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

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Date:	Fri, 12 Aug 2016 09:58:51 -0700
From:	William Tu <u9012063@...il.com>
To:	Alexei Starovoitov <alexei.starovoitov@...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.

Hi,

I've tested on ESXi version 5.5 and it seems OK.
- VM1: Ubuntu 14.04, kernel 3.19 ---> OK 3 cpu dirs, possible = 0-2
- VM2: Centos7, kernel 3.10 ---> OK 8 cpu dirs, possible = 0-7

I tried another MacBook with Fusion, same issue happens, the cpu[0-9]
dirs are not equal to /sys/devices/system/cpu/possible

Regards,
William

On Mon, Aug 1, 2016 at 9:30 AM, William Tu <u9012063@...il.com> wrote:
>>> And in my /sys/devices/system/cpu, I have cpu0 and cpu1,
>>> kernel_max = 63
>>> possible = 0-63
>>> present = 0-1
>>
>> glibc is doing
>> ls -d /sys/devices/system/cpu/cpu*
>> http://osxr.org:8080/glibc/source/sysdeps/unix/sysv/linux/getsysstats.c?v=glibc-2.14#0180
>> And /sys/devices/system/cpu/possible shows 0-63 while only two dirs 'cpu0' and 'cpu1'
>> are there?!
>
> yes, that's right.
> I only have cpu0 and cpu1 directories, while 'possible' shows 0-63.
>
>> If my understanding of cpu_dev_register_generic() in drivers/base/cpu.c
>> is correct the number of 'cpu*' dirs should be equal to possible_cpu.
>> Could you please debug why is that the case, because then it's probably
>> a bug on the kernel side.
>
> Thanks, I will do it.
>
>> I think it's correct for glibc to rely on the number of 'cpu*' dirs.
>> Did you boot with possible_cpus=64 command line arg by any chance?
>>
> No.
>
>>> So sysconf simply reads these entries configured by kernel. Looking at
>>> kernel code, "arch/x86/configs/x86_64_defconfig" sets
>>> CONFIG_NR_CPUS=64, and later on set_cpu_possible() is called at
>>> arch/x86/kernel/smpboot.c, which parses the ACPI multiprocessor table
>>> and configured new value. Based on these observations, I think
>>> different hypervisor may have different ways of emulating ACPI
>>> processor table or BIOS implementation thus these values differ.
>>
>> What behavior do you see in ESX ?
>> btw, rhel7 ships with nr_cpus=5120 and ubuntu default is 256,
>> so this lack of acpi in vmware fusion will lead to possible_cpu=5120,
>> a lot of pain in per-cpu allocator and linux VMs will not be happy.
>> I think vmware has to be fixed first regardless of what we find
>> out about 'cpu*' vs /sys/devices/system/cpu/possible
>>
>
> Thanks, I will debug it and update when I have more info.
> Regards,
> William