linux-kernel - Re: Question regarding MAX_ARG

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Date:   Fri, 30 Jun 2017 16:22:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
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 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.

> * Change the ratio test from 1/4th ?

-- 
Michal Hocko
SUSE Labs