Re: [PATCH] bpf: fix size of copy_to_user in percpu map.
From: Alexei Starovoitov <hidden>
Date: 2016-07-30 05:34:23
On Fri, Jul 29, 2016 at 10:23:06PM -0700, William Tu wrote:
On Fri, Jul 29, 2016 at 5:19 PM, Daniel Borkmann [off-list ref] wrote:quoted
On 07/29/2016 10:03 PM, William Tu wrote:quoted
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)
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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.
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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