This is a slight change from the fundamentals of HID-BPF. In theory, HID-BPF is abstract to the kernel itself, and makes only changes at the HID level (through report descriptors or events emitted to/from the device).
However, we have seen a few use cases where HID-BPF might interact with the running kernel when the target device is already handled by a specific device.
For example, the XP-Pen/Huion/UC-Logic tablets are handled by hid-uclogic but this driver is also doing a report descriptor fixup without checking if the device has already been fixed by HID-BPF.
In the same way, another recent example[0] was when a cheap foot pedal is used and tricks iPhones and Windows machines by presenting itself as a known Apple wireless keyboard. The problem is that this fake keyboard is not presenting a compatible report descriptor and hid-core merges all device nodes together making libinput ignore the keyboard part for historical reasons.
Last, there has been a long standing request to allow to disable the input part of a given gamepad while SDL or Steam opens the device through hidraw.
This series aims at tackling both of these problems: - first we had a new hook `hid_bpf_driver_probe` which allows the BPF program to decide if the curently probed driver should be used or not - then this same hook can also change the ->driver_data of the struct hid_device_id argument, and we teach hid-generic to use that field as the connect mask.
Basically, it means that when we insert a BPF program to fix a device, we can force hid-generic to handle the device, and thus preventing any other kernel driver to tamper with our device. We can also selectively decide to export the hidraw or input nodes when using hid-generic.
In the SDL/Steam use case, this would means that the gaming application will load one BPF program per input device it wants to open through hidraw, that BPF program reassigns the input device to hid-generic and disables hid-input, then it can open the new hidraw node. Once that program terminates, the BPF program is removed (either automatically because no-one has the fd of the links open, or manually by SDL/Steam), and the normal driver rebinds to the HID device, restoring full input functionality.
This branch is on top of the for-6.12/hidraw and for-6.12/constify-rdesc branches of hid.git, mainly because those branch would conflict otherwise.
[0] https://gitlab.freedesktop.org/libinput/libinput/-/issues/1014
Signed-off-by: Benjamin Tissoires bentiss@kernel.org --- Changes in v2: - Refactored the API to not use a new hook but hid_bpf_rdesc_fixup instead - Some cleanups in hid-core.c probe() device to not kmemdup multiple time the report descriptor when it's not required - I'm still not 100% sure the HID_QUIRK_IGNORE_HIDINPUT is that required, but I can not think of anything else at the moment to temporary disable any driver input device. - Link to v1: https://lore.kernel.org/r/20240903-hid-bpf-hid-generic-v1-0-9511a565b2da@ker...
--- Benjamin Tissoires (11): HID: bpf: move HID-BPF report descriptor fixup earlier HID: core: save one kmemdup during .probe() HID: core: remove one more kmemdup on .probe() HID: bpf: allow write access to quirks field in struct hid_device selftests/hid: add dependency on hid_common.h selftests/hid: cleanup C tests by adding a common struct uhid_device selftests/hid: allow to parametrize bus/vid/pid/rdesc on the test device HID: add per device quirk to force bind to hid-generic selftests/hid: add test for assigning a given device to hid-generic HID: add quirk to prevent hid-input to be used selftests/hid: add test to disable hid-input
drivers/hid/bpf/hid_bpf_dispatch.c | 8 +- drivers/hid/bpf/hid_bpf_struct_ops.c | 1 + drivers/hid/hid-core.c | 72 ++++++-- drivers/hid/hid-generic.c | 3 + include/linux/hid.h | 22 ++- include/linux/hid_bpf.h | 9 +- tools/testing/selftests/hid/Makefile | 2 +- tools/testing/selftests/hid/hid_bpf.c | 205 ++++++++++++++++----- tools/testing/selftests/hid/hid_common.h | 112 +++++++---- tools/testing/selftests/hid/hidraw.c | 36 +--- tools/testing/selftests/hid/progs/hid.c | 13 ++ .../testing/selftests/hid/progs/hid_bpf_helpers.h | 7 +- 12 files changed, 343 insertions(+), 147 deletions(-) --- base-commit: e1370d5de7b755600df050979e19fbcd625fb4c6 change-id: 20240829-hid-bpf-hid-generic-61579f5b5945
Best regards,