Re: [PATCH bpf-next v2 2/2] selftests/hid: Cover hid_bpf_get_data() size overflow

[Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>]

On Sat, Jun 20, 2026 at 03:11:00PM +0000, bot+bpf-ci@kernel.org wrote:
> That wrapping check was introduced in 658ee5a64fcf, and the fix commit
> carries a Fixes: tag for it. Should this test commit carry the same tag?
>
>   Fixes: 658ee5a64fcf ("HID: bpf: allocate data memory for device_event BPF programs")

Yes, I will add the same Fixes tag to the selftest patch in v3 so the
regression test is tied to the commit that introduced the wrapping check.

> This was raised on an earlier posting and does not look addressed:
> the callback stores the result in the get_data_overflow_check global
> and always returns 0, while the userspace test asserts on that global
> rather than on the callback return value.
>
> Could the callback just return the helper result so the test can check
> the return value directly, instead of routing it through a separate
> global?

The callback's return value is not something the test can observe
directly.  call_hid_bpf_rdesc_fixup() consumes it as a kernel-side
contract: a negative value falls back to the original report descriptor,
and a positive value becomes the new descriptor size (*size = ret).  Its
only userspace-visible effect is therefore the report descriptor size,
so checking the return value would amount to checking that size, which
means perturbing it.

Since the callback returns int and hid_bpf_get_data() returns a pointer,
the only way to return the helper result is to convert it, e.g.

	return !hid_bpf_get_data(hid_ctx, 2, ~0ULL);

On a fixed kernel the helper returns NULL, so this returns 1 and the
dispatch path truncates the report descriptor to 1 byte mid-test.  The
BSS variable records that the helper rejected the overflowing range
while the callback still returns 0 and leaves the descriptor untouched.

This matches the existing convention in progs/hid.c: callback_check and
callback2_check are BPF-side globals the userspace tests assert on, and
the existing hid_rdesc_fixup callback returns a positive size only when
it actually rewrites the descriptor (sizeof(rdesc) + 73).  The overflow
probe does not rewrite anything, so it returns 0 and reports the
helper's rejection through the BSS variable.

Thanks,
Yiyang