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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Jun 24, 2021 at 8:06 AM Daniel Borkmann <daniel@iogearbox.net> 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=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 | 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
>
> >       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):

I wonder what are the benefits of this approach, though. My
expectation is that if the user specifies a custom BTF path and BTF is
missing then the whole bpf_object load process should fail, but in
this case it will be silently ignored. Also, if custom BTF is
specified, that custom BTF has to be used even if
/sys/kernel/btf/vmlinux is present, but the patch below will still
prefer /sys/kernel/btf/vmlinux.

So the semantics is different. I'm not saying it's wrong, but I think
it means we need to discuss what behavior we are after first.

>
> 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;

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