libbpf: Don't assume SHT_GNU_verdef presence for SHT_GNU_versym section

Fix too eager assumption that SHT_GNU_verdef ELF section is going to be
present whenever binary has SHT_GNU_versym section. It seems like either
SHT_GNU_verdef or SHT_GNU_verneed can be used, so failing on missing
SHT_GNU_verdef actually breaks use cases in production.

One specific reported issue, which was used to manually test this fix,
was trying to attach to `readline` function in BASH binary.

Fixes: bb7fa093 ("libbpf: Support symbol versioning for uprobe")
Reported-by: Liam Wisehart <liamwisehart@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Manu Bretelle <chantr4@gmail.com>
Reviewed-by: Fangrui Song <maskray@google.com>
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Link: https://lore.kernel.org/bpf/20231016182840.4033346-1-andrii@kernel.org

tools/lib/bpf/elf.c

@@ -141,14 +141,15 @@ static int elf_sym_iter_new(struct elf_sym_iter *iter,
 	iter->versyms = elf_getdata(scn, 0);
 
 	scn = elf_find_next_scn_by_type(elf, SHT_GNU_verdef, NULL);
-	if (!scn) {
-		pr_debug("elf: failed to find verdef ELF sections in '%s'\n", binary_path);
-		return -ENOENT;
-	}
-	if (!gelf_getshdr(scn, &sh))
+	if (!scn)
+		return 0;
+
+	iter->verdefs = elf_getdata(scn, 0);
+	if (!iter->verdefs || !gelf_getshdr(scn, &sh)) {
+		pr_warn("elf: failed to get verdef ELF section in '%s'\n", binary_path);
 		return -EINVAL;
+	}
 	iter->verdef_strtabidx = sh.sh_link;
-	iter->verdefs = elf_getdata(scn, 0);
 
 	return 0;
 }
@@ -199,6 +200,9 @@ static const char *elf_get_vername(struct elf_sym_iter *iter, int ver)
 	GElf_Verdef verdef;
 	int offset;
 
+	if (!iter->verdefs)
+		return NULL;
+
 	offset = 0;
 	while (gelf_getverdef(iter->verdefs, offset, &verdef)) {
 		if (verdef.vd_ndx != ver) {