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Date:	Thu, 28 Jul 2016 23:47:24 -0700
From:	Alexei Starovoitov <>
To:	William Tu <>
Subject: Re: [PATCH] bpf: fix size of copy_to_user in percpu map.

On Thu, Jul 28, 2016 at 05:42:21PM -0700, William Tu wrote:
> The total size of value copy_to_user() writes to userspace should
> be the (current number of cpu) * (value size), instead of
> num_possible_cpus() * (value size).  Found by samples/bpf/test_maps.c,
> which always copies 512 byte to userspace, crashing the userspace
> program stack.

hmm. I'm missing something. The sample code assumes no cpu hutplug,
so sysconf(_SC_NPROCESSORS_CONF) == num_possible_cpu == num_online_cpu,
unless there is crazy INIT_ALL_POSSIBLE config option is used.

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