(struct bpf_object *obj, void *priv,
>  			 bpf_object_clear_priv_t clear_priv)
>  {
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index a1a424b9b8ff..cf9bc6f1f925 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -143,6 +143,7 @@ LIBBPF_API int bpf_object__unload(struct bpf_object *obj);
>  
>  LIBBPF_API const char *bpf_object__name(const struct bpf_object *obj);
>  LIBBPF_API unsigned int bpf_object__kversion(const struct bpf_object *obj);
> +LIBBPF_API int bpf_object__set_kversion(struct bpf_object *obj, __u32 kern_version);
>  
>  struct btf;
>  LIBBPF_API struct btf *bpf_object__btf(const struct bpf_object *obj);
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index 279ae861f568..f5990f7208ce 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -359,4 +359,5 @@ LIBBPF_0.4.0 {
>  		bpf_linker__finalize;
>  		bpf_linker__free;
>  		bpf_linker__new;
> +		bpf_object__set_kversion;
>  } LIBBPF_0.3.0;
> -- 
> 2.27.0
>