Commit d2836ddd authored Feb 11, 2021 by Martin KaFai Lau Committed by Andrii Nakryiko Feb 12, 2021

libbpf: Ignore non function pointer member in struct_ops



When libbpf initializes the kernel's struct_ops in
"bpf_map__init_kern_struct_ops()", it enforces all
pointer types must be a function pointer and rejects
others.  It turns out to be too strict.  For example,
when directly using "struct tcp_congestion_ops" from vmlinux.h,
it has a "struct module *owner" member and it is set to NULL
in a bpf_tcp_cc.o.

Instead, it only needs to ensure the member is a function
pointer if it has been set (relocated) to a bpf-prog.
This patch moves the "btf_is_func_proto(kern_mtype)" check
after the existing "if (!prog) { continue; }".  The original debug
message in "if (!prog) { continue; }" is also removed since it is
no longer valid.  Beside, there is a later debug message to tell
which function pointer is set.

The "btf_is_func_proto(mtype)" has already been guaranteed
in "bpf_object__collect_st_ops_relos()" which has been run
before "bpf_map__init_kern_struct_ops()".  Thus, this check
is removed.

v2:
- Remove outdated debug message (Andrii)
  Remove because there is a later debug message to tell
  which function pointer is set.
- Following mtype->type is no longer needed. Remove:
  "skip_mods_and_typedefs(btf, mtype->type, &mtype_id)"
- Do "if (!prog)" test before skip_mods_and_typedefs.

Fixes: 590a0088 ("bpf: libbpf: Add STRUCT_OPS support")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210212021030.266932-1-kafai@fb.com

parent 1e0aa3fb

tools/lib/bpf/libbpf.c

+11 −11

Original line number	Diff line number	Diff line
		@@ -884,22 +884,22 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map,
		if (btf_is_ptr(mtype)) {
		struct bpf_program *prog;

		mtype = skip_mods_and_typedefs(btf, mtype->type, &mtype_id);
		prog = st_ops->progs[i];
		if (!prog)
		continue;

		kern_mtype = skip_mods_and_typedefs(kern_btf,
		kern_mtype->type,
		&kern_mtype_id);
		if (!btf_is_func_proto(mtype) \|\|
		!btf_is_func_proto(kern_mtype)) {
		pr_warn("struct_ops init_kern %s: non func ptr %s is not supported\n",
		map->name, mname);
		return -ENOTSUP;
		}

		prog = st_ops->progs[i];
		if (!prog) {
		pr_debug("struct_ops init_kern %s: func ptr %s is not set\n",
		/* mtype->type must be a func_proto which was
		* guaranteed in bpf_object__collect_st_ops_relos(),
		* so only check kern_mtype for func_proto here.
		*/
		if (!btf_is_func_proto(kern_mtype)) {
		pr_warn("struct_ops init_kern %s: kernel member %s is not a func ptr\n",
		map->name, mname);
		continue;
		return -ENOTSUP;
		}

		prog->attach_btf_id = kern_type_id;