On Fri, Mar 18, 2022 at 9:16 AM Benjamin Tissoires benjamin.tissoires@redhat.com wrote:
+u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size) +{
int ret;struct hid_bpf_ctx_kern ctx = {.type = HID_BPF_RDESC_FIXUP,.hdev = hdev,.size = *size,};if (bpf_hid_link_empty(&hdev->bpf, BPF_HID_ATTACH_RDESC_FIXUP))goto ignore_bpf;ctx.data = kmemdup(rdesc, HID_MAX_DESCRIPTOR_SIZE, GFP_KERNEL);if (!ctx.data)goto ignore_bpf;ctx.allocated_size = HID_MAX_DESCRIPTOR_SIZE;ret = hid_bpf_run_progs(hdev, &ctx);if (ret)goto ignore_bpf;if (ctx.size > ctx.allocated_size)goto ignore_bpf;*size = ctx.size;if (*size) {rdesc = krealloc(ctx.data, *size, GFP_KERNEL);} else {rdesc = NULL;kfree(ctx.data);}return rdesc;- ignore_bpf:
kfree(ctx.data);return kmemdup(rdesc, *size, GFP_KERNEL);+}
int __init hid_bpf_module_init(void) { struct bpf_hid_hooks hooks = { .hdev_from_fd = hid_bpf_fd_to_hdev, .pre_link_attach = hid_bpf_pre_link_attach,
.post_link_attach = hid_bpf_post_link_attach, .array_detach = hid_bpf_array_detach, };diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c index 937fab7eb9c6..3182c39db006 100644 --- a/drivers/hid/hid-core.c +++ b/drivers/hid/hid-core.c @@ -1213,7 +1213,8 @@ int hid_open_report(struct hid_device *device) return -ENODEV; size = device->dev_rsize;
buf = kmemdup(start, size, GFP_KERNEL);
/* hid_bpf_report_fixup() ensures we work on a copy of rdesc */buf = hid_bpf_report_fixup(device, start, &size);
Looking at this patch and the majority of other patches... the code is doing a lot of work to connect HID side with bpf. At the same time the evolution of the patch series suggests that these hook points are not quite stable. More hooks and helpers are being added. It tells us that it's way too early to introduce a stable interface between HID and bpf. We suggest to use __weak global functions and unstable kfunc helpers to achieve the same goal. This way HID side and bpf side can evolve without introducing stable uapi burden. For example this particular patch can be compressed to: __weak int hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size) { return 0; } ALLOW_ERROR_INJECTION(ALLOW_ERROR_INJECTION, ERRNO);
- buf = kmemdup(start, size, GFP_KERNEL); + if (!hid_bpf_report_fixup(device, start, &size)) + buf = kmemdup(start, size, GFP_KERNEL);
Then bpf program can replace hid_bpf_report_fixup function and adjust its return value while reading args.
Similar approach can be done with all other hooks.
Once api between HID and bpf stabilizes we can replace nop functions with writeable tracepoints to make things a bit more stable while still allowing for change of the interface in the future.
The amount of bpf specific code in HID core will be close to zero while bpf can be used to flexibly tweak it.
kfunc is a corresponding mechanism to introduce unstable api from bpf into the kernel instead of stable helpers. Just whitelist some functions as unstable kfunc helpers and call them from bpf progs. See net/bpf/test_run.c and bpf_kfunc_call* for inspiration.