Re: [PACTH v4 1/3] mm, proc: Implement /proc/<pid>/totmaps

[Robert Foss]

On 2016-08-31 01:04 PM, Mateusz Guzik wrote:
> On Wed, Aug 31, 2016 at 12:36:26PM -0400, Robert Foss wrote:
> > On 2016-08-31 05:45 AM, Jacek Anaszewski wrote:
> > > > +static void *m_totmaps_start(struct seq_file *p, loff_t *pos)
> > > > +{
> > > > +    return NULL + (*pos == 0);
> > > > +}
> > > > +
> > > > +static void *m_totmaps_next(struct seq_file *p, void *v, loff_t *pos)
> > > > +{
> > > > +    ++*pos;
> > > > +    return NULL;
> > > > +}
> > > > +
> > >
> > > When reading totmaps of kernel processes the following NULL pointer
> > > dereference occurs:
> > >
> > > Unable to handle kernel NULL pointer dereference at virtual address
> > > 00000044
> > > [<c06c01f4>] (down_read) from [<c022a154>] (totmaps_proc_show+0x2c/0x1e8)
> > > [<c022a154>] (totmaps_proc_show) from [<c01fe564>] (seq_read+0x1c8/0x4b8)
> > > [<c01fe564>] (seq_read) from [<c01dc850>] (__vfs_read+0x2c/0x110)
> > > [<c01dc850>] (__vfs_read) from [<c01ddca0>] (vfs_read+0x8c/0x110)
> > > [<c01ddca0>] (vfs_read) from [<c01ddd64>] (SyS_read+0x40/0x8c)
> > > [<c01ddd64>] (SyS_read) from [<c0107900>] (ret_fast_syscall+0x0/0x3c)
> > >
> > > It seems that some protection is needed for such processes, so that
> > > totmaps would return empty string then, like in case of smaps.
> >
> > Thanks for the testing Jacek!
> >
> > I had a look around the corresponding smaps code, but I'm not seeing any
> > checks, do you know where that check actually is made?
>
> See m_start in f/sproc/task_mmu.c. It not only check for non-null mm,
> but also tries to bump ->mm_users and only then proceeds to walk the mm.

So a m_totmaps_start that looks something like the below would be 
enough? And if so, would mm->mm_users need to be decrement inside of
m_totmaps_start?

static void *m_totmaps_start(struct seq_file *p, loff_t *pos)
{
	struct proc_maps_private *priv = m->private;
	struct mm_struct *mm;

	mm = priv->mm;
	if (!mm || !atomic_inc_not_zero(&mm->mm_users))
		return NULL;

	return NULL + (*pos == 0);
}