netdev - [bpf-next V2 PATCH 5/5] tools/libbpf: handle issues with bpf ELF objects containing .eh

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Date:   Tue, 06 Feb 2018 15:54:28 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        wangnan0@...wei.com
Cc:     jakub.kicinski@...ronome.com, joe@....org, acme@...hat.com,
        eric@...it.org, Jesper Dangaard Brouer <brouer@...hat.com>,
        yhs@...com
Subject: [bpf-next V2 PATCH 5/5] tools/libbpf: handle issues with bpf ELF
 objects containing .eh_frames

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).

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 <brouer@...hat.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) {