Gives a primer on HID-BPF.
Signed-off-by: Benjamin Tissoires benjamin.tissoires@redhat.com
---
changes in v6: - amended the example now that we can directly use the data from the syscall context
changes in v5: - amended for new API - reworded most of the sentences (thanks to Peter Hutterer for the review)
changes in v4: - fixed typos
new in v3 --- Documentation/hid/hid-bpf.rst | 512 ++++++++++++++++++++++++++++++++++ Documentation/hid/index.rst | 1 + 2 files changed, 513 insertions(+) create mode 100644 Documentation/hid/hid-bpf.rst
diff --git a/Documentation/hid/hid-bpf.rst b/Documentation/hid/hid-bpf.rst new file mode 100644 index 000000000000..75e65c135925 --- /dev/null +++ b/Documentation/hid/hid-bpf.rst @@ -0,0 +1,512 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======= +HID-BPF +======= + +HID is a standard protocol for input devices but some devices may require +custom tweaks, traditionally done with a kernel driver fix. Using the eBPF +capabilities instead speeds up development and adds new capabilities to the +existing HID interfaces. + +.. contents:: + :local: + :depth: 2 + + +When (and why) to use HID-BPF +============================= + +We can enumerate several use cases for when using HID-BPF is better than +using a standard kernel driver fix: + +Dead zone of a joystick +----------------------- + +Assuming you have a joystick that is getting older, it is common to see it +wobbling around its neutral point. This is usually filtered at the application +level by adding a *dead zone* for this specific axis. + +With HID-BPF, we can apply this filtering in the kernel directly so userspace +does not get woken up when nothing else is happening on the input controller. + +Of course, given that this dead zone is specific to an individual device, we +can not create a generic fix for all of the same joysticks. Adding a custom +kernel API for this (e.g. by adding a sysfs entry) does not guarantee this new +kernel API will be broadly adopted and maintained. + +HID-BPF allows the userspace program to load the program itself, ensuring we +only load the custom API when we have a user. + +Simple fixup of report descriptor +--------------------------------- + +In the HID tree, half of the drivers only fix one key or one byte +in the report descriptor. These fixes all require a kernel patch and the +subsequent shepherding into a release, a long and painful process for users. + +We can reduce this burden by providing an eBPF program instead. Once such a +program has been verified by the user, we can embed the source code into the +kernel tree and ship the eBPF program and load it directly instead of loading +a specific kernel module for it. + +Note: distribution of eBPF programs and their inclusion in the kernel is not +yet fully implemented + +Add a new feature that requires a new kernel API +------------------------------------------------ + +An example for such a feature are the Universal Stylus Interface (USI) pens. +Basically, USI pens require a new kernel API because there are new +channels of communication that our HID and input stack do not support. +Instead of using hidraw or creating new sysfs entries or ioctls, we can rely +on eBPF to have the kernel API controlled by the consumer and to not +impact the performances by waking up userspace every time there is an +event. + +Morph a device into something else and control that from userspace +------------------------------------------------------------------ + +The kernel has a relatively static mapping of HID items to evdev bits. +It cannot decide to dynamically transform a given device into something else +as it does not have the required context and any such transformation cannot be +undone (or even discovered) by userspace. + +However, some devices are useless with that static way of defining devices. For +example, the Microsoft Surface Dial is a pushbutton with haptic feedback that +is barely usable as of today. + +With eBPF, userspace can morph that device into a mouse, and convert the dial +events into wheel events. Also, the userspace program can set/unset the haptic +feedback depending on the context. For example, if a menu is visible on the +screen we likely need to have a haptic click every 15 degrees. But when +scrolling in a web page the user experience is better when the device emits +events at the highest resolution. + +Firewall +-------- + +What if we want to prevent other users to access a specific feature of a +device? (think a possibly broken firmware update entry point) + +With eBPF, we can intercept any HID command emitted to the device and +validate it or not. + +This also allows to sync the state between the userspace and the +kernel/bpf program because we can intercept any incoming command. + +Tracing +------- + +The last usage is tracing events and all the fun we can do we BPF to summarize +and analyze events. + +Right now, tracing relies on hidraw. It works well except for a couple +of issues: + +1. if the driver doesn't export a hidraw node, we can't trace anything + (eBPF will be a "god-mode" there, so this may raise some eyebrows) +2. hidraw doesn't catch other processes' requests to the device, which + means that we have cases where we need to add printks to the kernel + to understand what is happening. + +High-level view of HID-BPF +========================== + +The main idea behind HID-BPF is that it works at an array of bytes level. +Thus, all of the parsing of the HID report and the HID report descriptor +must be implemented in the userspace component that loads the eBPF +program. + +For example, in the dead zone joystick from above, knowing which fields +in the data stream needs to be set to ``0`` needs to be computed by userspace. + +A corollary of this is that HID-BPF doesn't know about the other subsystems +available in the kernel. *You can not directly emit input event through the +input API from eBPF*. + +When a BPF program needs to emit input events, it needs to talk HID, and rely +on the HID kernel processing to translate the HID data into input events. + +Available types of programs +=========================== + +HID-BPF is built "on top" of BPF, meaning that we use tracing method to +declare our programs. + +HID-BPF has the following attachment types available: + +1. event processing/filtering with ``SEC("fmod_ret/hid_bpf_device_event")`` in libbpf +2. actions coming from userspace with ``SEC("syscall")`` in libbpf +3. change of the report descriptor with ``SEC("fmod_ret/hid_bpf_rdesc_fixup")`` in libbpf + +A ``hid_bpf_device_event`` is calling a BPF program when an event is received from +the device. Thus we are in IRQ context and can act on the data or notify userspace. +And given that we are in IRQ context, we can not talk back to the device. + +A ``syscall`` means that userspace called the syscall ``BPF_PROG_RUN`` facility. +This time, we can do any operations allowed by HID-BPF, and talking to the device is +allowed. + +Last, ``hid_bpf_rdesc_fixup`` is different from the others as there can be only one +BPF program of this type. This is called on ``probe`` from the driver and allows to +change the report descriptor from the BPF program. Once a ``hid_bpf_rdesc_fixup`` +program has been loaded, it is not possible to overwrite it unless the program which +inserted it allows us by pinning the program and closing all of its fds pointing to it. + +Developer API: +============== + +User API data structures available in programs: +----------------------------------------------- + +.. kernel-doc:: include/uapi/linux/hid_bpf.h +.. kernel-doc:: include/linux/hid_bpf.h + +Available tracing functions to attach a HID-BPF program: +-------------------------------------------------------- + +.. kernel-doc:: drivers/hid/bpf/hid_bpf_dispatch.c + :functions: hid_bpf_device_event hid_bpf_rdesc_fixup + +Available API that can be used in all HID-BPF programs: +------------------------------------------------------- + +.. kernel-doc:: drivers/hid/bpf/hid_bpf_dispatch.c + :functions: hid_bpf_get_data + +Available API that can be used in syscall HID-BPF programs: +----------------------------------------------------------- + +.. kernel-doc:: drivers/hid/bpf/hid_bpf_dispatch.c + :functions: hid_bpf_attach_prog hid_bpf_hw_request hid_bpf_allocate_context hid_bpf_release_context + +General overview of a HID-BPF program +===================================== + +Accessing the data attached to the context +------------------------------------------ + +The ``struct hid_bpf_ctx`` doesn't export the ``data`` fields directly and to access +it, a bpf program needs to first call :c:func:`hid_bpf_get_data`. + +``offset`` can be any integer, but ``size`` needs to be constant, known at compile +time. + +This allows the following: + +1. for a given device, if we know that the report length will always be of a certain value, + we can request the ``data`` pointer to point at the full report length. + + The kernel will ensure we are using a correct size and offset and eBPF will ensure + the code will not attempt to read or write outside of the boundaries:: + + __u8 *data = hid_bpf_get_data(ctx, 0 /* offset */, 256 /* size */); + + if (!data) + return 0; /* ensure data is correct, now the verifier knows we + * have 256 bytes available */ + + bpf_printk("hello world: %02x %02x %02x", data[0], data[128], data[255]); + +2. if the report length is variable, but we know the value of ``X`` is always a 16-bit + integer, we can then have a pointer to that value only:: + + __u16 *x = hid_bpf_get_data(ctx, offset, sizeof(*x)); + + if (!x) + return 0; /* something went wrong */ + + *x += 1; /* increment X by one */ + +Effect of a HID-BPF program +--------------------------- + +For all HID-BPF attachment types except for :c:func:`hid_bpf_rdesc_fixup`, several eBPF +programs can be attached to the same device. + +Unless ``HID_BPF_FLAG_INSERT_HEAD`` is added to the flags while attaching the +program, the new program is appended at the end of the list. +``HID_BPF_FLAG_INSERT_HEAD`` will insert the new program at the beginning of the +list which is useful for e.g. tracing where we need to get the unprocessed events +from the device. + +Note that if there are multiple programs using the ``HID_BPF_FLAG_INSERT_HEAD`` flag, +only the most recently loaded one is actually the first in the list. + +``SEC("fmod_ret/hid_bpf_device_event")`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Whenever a matching event is raised, the eBPF programs are called one after the other +and are working on the same data buffer. + +If a program changes the data associated with the context, the next one will see +the modified data but it will have *no* idea of what the original data was. + +Once all the programs are run and return ``0`` or a positive value, the rest of the +HID stack will work on the modified data, with the ``size`` field of the last hid_bpf_ctx +being the new size of the input stream of data. + +A BPF program returning a negative error discards the event, i.e. this event will not be +processed by the HID stack. Clients (hidraw, input, LEDs) will **not** see this event. + +``SEC("syscall")`` +~~~~~~~~~~~~~~~~~~ + +``syscall`` are not attached to a given device. To tell which device we are working +with, userspace needs to refer to the device by its unique system id (the last 4 numbers +in the sysfs path: ``/sys/bus/hid/devices/xxxx:yyyy:zzzz:0000``). + +To retrieve a context associated with the device, the program must call +:c:func:`hid_bpf_allocate_context` and must release it with :c:func:`hid_bpf_release_context` +before returning. +Once the context is retrieved, one can also request a pointer to kernel memory with +:c:func:`hid_bpf_get_data`. This memory is big enough to support all input/output/feature +reports of the given device. + +``SEC("fmod_ret/hid_bpf_rdesc_fixup")`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``hid_bpf_rdesc_fixup`` program works in a similar manner to +``.report_fixup`` of ``struct hid_driver``. + +When the device is probed, the kernel sets the data buffer of the context with the +content of the report descriptor. The memory associated with that buffer is +``HID_MAX_DESCRIPTOR_SIZE`` (currently 4kB). + +The eBPF program can modify the data buffer at-will and the kernel uses the +modified content and size as the report descriptor. + +Whenever a ``SEC("fmod_ret/hid_bpf_rdesc_fixup")`` program is attached (if no +program was attached before), the kernel immediately disconnects the HID device +and does a reprobe. + +In the same way, when the ``SEC("fmod_ret/hid_bpf_rdesc_fixup")`` program is +detached, the kernel issues a disconnect on the device. + +There is no ``detach`` facility in HID-BPF. Detaching a program happens when +all the user space file descriptors pointing at a program are closed. +Thus, if we need to replace a report descriptor fixup, some cooperation is +required from the owner of the original report descriptor fixup. +The previous owner will likely pin the program in the bpffs, and we can then +replace it through normal bpf operations. + +Attaching a bpf program to a device +=================================== + +``libbpf`` does not export any helper to attach a HID-BPF program. +Users need to use a dedicated ``syscall`` program which will call +``hid_bpf_attach_prog(hid_id, program_fd, flags)``. + +``hid_id`` is the unique system ID of the HID device (the last 4 numbers in the +sysfs path: ``/sys/bus/hid/devices/xxxx:yyyy:zzzz:0000``) + +``progam_fd`` is the opened file descriptor of the program to attach. + +``flags`` is of type ``enum hid_bpf_attach_flags``. + +We can not rely on hidraw to bind a BPF program to a HID device. hidraw is an +artefact of the processing of the HID device, and is not stable. Some drivers +even disable it, so that removes the tracing capabilies on those devices +(where it is interesting to get the non-hidraw traces). + +On the other hand, the ``hid_id`` is stable for the entire life of the HID device, +even if we change its report descriptor. + +Given that hidraw is not stable when the device disconnects/reconnects, we recommend +accessing the current report descriptor of the device through the sysfs. +This is available at ``/sys/bus/hid/devices/BUS:VID:PID.000N/report_descriptor`` as a +binary stream. + +Parsing the report descriptor is the responsibility of the BPF programmer or the userspace +component that loads the eBPF program. + +An (almost) complete example of a BPF enhanced HID device +========================================================= + +*Foreword: for most parts, this could be implemented as a kernel driver* + +Let's imagine we have a new tablet device that has some haptic capabilities +to simulate the surface the user is scratching on. This device would also have +a specific 3 positions switch to toggle between *pencil on paper*, *cray on a wall* +and *brush on a painting canvas*. To make things even better, we can control the +physical position of the switch through a feature report. + +And of course, the switch is relying on some userspace component to control the +haptic feature of the device itself. + +Filtering events +---------------- + +The first step consists in filtering events from the device. Given that the switch +position is actually reported in the flow of the pen events, using hidraw to implement +that filtering would mean that we wake up userspace for every single event. + +This is OK for libinput, but having an external library that is just interested in +one byte in the report is less than ideal. + +For that, we can create a basic skeleton for our BPF program:: + + #include "vmlinux.h" + #include <bpf/bpf_helpers.h> + #include <bpf/bpf_tracing.h> + + /* HID programs need to be GPL */ + char _license[] SEC("license") = "GPL"; + + /* HID-BPF kfunc API definitions */ + extern __u8 *hid_bpf_get_data(struct hid_bpf_ctx *ctx, + unsigned int offset, + const size_t __sz) __ksym; + extern int hid_bpf_attach_prog(unsigned int hid_id, int prog_fd, u32 flags) __ksym; + + struct { + __uint(type, BPF_MAP_TYPE_RINGBUF); + __uint(max_entries, 4096 * 64); + } ringbuf SEC(".maps"); + + struct attach_prog_args { + int prog_fd; + unsigned int hid; + unsigned int flags; + int retval; + }; + + SEC("syscall") + int attach_prog(struct attach_prog_args *ctx) + { + ctx->retval = hid_bpf_attach_prog(ctx->hid, + ctx->prog_fd, + ctx->flags); + return 0; + } + + __u8 current_value = 0; + + SEC("?fmod_ret/hid_bpf_device_event") + int BPF_PROG(filter_switch, struct hid_bpf_ctx *hid_ctx) + { + __u8 *data = hid_bpf_get_data(hid_ctx, 0 /* offset */, 192 /* size */); + __u8 *buf; + + if (!data) + return 0; /* EPERM check */ + + if (current_value != data[152]) { + buf = bpf_ringbuf_reserve(&ringbuf, 1, 0); + if (!buf) + return 0; + + *buf = data[152]; + + bpf_ringbuf_commit(buf, 0); + + current_value = data[152]; + } + + return 0; + } + +To attach ``filter_switch``, userspace needs to call the ``attach_prog`` syscall +program first:: + + static int attach_filter(struct hid *hid_skel, int hid_id) + { + int err, prog_fd; + int ret = -1; + struct attach_prog_args args = { + .hid = hid_id, + }; + DECLARE_LIBBPF_OPTS(bpf_test_run_opts, tattrs, + .ctx_in = &args, + .ctx_size_in = sizeof(args), + ); + + args.prog_fd = bpf_program__fd(hid_skel->progs.filter_switch); + + prog_fd = bpf_program__fd(hid_skel->progs.attach_prog); + + err = bpf_prog_test_run_opts(prog_fd, &tattrs); + return err; + } + +Our userspace program can now listen to notifications on the ring buffer, and +is awaken only when the value changes. + +Controlling the device +---------------------- + +To be able to change the haptic feedback from the tablet, the userspace program +needs to emit a feature report on the device itself. + +Instead of using hidraw for that, we can create a ``SEC("syscall")`` program +that talks to the device:: + + /* some more HID-BPF kfunc API definitions */ + extern struct hid_bpf_ctx *hid_bpf_allocate_context(unsigned int hid_id) __ksym; + extern void hid_bpf_release_context(struct hid_bpf_ctx *ctx) __ksym; + extern int hid_bpf_hw_request(struct hid_bpf_ctx *ctx, + __u8* data, + size_t len, + enum hid_report_type type, + enum hid_class_request reqtype) __ksym; + + + struct hid_send_haptics_args { + /* data needs to come at offset 0 so we can do a memcpy into it */ + __u8 data[10]; + unsigned int hid; + }; + + SEC("syscall") + int send_haptic(struct hid_send_haptics_args *args) + { + struct hid_bpf_ctx *ctx; + int ret = 0; + + ctx = hid_bpf_allocate_context(args->hid); + if (!ctx) + return 0; /* EPERM check */ + + ret = hid_bpf_hw_request(ctx, + args->data, + 10, + HID_FEATURE_REPORT, + HID_REQ_SET_REPORT); + + hid_bpf_release_context(ctx); + + return ret; + } + +And then userspace needs to call that program directly:: + + static int set_haptic(struct hid *hid_skel, int hid_id, __u8 haptic_value) + { + int err, prog_fd; + int ret = -1; + struct hid_send_haptics_args args = { + .hid = hid_id, + }; + DECLARE_LIBBPF_OPTS(bpf_test_run_opts, tattrs, + .ctx_in = &args, + .ctx_size_in = sizeof(args), + ); + + args.data[0] = 0x02; /* report ID of the feature on our device */ + args.data[1] = haptic_value; + + prog_fd = bpf_program__fd(hid_skel->progs.set_haptic); + + err = bpf_prog_test_run_opts(prog_fd, &tattrs); + return err; + } + +Now our userspace program is aware of the haptic state and can control it. The +program could make this state further available to other userspace programs +(e.g. via a DBus API). + +The interesting bit here is that we did not created a new kernel API for this. +Which means that if there is a bug in our implementation, we can change the +interface with the kernel at-will, because the userspace application is +responsible for its own usage. diff --git a/Documentation/hid/index.rst b/Documentation/hid/index.rst index e50f513c579c..b2028f382f11 100644 --- a/Documentation/hid/index.rst +++ b/Documentation/hid/index.rst @@ -11,6 +11,7 @@ Human Interface Devices (HID) hidraw hid-sensor hid-transport + hid-bpf
uhid