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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Martin KaFai Lau <kafai@fb.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations
Date: Fri, 20 Nov 2020 15:12:41 -0800	[thread overview]
Message-ID: <CAEf4BzYU8h6CaruMasZW8C2CF27GVhV-2mnM5wz4rpLzyNSYtg@mail.gmail.com> (raw)
In-Reply-To: <20201120230549.37k4zsjsrxbyjin3@kafai-mbp.dhcp.thefacebook.com>

On Fri, Nov 20, 2020 at 3:06 PM Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Thu, Nov 19, 2020 at 03:22:42PM -0800, Andrii Nakryiko wrote:
> [ ... ]
>
> > +static int load_module_btfs(struct bpf_object *obj)
> > +{
> > +     struct bpf_btf_info info;
> > +     struct module_btf *mod_btf;
> > +     struct btf *btf;
> > +     char name[64];
> > +     __u32 id, len;
> > +     int err, fd;
> > +
> > +     if (obj->btf_modules_loaded)
> > +             return 0;
> > +
> > +     /* don't do this again, even if we find no module BTFs */
> > +     obj->btf_modules_loaded = true;
> > +
> > +     /* kernel too old to support module BTFs */
> > +     if (!kernel_supports(FEAT_MODULE_BTF))
> > +             return 0;
> > +
> > +     while (true) {
> > +             err = bpf_btf_get_next_id(id, &id);
> > +             if (err && errno == ENOENT)
> > +                     return 0;
> > +             if (err) {
> > +                     err = -errno;
> > +                     pr_warn("failed to iterate BTF objects: %d\n", err);
> > +                     return err;
> > +             }
> > +
> > +             fd = bpf_btf_get_fd_by_id(id);
> > +             if (fd < 0) {
> > +                     if (errno == ENOENT)
> > +                             continue; /* expected race: BTF was unloaded */
> > +                     err = -errno;
> > +                     pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
> > +                     return err;
> > +             }
> > +
> > +             len = sizeof(info);
> > +             memset(&info, 0, sizeof(info));
> > +             info.name = ptr_to_u64(name);
> > +             info.name_len = sizeof(name);
> > +
> > +             err = bpf_obj_get_info_by_fd(fd, &info, &len);
> > +             if (err) {
> > +                     err = -errno;
> > +                     pr_warn("failed to get BTF object #%d info: %d\n", id, err);
>
>                         close(fd);
>
> > +                     return err;
> > +             }
> > +
> > +             /* ignore non-module BTFs */
> > +             if (!info.kernel_btf || strcmp(name, "vmlinux") == 0) {
> > +                     close(fd);
> > +                     continue;
> > +             }
> > +
>
> [ ... ]
>
> > @@ -8656,9 +8815,6 @@ static inline int __find_vmlinux_btf_id(struct btf *btf, const char *name,
> >       else
> >               err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
> >
> > -     if (err <= 0)
> > -             pr_warn("%s is not found in vmlinux BTF\n", name);
> > -
> >       return err;
> >  }
> >
> > @@ -8675,6 +8831,9 @@ int libbpf_find_vmlinux_btf_id(const char *name,
> >       }
> >
> >       err = __find_vmlinux_btf_id(btf, name, attach_type);
> > +     if (err <= 0)
> > +             pr_warn("%s is not found in vmlinux BTF\n", name);
> > +
> Please explain this move in the commit message.

ok, I'll add something about that. The short answer is that
__find_vmlinux_btf_id() is now expected to not find a type in vmlinux
BTF, so emitting error would be wrong. So I moved it up a level where
it's not expected.

>
> >       btf__free(btf);
> >       return err;
> >  }
> > --
> > 2.24.1
> >