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

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@redhat.com

Acked-by: Alexei Starovoitov ast@kernel.org