[PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0
From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: 2021-10-06 00:29:13
Also in:
bpf
Subsystem:
bpf [general] (safe dynamic programs and tools), bpf [library] (libbpf), the rest · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Linus Torvalds
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@gmail.com> --- 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));
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2.33.0