Re: [PATCH] mm: mempolicy: Fix wrong use of maxnode in mempolicy API

["lixinhai.lxh@gmail.com" <lixinhai.lxh@gmail.com>]

On 2019-11-09 at 00:04 Michal Hocko wrote:
>On Fri 08-11-19 23:49:29, Li Xinhai wrote:
>> The maxnode used by mbind(), set_mempolicy() and migrate_pages() is not
>> correctly handled in get_nodes(), where --maxnode cause the calculation
>>   endmask = (1UL << (maxnode % BITS_PER_LONG)) - 1;
>> ignores the highest node ID bit. Then, at
>>   nodes_addr(*nodes)[nlongs-1] &= endmask;
>> the highest node ID bit is cleared. Finally, cause mpol_new() think user
>> does not pass in any node ID, and return EINVAL.
>>
>> The results are:
>> - Application receives EINVAL when only the highest node ID bit is set.
>> - Application recevies no error when other bits set together with the
>>   highest node ID bit, but that highest ID is ignored.
>
>A similar patch has been posted [1] and rejected [2].
>
>[1] http://lkml.kernel.org/r/1570882789-20579-1-git-send-email-zhangpan26@huawei.com
>[2] http://lkml.kernel.org/r/20191014091243.GD317@dhcp22.suse.cz
>
>
>--
>Michal Hocko
>SUSE Labs 

I just checked the code in 'history' log for curiosity, this --maxnode seems 
appeared since the very first commit. Mention about this special point of 
maxnode in manpage of these three APIs would be one solution, :).

Xinhai