Since the code assumes in various places that BTF fd for modules is
never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though
fd 0 being free for allocation is usually an application error, it is
still possible that we end up getting fd 0 if the application explicitly
closes its stdin. Deal with this by getting a new fd using dup and
closing fd 0.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d286dec73b5f..3e5e460fe63e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj)
pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
return err;
}
+ /* Make sure module BTF fd is never 0, as kernel depends on it
+ * being > 0 to distinguish between vmlinux and module BTFs,
+ * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms).
+ */
+ if (!fd) {
+ fd = dup(0);
+ if (fd < 0) {
+ err = -errno;
+ pr_warn("failed to dup BTF object #%d FD 0 to FD > 0: %d\n", id, err);
+ close(0);
+ return err;
+ }
+ close(0);
+ }
len = sizeof(info);
memset(&info, 0, sizeof(info));
--
2.33.0
