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

[Shuyi Cheng]

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=Q@xxxxxxxxxxxxxx/#t 
> >
> >     [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@xxxxxxxxxxxxxx/ 
> >
> >
> > Signed-off-by: Shuyi Cheng <chengshuyi@xxxxxxxxxxxxxxxxx>
> > ---
> >   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;