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

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Date:   Sun, 15 Jan 2023 09:59:50 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Tero Kristo <tero.kristo@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: 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@...hat.com>

Acked-by: Alexei Starovoitov <ast@...nel.org>