On Tue, Feb 06, 2018 at 03:54:28PM +0100, Jesper Dangaard Brouer wrote:
> If clang >= 4.0.1 is missing the option '-target bpf', it will cause
> llc/llvm to create two ELF sections for "Exception Frames", with
> section names '.eh_frame' and '.rel.eh_frame'.
>
> The BPF ELF loader library libbpf fails when loading files with these
> sections. The other in-kernel BPF ELF loader in samples/bpf/bpf_load.c,
> handle this gracefully. And iproute2 loader also seems to work with these
> "eh" sections.
>
> The issue in libbpf is caused by bpf_object__elf_collect() skip the
> '.eh_frame' and thus doesn't create an internal data structure
> pointing to this ELF section index. Later when the relocation section
> '.rel.eh_frame' is processed, it tries to find the '.eh_frame' via the
> ELF section idx, which is that fails (in bpf_object__collect_reloc).
>
> I couldn't find a way to see that the '.rel.eh_frame' was irrelevant
> (that is only determined by looking at the section it reference, which
> we no longer have info available on).
but does this approach work for all extra sections and relocations emitted
when source is compiled with -g ?
To address this case bpf_load.c does:
if (shdr.sh_type == SHT_REL) {
struct bpf_insn *insns;
/* locate prog sec that need map fixup (relocations) */
if (get_sec(elf, shdr.sh_info, &ehdr, &shname_prog,
&shdr_prog, &data_prog))
continue;
if (shdr_prog.sh_type != SHT_PROGBITS ||
!(shdr_prog.sh_flags & SHF_EXECINSTR))
continue;
why the same approach is not applicable here?
I guess we can apply this workaround as-is but it looks incomplete.
> Thus, my solution is simply to match on the name of the relocation
> section, to skip that too.
>
> Note, for samples/bpf/ the '-target bpf' parameter to clang cannot be used
> due to incompatibility with asm embedded headers, that some of the samples
> include. This is explained in more details by Yonghong Song in bpf_devel_QA.
>
> Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com>
> ---
> tools/lib/bpf/libbpf.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index b4eeaa3ebff5..84e8bbe07347 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -822,6 +822,13 @@ static int bpf_object__elf_collect(struct bpf_object
> *obj)
> void *reloc = obj->efile.reloc;
> int nr_reloc = obj->efile.nr_reloc + 1;
>
> + /* Skip decoding of "eh" exception frames */
> + if (strcmp(name, ".rel.eh_frame") == 0) {
> + pr_debug("skip relo section %s(%d) for
> section(%d)\n",
> + name, idx, sh.sh_info);
> + continue;
> + }
> +
> reloc = realloc(reloc,
> sizeof(*obj->efile.reloc) * nr_reloc);
> if (!reloc) {
>
