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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Nov 19, 2020 at 05:24:43PM -0800, Andrii Nakryiko wrote:
> On Thu, Nov 19, 2020 at 4:55 PM Maciej Fijalkowski
> <maciej.fijalkowski@intel.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@kernel.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?

Hm, just to make use of local var? We might argue that current version is
more readable, but then again I would question the len's existence.

Do whatever you want, these were just nits :)

> 
> >
> > > +     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;
> > > +}
> >
> > [...]