netdev - Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations

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Date:   Thu, 19 Nov 2020 17:24:43 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for
 CO-RE relocations

On Thu, Nov 19, 2020 at 4:55 PM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Thu, Nov 19, 2020 at 03:22:42PM -0800, Andrii Nakryiko wrote:
> > Teach libbpf to search for candidate types for CO-RE relocations across kernel
> > modules BTFs, in addition to vmlinux BTF. If at least one candidate type is
> > found in vmlinux BTF, kernel module BTFs are not iterated. If vmlinux BTF has
> > no matching candidates, then find all kernel module BTFs and search for all
> > matching candidates across all of them.
> >
> > Kernel's support for module BTFs are inferred from the support for BTF name
> > pointer in BPF UAPI.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >  tools/lib/bpf/libbpf.c | 185 ++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 172 insertions(+), 13 deletions(-)
> >
>
> [...]
>
> > +static int probe_module_btf(void)
> > +{
> > +     static const char strs[] = "\0int";
> > +     __u32 types[] = {
> > +             /* int */
> > +             BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
> > +     };
> > +     struct bpf_btf_info info;
> > +     __u32 len = sizeof(info);
> > +     char name[16];
> > +     int fd, err;
> > +
> > +     fd = libbpf__load_raw_btf((char *)types, sizeof(types), strs, sizeof(strs));
> > +     if (fd < 0)
> > +             return 0; /* BTF not supported at all */
> > +
> > +     len = sizeof(info);
>
> nit: reinit of len
>

oops, right, I'll remove it


> > +     memset(&info, 0, sizeof(info));
>
> use len in memset

why?

>
> > +     info.name = ptr_to_u64(name);
> > +     info.name_len = sizeof(name);
> > +
> > +     /* check that BPF_OBJ_GET_INFO_BY_FD supports specifying name pointer;
> > +      * kernel's module BTF support coincides with support for
> > +      * name/name_len fields in struct bpf_btf_info.
> > +      */
> > +     err = bpf_obj_get_info_by_fd(fd, &info, &len);
> > +     close(fd);
> > +     return !err;
> > +}
>
> [...]