linux-kernel - [PATCH] hid: bpf: avoid building struct ops without JIT

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Message-Id: <20240719095117.3482509-1-arnd@kernel.org>
Date: Fri, 19 Jul 2024 11:51:07 +0200
From: Arnd Bergmann <arnd@...nel.org>
To: Jiri Kosina <jikos@...nel.org>,
	Benjamin Tissoires <bentiss@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>,
	linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: [PATCH] hid: bpf: avoid building struct ops without JIT

From: Arnd Bergmann <arnd@...db.de>

The module does not do anything when the JIT is disabled, but instead
causes a warning:

In file included from include/linux/bpf_verifier.h:7,
                 from drivers/hid/bpf/hid_bpf_struct_ops.c:10:
drivers/hid/bpf/hid_bpf_struct_ops.c: In function 'hid_bpf_struct_ops_init':
include/linux/bpf.h:1853:50: error: statement with no effect [-Werror=unused-value]
 1853 | #define register_bpf_struct_ops(st_ops, type) ({ (void *)(st_ops); 0; })
      |                                                  ^~~~~~~~~~~~~~~~
drivers/hid/bpf/hid_bpf_struct_ops.c:305:16: note: in expansion of macro 'register_bpf_struct_ops'
  305 |         return register_bpf_struct_ops(&bpf_hid_bpf_ops, hid_bpf_ops);
      |                ^~~~~~~~~~~~~~~~~~~~~~~

This could be avoided by making HID-BPF just depend on JIT, but that
is probably not what we want here. Checking the other users of struct_ops,
I see that those just leave out the struct_ops usage, so do the same here.

Fixes: ebc0d8093e8c ("HID: bpf: implement HID-BPF through bpf_struct_ops")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/hid/bpf/Makefile           | 3 ++-
 drivers/hid/bpf/hid_bpf_dispatch.h | 6 ++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/bpf/Makefile b/drivers/hid/bpf/Makefile
index d1f2b81788ca..7566be8eefba 100644
--- a/drivers/hid/bpf/Makefile
+++ b/drivers/hid/bpf/Makefile
@@ -8,4 +8,5 @@ LIBBPF_INCLUDE = $(srctree)/tools/lib
 obj-$(CONFIG_HID_BPF) += hid_bpf.o
 CFLAGS_hid_bpf_dispatch.o += -I$(LIBBPF_INCLUDE)
 CFLAGS_hid_bpf_jmp_table.o += -I$(LIBBPF_INCLUDE)
-hid_bpf-objs += hid_bpf_dispatch.o hid_bpf_struct_ops.o
+hid_bpf-y += hid_bpf_dispatch.o
+hid_bpf-$(CONFIG_BPF_JIT) += hid_bpf_struct_ops.o
diff --git a/drivers/hid/bpf/hid_bpf_dispatch.h b/drivers/hid/bpf/hid_bpf_dispatch.h
index 44c6ea22233f..577572f41454 100644
--- a/drivers/hid/bpf/hid_bpf_dispatch.h
+++ b/drivers/hid/bpf/hid_bpf_dispatch.h
@@ -14,7 +14,13 @@ struct hid_bpf_ctx_kern {
 struct hid_device *hid_get_device(unsigned int hid_id);
 void hid_put_device(struct hid_device *hid);
 int hid_bpf_allocate_event_data(struct hid_device *hdev);
+#ifdef CONFIG_BPF_JIT
 void __hid_bpf_ops_destroy_device(struct hid_device *hdev);
+#else
+static inline void __hid_bpf_ops_destroy_device(struct hid_device *hdev)
+{
+}
+#endif
 int hid_bpf_reconnect(struct hid_device *hdev);
 
 struct bpf_prog;
-- 
2.39.2