Re: [PATCH HID for-next v2 6/9] HID: bpf: rework how programs are attached and stored in the kernel

[Alexei Starovoitov]

On Fri, Jan 13, 2023 at 10:09:32AM +0100, Benjamin Tissoires wrote:
> Previously, HID-BPF was relying on a bpf tracing program to be notified
> when a program was released from userspace. This is error prone, as
> LLVM sometimes inline the function and sometimes not.
> 
> So instead of messing up with the bpf prog ref count, we can use the
> bpf_link concept which actually matches exactly what we want:
> - a bpf_link represents the fact that a given program is attached to a
>   given HID device
> - as long as the bpf_link has fd opened (either by the userspace program
>   still being around or by pinning the bpf object in the bpffs), the
>   program stays attached to the HID device
> - once every user has closed the fd, we get called by
>   hid_bpf_link_release() that we no longer have any users, and we can
>   disconnect the program to the device in 2 passes: first atomically clear
>   the bit saying that the link is active, and then calling release_work in
>   a scheduled work item.
> 
> This solves entirely the problems of BPF tracing not showing up and is
> definitely cleaner.
> 
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>

Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>