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.

When (and why) to use HID-BPF ¶

There are several use cases when using HID-BPF is better than 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:

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)
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 with the HID protocol, 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:

event processing/filtering with SEC("fmod_ret/hid_bpf_device_event") in libbpf
actions coming from userspace with SEC("syscall") in libbpf
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:¶

struct hid_bpf_ctx¶: User accessible data for all HID programs

Definition:

struct hid_bpf_ctx {
    __u32 index;
    const struct hid_device *hid;
    __u32 allocated_size;
    enum hid_report_type report_type;
    union {
        __s32 retval;
        __s32 size;
    };
};

Members

index

program index in the jump table. No special meaning (a smaller index doesn't mean the program will be executed before another program with a bigger index).

hid

the struct hid_device representing the device itself

allocated_size

Allocated size of data.

This is how much memory is available and can be requested by the HID program. Note that for HID_BPF_RDESC_FIXUP, that memory is set to 4096 (4 KB)

report_type

used for hid_bpf_device_event()

{unnamed_union}

anonymous

retval

Return value of the previous program.

size

Valid data in the data field.

Programs can get the available valid size in data by fetching this field. Programs can also change this value by returning a positive number in the program. To discard the event, return a negative error code.

size must always be less or equal than allocated_size (it is enforced once all BPF programs have been run).

Description

data is not directly accessible from the context. We need to issue a call to hid_bpf_get_data() in order to get a pointer to that field.

All of these fields are currently read-only.

enum hid_bpf_attach_flags¶: flags used when attaching a HIF-BPF program

Constants

HID_BPF_FLAG_NONE: no specific flag is used, the kernel choses where to insert the program
HID_BPF_FLAG_INSERT_HEAD: insert the given program before any other program currently attached to the device. This doesn't guarantee that this program will always be first
HID_BPF_FLAG_MAX: sentinel value, not to be used by the callers

Available tracing functions to attach a HID-BPF program:¶

noinline int hid_bpf_device_event(struct hid_bpf_ctx *ctx)¶: Called whenever an event is coming in from the device

Parameters

struct hid_bpf_ctx *ctx: The HID-BPF context

Description

return 0 on success and keep processing; a positive value to change the incoming size buffer; a negative error code to interrupt the processing of this event

Declare an fmod_ret tracing bpf program to this function and attach this program through hid_bpf_attach_prog() to have this helper called for any incoming event from the device itself.

The function is called while on IRQ context, so we can not sleep.

noinline int hid_bpf_rdesc_fixup(struct hid_bpf_ctx *ctx)¶: Called when the probe function parses the report descriptor of the HID device

Parameters

struct hid_bpf_ctx *ctx: The HID-BPF context

Description

return 0 on success and keep processing; a positive value to change the incoming size buffer; a negative error code to interrupt the processing of this event

Declare an fmod_ret tracing bpf program to this function and attach this program through hid_bpf_attach_prog() to have this helper called before any parsing of the report descriptor by HID.

Available API that can be used in all HID-BPF programs:¶

__u8 *hid_bpf_get_data(struct hid_bpf_ctx *ctx, unsigned int offset, const size_t rdwr_buf_size)¶: Get the kernel memory pointer associated with the context ctx

Parameters

struct hid_bpf_ctx *ctx: The HID-BPF context
unsigned int offset: The offset within the memory
const size_t rdwr_buf_size: the const size of the buffer

Description

returns NULL on error, an __u8 memory pointer on success

Available API that can be used in syscall HID-BPF programs:¶

int hid_bpf_attach_prog(unsigned int hid_id, int prog_fd, __u32 flags)¶: Attach the given prog_fd to the given HID device

Parameters

unsigned int hid_id: the system unique identifier of the HID device
int prog_fd: an fd in the user process representing the program to attach
__u32 flags: any logical OR combination of enum hid_bpf_attach_flags

Description

returns an fd of a bpf_link object on success (> 0), an error code otherwise. Closing this fd will detach the program from the HID device (unless the bpf_link is pinned to the BPF file system).

struct hid_bpf_ctx *hid_bpf_allocate_context(unsigned int hid_id)¶: Allocate a context to the given HID device

Parameters

unsigned int hid_id: the system unique identifier of the HID device

Description

returns A pointer to struct hid_bpf_ctx on success, NULL on error.

void hid_bpf_release_context(struct hid_bpf_ctx *ctx)¶: Release the previously allocated context ctx

Parameters

struct hid_bpf_ctx *ctx: the HID-BPF context to release

int hid_bpf_hw_request(struct hid_bpf_ctx *ctx, __u8 *buf, size_t buf__sz, enum hid_report_type rtype, enum hid_class_request reqtype)¶: Communicate with a HID device

Parameters

struct hid_bpf_ctx *ctx: the HID-BPF context previously allocated in hid_bpf_allocate_context()
__u8 *buf: a PTR_TO_MEM buffer
size_t buf__sz: the size of the data to transfer
enum hid_report_type rtype: the type of the report (HID_INPUT_REPORT, HID_FEATURE_REPORT, HID_OUTPUT_REPORT)
enum hid_class_request reqtype: the type of the request (HID_REQ_GET_REPORT, HID_REQ_SET_REPORT, ...)

Description

returns 0 on success, a negative error code otherwise.

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 hid_bpf_get_data().

offset can be any integer, but size needs to be constant, known at compile time.

This allows the following:

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]);
```

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 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 hid_bpf_allocate_context() and must release it with hid_bpf_release_context() before returning. Once the context is retrieved, one can also request a pointer to kernel memory with 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 capabilities 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);
      if (err)
              return err;

      return args.retval; /* the fd of the created bpf_link */
}

Our userspace program can now listen to notifications on the ring buffer, and is awaken only when the value changes.

When the userspace program doesn't need to listen to events anymore, it can just close the returned fd from attach_filter(), which will tell the kernel to detach the program from the HID device.

Of course, in other use cases, the userspace program can also pin the fd to the BPF filesystem through a call to bpf_obj_pin(), as with any bpf_link.

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.

The Linux Kernel

Contents

This Page

HID-BPF¶

When (and why) to use HID-BPF ¶

Dead zone of a joystick ¶

Simple fixup of report descriptor ¶

Add a new feature that requires a new kernel API ¶

Morph a device into something else and control that from userspace ¶

Firewall ¶

Tracing ¶

High-level view of HID-BPF ¶

Available types of programs ¶

Developer API:¶

User API data structures available in programs:¶

Available tracing functions to attach a HID-BPF program:¶

Available API that can be used in all HID-BPF programs:¶

Available API that can be used in syscall HID-BPF programs:¶

General overview of a HID-BPF program ¶

Accessing the data attached to the context ¶

Effect of a HID-BPF program ¶

`SEC("fmod_ret/hid_bpf_device_event")`¶

`SEC("syscall")`¶

`SEC("fmod_ret/hid_bpf_rdesc_fixup")`¶

Attaching a bpf program to a device ¶

An (almost) complete example of a BPF enhanced HID device ¶

Filtering events ¶

Controlling the device ¶

HID-BPF¶

SEC("fmod_ret/hid_bpf_device_event")¶

SEC("syscall")¶

SEC("fmod_ret/hid_bpf_rdesc_fixup")¶

`SEC("fmod_ret/hid_bpf_device_event")`¶

`SEC("syscall")`¶

`SEC("fmod_ret/hid_bpf_rdesc_fixup")`¶