[PATCH v2 bpf 1/2] 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no longer valid.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- Remove outdated debug message (Andrii)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: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Martin KaFai Lau <redacted>
---
 tools/lib/bpf/libbpf.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6ae748f6ea11..a0d4fc4de402 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -883,24 +883,24 @@ 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",
+
+			/* 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);
 				return -ENOTSUP;
 			}
 
-			prog = st_ops->progs[i];
-			if (!prog) {
-				pr_debug("struct_ops init_kern %s: func ptr %s is not set\n",
-					 map->name, mname);
-				continue;
-			}
-
 			prog->attach_btf_id = kern_type_id;
 			prog->expected_attach_type = kern_member_idx;
 

-- 
2.24.1

This patch adds a "void *owner" member.  The existing
bpf_tcp_ca test will ensure the bpf_cubic.o and bpf_dctcp.o
can be loaded.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Martin KaFai Lau <redacted>
---
 tools/testing/selftests/bpf/bpf_tcp_helpers.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/bpf/bpf_tcp_helpers.h b/tools/testing/selftests/bpf/bpf_tcp_helpers.h
index 6a9053162cf2..91f0fac632f4 100644
--- a/tools/testing/selftests/bpf/bpf_tcp_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_tcp_helpers.h

@@ -177,6 +177,7 @@ struct tcp_congestion_ops {
 	 * after all the ca_state processing. (optional)
 	 */
 	void (*cong_control)(struct sock *sk, const struct rate_sample *rs);
+	void *owner;
 };
 
 #define min(a, b) ((a) < (b) ? (a) : (b))

-- 
2.24.1

Hello:

This series was applied to bpf/bpf-next.git (refs/heads/master):

On Thu, 11 Feb 2021 18:10:30 -0800 you wrote:

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.

[...]

Here is the summary with links:
  - [v2,bpf,1/2] libbpf: Ignore non function pointer member in struct_ops
    https://git.kernel.org/bpf/bpf-next/c/d2836dddc95d
  - [v2,bpf,2/2] bpf: selftests: Add non function pointer test to struct_ops
    https://git.kernel.org/bpf/bpf-next/c/a79e88dd2ca6

You are awesome, thank you!
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