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

On Tue, Jan 19, 2021 at 05:22:08PM -0800, Andrii Nakryiko wrote:

On Tue, Jan 19, 2021 at 2:15 PM Jiri Olsa [off-list ref] wrote:

quoted

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;

just use elf_getshdrstrndx() unconditionally, it works for extended
and non-extended numbering (see libbpf.c).

I did not see that, ok

thanks,
jirka

quoted

+
+       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