selftests/hid: ensure we can compile the tests on kernels pre-6.3

For the hid-bpf tests to compile, we need to have the definition of
struct hid_bpf_ctx. This definition is an internal one from the kernel
and it is supposed to be defined in the generated vmlinux.h.

This vmlinux.h header is generated based on the currently running kernel
or if the kernel was already compiled in the tree. If you just compile
the selftests without compiling the kernel beforehand and you are running
on a 6.2 kernel, you'll end up with a vmlinux.h without the hid_bpf_ctx
definition.

Use the clever trick from tools/testing/selftests/bpf/progs/bpf_iter.h
to force the definition of that symbol in case we don't find it in the
BTF and also add __attribute__((preserve_access_index)) to further
support CO-RE functionality for these tests.

Signed-off-by: Justin Stitt <justinstitt@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com> # Build
Tested-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230825-wip-selftests-v3-1-639963c54109@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

selftests: hid: attach/detach 2 bpf programs, not just one

Add a second BPF program to attach to the device, as the development of
this feature showed that we also need to ensure we can detach multiple
programs to a device (hid_bpf_link->hid_table_index was actually not set
initially, and this lead to any BPF program not being released except for
the first one).

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

selftests/hid: Add a test for BPF_F_INSERT_HEAD

Insert 3 programs to check that we are doing the correct thing:
'2', '1', '3' are inserted, but '1' is supposed to be executed first.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

selftests/hid: add report descriptor fixup tests

Simple report descriptor override in HID: replace part of the report
descriptor from a static definition in the bpf kernel program.

Note that this test should be run last because we disconnect/reconnect
the device, meaning that it changes the overall uhid device.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

selftests/hid: add tests for bpf_hid_hw_request

Add tests for the newly implemented function.
We test here only the GET_REPORT part because the other calls are pure
HID protocol and won't infer the result of the test of the bpf hook.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

selftests/hid: add test to change the report size

Use a different report with a bigger size and ensures we are doing
things properly.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

selftests: add tests for the HID-bpf initial implementation

The tests are pretty basic:
- create a virtual uhid device that no userspace will like (to not mess
up the running system)
- attach a BPF prog to it
- open the matching hidraw node
- inject one event and check:
* that the BPF program can do something on the event stream
* can modify the event stream
- add another test where we attach/detach BPF programs to see if we get
errors

Note: the Makefile is extracted from selftests/bpf so we can rebuild
the libbpf and bpftool components from the current kernel tree without
relying on system installed components.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>