linux-kernel - Re: [patch 2/2] fs, proc: Introduce the /proc/<pid>/map

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Message-ID: <20110907224234.GD28162@sun>
Date:	Thu, 8 Sep 2011 02:42:34 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vasiliy Kulikov <segoon@...nwall.com>, Tejun Heo <tj@...nel.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	containers@...ts.osdl.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Nathan Lynch <ntl@...ox.com>,
	kernel-hardening@...ts.openwall.com,
	Oren Laadan <orenl@...columbia.edu>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Glauber Costa <glommer@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [patch 2/2] fs, proc: Introduce the /proc/<pid>/map_files/
 directory v6

On Wed, Sep 07, 2011 at 03:13:23PM -0700, Andrew Morton wrote:
...
> > > 
> > > Andrew, are you OK with closing the hole with pid_no_revalidate()
> > > and 0600 /proc/slabinfo?  If so, I feel I have to start this discussion
> > > with people participating in the discussion above: Theodore, Dan, Linus, etc.
> 
> I fell asleep a long time ago and don't know what pid_no_revalidate()
> and slabinfo permissions have to do with this.  Perhaps summarising the
> issues in the changelog would be appropriate, dunno.

Well, time to poke Vasiliy ;)

...
> > 
> > I fear we still need to use two passes in proc_map_files_readdir, I found no way
> > to escape lockdep complains when doing all work in one pass with mmap_sem taken.
> > The /maps does the same thing -- ie it fills maps file with mmap_sem taken to produce
> > robust data.
> 
> The code's using three passes.

Yes, and I didn't find thy way to escape it (actually if there would not
be filldir+might_fault tuple I would create this all under mmap_sem and
would not need this flex_array or any temporary storage at all and code
would be a way simplier).

> 
> > And I'm not really sure what you mean with problematic put_filp?
> 
> I was thinking fput(), which can do a hell of a lot of stuff if it's
> the final put on the inode.

Ouch, somehow missed it, thanks!

> > +err:
> > +		up_read(&mm->mmap_sem);
> > +
> > +		for (i = 0; i < used && !ret; i++) {
> 
> The "&& !ret" is unneeded?

No, it's needed, since it makes sure that if "impossible"
scenario happens and flex-arrays fails with preallocated
data so we will reach this point with used > 0 and ret = -ENOMEM
and thus will not call for proc_map_files_instantiate as needed.

> 
> > +			p = flex_array_get(fa, i);
> > +			ret = proc_fill_cache(filp, dirent, filldir,
> > +					      p->name, p->len,
> > +					      proc_map_files_instantiate,
> > +					      task, p->file);
> > +			if (ret)
> > +				break;

1: Say we failed here

> > +			filp->f_pos++;
> > +			put_filp(p->file);
> > +		}
> > +
> > +		for (; i < used; i++) {
> > +			p = flex_array_get(fa, i);
> > +			put_filp(p->file);
> > +		}
> 
> Still unclear why we need the third loop.

Due to (1) -- so we will have a number of files reference
taken and need to put them back.

	Cyrill
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