linux-kernel - Re: [PATCH] proc: Enable smaps

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Date:   Tue, 13 Sep 2022 15:07:20 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     kernel <kernel@...s.com>,
        "adobriyan@...il.com" <adobriyan@...il.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] proc: Enable smaps_rollup without ptrace rights

On Thu, Sep 08, 2022 at 11:59:34PM +0200, Andrew Morton wrote:
> On Thu, 8 Sep 2022 11:39:19 +0200 Vincent Whitchurch <vincent.whitchurch@...s.com> wrote:
> > smaps_rollup is currently only allowed on processes which the user has
> > ptrace permissions for, since it uses a common proc open function used
> > by other files like mem and smaps.
> > 
> > However, while smaps provides detailed, individual information about
> > each memory map in the process (justifying its ptrace rights
> > requirement), smaps_rollup only provides a summary of the memory usage,
> > which is not unlike the information available from other places like the
> > status and statm files, which do not need ptrace permissions.
> > 
> > The first line of smaps_rollup could however be sensitive, since it
> > exposes the randomized start and end of the process' address space.
> > This information however does not seem essential to smap_rollup's
> > purpose and could be replaced with placeholder values to preserve the
> > format without leaking information.  (I could not find any user space in
> > Debian or Android which uses the information in the first line.)
> > 
> > Replace the start with 0 and end with ~0 and allow smaps_rollup to be
> > opened and read regardless of ptrace permissions.
> 
> What is the motivation for this?  Use case?  End-user value and such?

My use case is similar to Sergey's[0]: to be able to gather memory usage
information from a daemon/script running without root permissions or
ptrace rights.  Values like Pss are only available from smaps_rollup,
and not from the other files like status and statm which already provide
memory usage information without requiring elevated privileges.

[0] https://lore.kernel.org/lkml/20200929024018.GA529@jagdpanzerIV.localdomain/

smaps_rollup is however much more expensive than those other files, so I
guess that could be an argument for treating it differently, even if the
content itself does not need to be protected.