Change proc/<pid>/cmdline to 8k

[gregkh@linuxfoundation.org (Greg KH)]

On Thu, Jun 04, 2015 at 09:06:10PM +0530, Navin P wrote:
> On Thu, Jun 4, 2015 at 1:34 PM, Greg KH <gregkh@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@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 .

Applications shouldn't be messing around with the kernel command line.

> 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.

You can not easily, if at all, change the PAGE_SIZE of an architecture.
That is a non-trivial task that takes a ton of knowledge about the
platform.

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

A pid is a number, not a string.  What do you mean by "pid" in this
context?

greg k-h