Change proc/<pid>/cmdline to 8k

[Navin P]

On Thu, Jun 4, 2015 at 1:34 PM, Greg KH <gregkh at linuxfoundation.org> wrote:
> On Thu, Jun 04, 2015 at 01:11:48PM +0530, Navin P wrote:
>> On Thu, Jun 4, 2015 at 10:47 AM, Greg KH <gregkh at linuxfoundation.org> wrote:
>> > On Thu, Jun 04, 2015 at 09:52:08AM +0530, Navin P wrote:
>> >> Hi,
>> >>
>> >>   I want to change the cmdline of a process to support 8096 . It works
>> >> well on ppc where the page size is 65k.
>> >>   But when i try to increase it on x86 (i686)
>> >>
>> >> http://lxr.free-electrons.com/source/fs/proc/base.c  by doing
>> >> 6*PAGE_SIZE, i get kernel panic after some time
>> >>
>> >> 199 static int proc_pid_cmdline(struct seq_file *m, struct pid_namespace *ns,
>> >> 200                             struct pid *pid, struct task_struct *task)
>> >> 201 {
>> >> 202         /*
>> >> 203          * Rely on struct seq_operations::show() being called once
>> >> 204          * per internal buffer allocation. See single_open(), traverse().
>> >> 205          */
>> >> 206         BUG_ON(m->size < PAGE_SIZE);
>> >> 207         m->count += get_cmdline(task, m->buf, 6*PAGE_SIZE);
>> >> 208         return 0;
>> >> 209 }
>> >> 210
>> >
>> > That shows you that this will not work, sorry.
>>
>> Can you please help me and guide me into achieving this ?
>
> Why do you want this?  What problem are you trying to solve that you
> have come to the conclusion that increasing the size of the command line
> is the solution?
>
I'm trying to learn things so that default cmd linux across all
distributions for ia64 and ppc which has PAGE_SIZE has 64k works . We
have this 3rd party application with deeply mounted dir that runs as a
testcase more than 5k chars . We can change it , i thought if that was
the problem ? So just trying to learn. I tried changing the PAGE_SHIFT
but that made the kernel non-bootable or stuck at booting.

This panics only when you access the pid with more than 4k chars.