linux-kernel - Re: Question regarding MAX_ARG

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Date:   Mon, 3 Jul 2017 13:58:59 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: Question regarding MAX_ARG_STRLEN with execve()

On 06/30/2017 07:52 PM, Michal Hocko wrote:
> On Fri 30-06-17 11:59:37, Anshuman Khandual wrote:
>> Hello,
>>
>> execve() system call should support argument length of
>> MAX_ARG_STRLEN (PAGE_SIZE * 32). On 64K page size systems, we
>> are not able to pass 32 * PAGE_SIZE arguments into the execve()
>> system call because of the following reasons.
>>
>> * struct linux_binprm's vma starts with a size of PAGE_SIZE
>>
>> 	vma->vm_end = STACK_TOP_MAX;
>> 	vma->vm_start = vma->vm_end - PAGE_SIZE;
>>
>> * The VMA expands as much depending upon the argument size. So
>>   for 32 * PAGE_SIZE argument, it becomes 33 * PAGE_SIZE.
>>
>> * 33 * PAGE_SIZE with 64K pages fails the following test in
>>   get_arg_page() function. 33 * PAGE_SIZE is more than 2MB
>>   (8 MB /4) with 64K page size.
>>
>>    if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
>>
>> * Right now RLIMIT_STACK is hard coded 8MB which does not take
>>   PAGE_SIZE into account. 
>>
>> Wondering what should be the solution for this problem ?
>>
>> * Change the default stack size from 8MB ?
> just increase the ulimit if you want to use such a large arguments.
> 

Yeah that is possible but it does not still offset the fact that
the calculation is broken on the page size of 64K. I mean, yeah
its not practical to have such a large argument. But the point
is whether we would want to support the MAX_ARG_STRLEN semantic
for execve system call or not. At present its broken for 64K
and I am asking whether we will be willing to revisit the
'1/4th of the stack' condition.