tools/libbpf: handle issues with bpf ELF objects containing .eh_frames

Web:        https://git.kernel.org/torvalds/c/e3d91b0ca523d53158f435a3e13df7f0cb360ea2
Commit:     e3d91b0ca523d53158f435a3e13df7f0cb360ea2
Parent:     f09b2e382e9a7053e3ae6f2fb6535efd5760cf5d
Refname:    refs/heads/master
Author:     Jesper Dangaard Brouer <[email protected]>
AuthorDate: Thu Feb 8 12:48:32 2018 +0100
Committer:  Daniel Borkmann <[email protected]>
CommitDate: Fri Feb 9 00:25:12 2018 +0100

    tools/libbpf: handle issues with bpf ELF objects containing .eh_frames
    
    V3: More generic skipping of relo-section (suggested by Daniel)
    
    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() skipping
    some sections, and later when performing relocation it will be
    pointing to a skipped section, as these sections cannot be found by
    bpf_object__find_prog_by_idx() in bpf_object__collect_reloc().
    
    This is a general issue that also occurs for other sections, like
    debug sections which are also skipped and can have relo section.
    
    As suggested by Daniel.  To avoid keeping state about all skipped
    sections, instead perform a direct qlookup in the ELF object.  Lookup
    the section that the relo-section points to and check if it contains
    executable machine instructions (denoted by the sh_flags
    SHF_EXECINSTR).  Use this check to also skip irrelevant relo-sections.
    
    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 <[email protected]>
    Signed-off-by: Daniel Borkmann <[email protected]>
---
 tools/lib/bpf/libbpf.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 0c794f7fc050..97073d649c1a 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -742,6 +742,24 @@ bpf_object__init_maps(struct bpf_object *obj)
 	return 0;
 }
 
+static bool section_have_execinstr(struct bpf_object *obj, int idx)
+{
+	Elf_Scn *scn;
+	GElf_Shdr sh;
+
+	scn = elf_getscn(obj->efile.elf, idx);
+	if (!scn)
+		return false;
+
+	if (gelf_getshdr(scn, &sh) != &sh)
+		return false;
+
+	if (sh.sh_flags & SHF_EXECINSTR)
+		return true;
+
+	return false;
+}
+
 static int bpf_object__elf_collect(struct bpf_object *obj)
 {
 	Elf *elf = obj->efile.elf;
@@ -825,6 +843,14 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		} else if (sh.sh_type == SHT_REL) {
 			void *reloc = obj->efile.reloc;
 			int nr_reloc = obj->efile.nr_reloc + 1;
+			int sec = sh.sh_info; /* points to other section */
+
+			/* Only do relo for section with exec instructions */
+			if (!section_have_execinstr(obj, sec)) {
+				pr_debug("skip relo %s(%d) for section(%d)\n",
+					 name, idx, sec);
+				continue;
+			}
 
 			reloc = realloc(reloc,
 					sizeof(*obj->efile.reloc) * nr_reloc);
--
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