Re: [PATCH hid v12 03/15] HID: initial BPF implementation

[Jon Hunter <jonathanh@nvidia.com>]

On 03/11/2022 15:57, Benjamin Tissoires wrote:
> Declare an entry point that can use fmod_ret BPF programs, and
> also an API to access and change the incoming data.
> 
> A simpler implementation would consist in just calling
> hid_bpf_device_event() for any incoming event and let users deal
> with the fact that they will be called for any event of any device.
> 
> The goal of HID-BPF is to partially replace drivers, so this situation
> can be problematic because we might have programs which will step on
> each other toes.
> 
> For that, we add a new API hid_bpf_attach_prog() that can be called
> from a syscall and we manually deal with a jump table in hid-bpf.
> 
> Whenever we add a program to the jump table (in other words, when we
> attach a program to a HID device), we keep the number of time we added
> this program in the jump table so we can release it whenever there are
> no other users.
> 
> HID devices have an RCU protected list of available programs in the
> jump table, and those programs are called one after the other thanks
> to bpf_tail_call().
> 
> To achieve the detection of users losing their fds on the programs we
> attached, we add 2 tracing facilities on bpf_prog_release() (for when
> a fd is closed) and bpf_free_inode() (for when a pinned program gets
> unpinned).
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>

...

> +static int __init hid_bpf_init(void)
> +{
> +	int err;
> +
> +	/* Note: if we exit with an error any time here, we would entirely break HID, which
> +	 * is probably not something we want. So we log an error and return success.
> +	 *
> +	 * This is not a big deal: the syscall allowing to attach a BPF program to a HID device
> +	 * will not be available, so nobody will be able to use the functionality.
> +	 */
> +
> +	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &hid_bpf_kfunc_set);
> +	if (err) {
> +		pr_warn("error while setting HID BPF tracing kfuncs: %d", err);
> +		return 0;
> +	}
> +
> +	err = hid_bpf_preload_skel();
> +	if (err) {
> +		pr_warn("error while preloading HID BPF dispatcher: %d", err);
> +		return 0;
> +	}
> +
> +	/* register syscalls after we are sure we can load our preloaded bpf program */
> +	err = register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &hid_bpf_syscall_kfunc_set);
> +	if (err) {
> +		pr_warn("error while setting HID BPF syscall kfuncs: %d", err);
> +		return 0;
> +	}
> +
> +	return 0;
> +}


We have a kernel test that checks for new warning and error messages on 
boot and with this change I am now seeing the following error message on 
our Tegra platforms ...

  WARNING KERN hid_bpf: error while preloading HID BPF dispatcher: -13

I have a quick look at the code, but I can't say I am familiar with 
this. So I wanted to ask if a way to fix this or avoid this? I see the 
code returns 0, so one option would be to make this an informational or 
debug print.

Thanks
Jon

-- 
nvpublic