[PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed

[Jiri Olsa <jolsa@kernel.org>]

For very large ELF objects (with many sections), we could
get special value SHN_XINDEX (65535) for elf object's string
table index - e_shstrndx.

In such case we need to call elf_getshdrstrndx to get the
proper string table index.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 tools/lib/bpf/btf.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 3c3f2bc6c652..4fe987846bc0 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -863,6 +863,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 	Elf_Scn *scn = NULL;
 	Elf *elf = NULL;
 	GElf_Ehdr ehdr;
+	size_t shstrndx;
 
 	if (elf_version(EV_CURRENT) == EV_NONE) {
 		pr_warn("failed to init libelf for %s\n", path);
@@ -887,7 +888,16 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 		pr_warn("failed to get EHDR from %s\n", path);
 		goto done;
 	}
-	if (!elf_rawdata(elf_getscn(elf, ehdr.e_shstrndx), NULL)) {
+
+	/*
+	 * Get string table index from extended section index
+	 * table if needed.
+	 */
+	shstrndx = ehdr.e_shstrndx;
+	if (shstrndx == SHN_XINDEX && elf_getshdrstrndx(elf, &shstrndx))
+		goto done;
+
+	if (!elf_rawdata(elf_getscn(elf, shstrndx), NULL)) {
 		pr_warn("failed to get e_shstrndx from %s\n", path);
 		goto done;
 	}
@@ -902,7 +912,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 				idx, path);
 			goto done;
 		}
-		name = elf_strptr(elf, ehdr.e_shstrndx, sh.sh_name);
+		name = elf_strptr(elf, shstrndx, sh.sh_name);
 		if (!name) {
 			pr_warn("failed to get section(%d) name from %s\n",
 				idx, path);
-- 
2.27.0