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

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

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.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@arndb.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

Re: [PATCH] hid: bpf: avoid building struct ops without JIT

[Benjamin Tissoires]

On Jul 19 2024, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.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.

Actually, if we make the struct_ops part only depend on JIT HID-BPF is
kind of moot. All we could do is use HID-BPF to communicate with the
device, without getting any feedback, so nothing much more than what
hidraw provides.

The only "interesting" bit we could do is inject a new event on a device
as if it were originated from the device itself, but I really do not see
the point without the struct_ops hooks.

So I think struct_ops is now the base for HID-BPF, and if it's not
available, we should not have HID-BPF at all.

> 
> Fixes: ebc0d8093e8c ("HID: bpf: implement HID-BPF through bpf_struct_ops")
> Signed-off-by: Arnd Bergmann <arnd@arndb.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)

Unrelated to your patch, but looks like I forgot to remove that entry
from the Makefile now that hid_bpf_jmp_table.c is not available :)

Cheers,
Benjamin

> -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
> 

Re: [PATCH] hid: bpf: avoid building struct ops without JIT

[Arnd Bergmann]

On Fri, Jul 19, 2024, at 15:52, Benjamin Tissoires wrote:
> On Jul 19 2024, Arnd Bergmann wrote:
>> 
>> 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.
>
> Actually, if we make the struct_ops part only depend on JIT HID-BPF is
> kind of moot. All we could do is use HID-BPF to communicate with the
> device, without getting any feedback, so nothing much more than what
> hidraw provides.
>
> The only "interesting" bit we could do is inject a new event on a device
> as if it were originated from the device itself, but I really do not see
> the point without the struct_ops hooks.
>
> So I think struct_ops is now the base for HID-BPF, and if it's not
> available, we should not have HID-BPF at all.
>

Ok, got it. So my original patch was correct after all.
I had tried this version and then discarded it.

    Arnd

8<------
Subject: [PATCH] hid: bpf: add BPF_JIT dependency

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);
      |                ^~~~~~~~~~~~~~~~~~~~~~~

Add a Kconfig dependency to only allow building the HID-BPF support
when a JIT is enabled.

Fixes: ebc0d8093e8c ("HID: bpf: implement HID-BPF through bpf_struct_ops")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
----
diff --git a/drivers/hid/bpf/Kconfig b/drivers/hid/bpf/Kconfig
index 83214bae6768..d65482e02a6c 100644
--- a/drivers/hid/bpf/Kconfig
+++ b/drivers/hid/bpf/Kconfig
@@ -3,7 +3,7 @@ menu "HID-BPF support"
 
 config HID_BPF
        bool "HID-BPF support"
-       depends on BPF
+       depends on BPF_JIT
        depends on BPF_SYSCALL
        depends on DYNAMIC_FTRACE_WITH_DIRECT_CALLS
        help

Re: [PATCH] hid: bpf: avoid building struct ops without JIT

[Benjamin Tissoires]

On Jul 19 2024, Arnd Bergmann wrote:
> On Fri, Jul 19, 2024, at 15:52, Benjamin Tissoires wrote:
> > On Jul 19 2024, Arnd Bergmann wrote:
> >> 
> >> 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.
> >
> > Actually, if we make the struct_ops part only depend on JIT HID-BPF is
> > kind of moot. All we could do is use HID-BPF to communicate with the
> > device, without getting any feedback, so nothing much more than what
> > hidraw provides.
> >
> > The only "interesting" bit we could do is inject a new event on a device
> > as if it were originated from the device itself, but I really do not see
> > the point without the struct_ops hooks.
> >
> > So I think struct_ops is now the base for HID-BPF, and if it's not
> > available, we should not have HID-BPF at all.
> >
> 
> Ok, got it. So my original patch was correct after all.
> I had tried this version and then discarded it.
> 
>     Arnd
> 
> 8<------
> Subject: [PATCH] hid: bpf: add BPF_JIT dependency
> 
> 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);
>       |                ^~~~~~~~~~~~~~~~~~~~~~~
> 
> Add a Kconfig dependency to only allow building the HID-BPF support
> when a JIT is enabled.
> 
> Fixes: ebc0d8093e8c ("HID: bpf: implement HID-BPF through bpf_struct_ops")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ----
> diff --git a/drivers/hid/bpf/Kconfig b/drivers/hid/bpf/Kconfig
> index 83214bae6768..d65482e02a6c 100644
> --- a/drivers/hid/bpf/Kconfig
> +++ b/drivers/hid/bpf/Kconfig
> @@ -3,7 +3,7 @@ menu "HID-BPF support"
>  
>  config HID_BPF
>         bool "HID-BPF support"
> -       depends on BPF
> +       depends on BPF_JIT
>         depends on BPF_SYSCALL
>         depends on DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>         help

Thanks. I've applied this patch to for-6.11/upstream-fixes in the HID
tree.

Cheers,
Benjamin