netdev - Re: [PATCH] bpf: fix size of copy_to

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Date:	Fri, 29 Jul 2016 22:34:16 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	William Tu <u9012063@...il.com>
Cc:	Daniel Borkmann <daniel@...earbox.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] bpf: fix size of copy_to_user in percpu map.

On Fri, Jul 29, 2016 at 10:23:06PM -0700, William Tu wrote:
> On Fri, Jul 29, 2016 at 5:19 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> > On 07/29/2016 10:03 PM, William Tu wrote:
> >>
> >> I'm not using ARM. It's x86 in a VM with 2 vcpu. By printk() in kernel, I
> >> got
> >>    num_possible_cpu == 64
> >>    num_online_cpu == 2 == sysconf(_SC_NPROCESSORS_CONF)
...
> >> To fix it, I could either
> >> 1). declare values array based on num_possible_cpu in test_map.c,
> >>    long values[64];
> >> or 2) in kernel, only copying 8*2 = 16 byte from kernel to user.
...
> Since percpu array adds variable length of data passing between kernel
> and userspace, I wonder if we should add a 'value_len' field in 'union
> bpf_attr' so kernel knows how much data to copy to user?

I think the first step is to figure out why num_possible is 64,
since it hurts all per-cpu allocations. If it is a widespread issue,
it hurts a lot of VMs.
Hopefully it's not the case, since in my kvm setup num_possible==num_online
qemu version 2.4.0
booting with -enable-kvm -smp N