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

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Date:	Wed, 31 Aug 2011 23:07:05 -0400
From:	Kyle Moffett <kyle@...fetthome.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	containers@...ts.osdl.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Nathan Lynch <ntl@...ox.com>,
	Oren Laadan <orenl@...columbia.edu>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Glauber Costa <glommer@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Tejun Heo <tj@...nel.org>,
	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, Aug 31, 2011 at 18:10, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Wed, 31 Aug 2011 18:26:22 +0400  Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> This one behaves similarly to the /proc/<pid>/fd/ one - it contains symlinks
>> one for each mapping with file, the name of a symlink is "vma->vm_start-vma->vm_end",
>> the target is the file. Opening a symlink results in a file that point exactly
>> to the same inode as them vma's one.
>>
>> For example the ls -l of some arbitrary /proc/<pid>/map_files/
>>
>>  | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80403000-7f8f80404000 -> /lib64/libc-2.5.so
>>  | lr-x------ 1 root root 64 Aug 26 06:40 7f8f8061e000-7f8f80620000 -> /lib64/libselinux.so.1
>>  | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80826000-7f8f80827000 -> /lib64/libacl.so.1.1.0
>>  | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a2f000-7f8f80a30000 -> /lib64/librt-2.5.so
>>  | lr-x------ 1 root root 64 Aug 26 06:40 7f8f80a30000-7f8f80a4c000 -> /lib64/ld-2.5.so
>
> This particular patch introduces a distressing amount of duplication of
> /proc/pid/maps.  The changelog should provide a really good
> justification for doing this: why is /proc/pid/maps (and smaps!)
> unsuitable and why cannot maps/smaps be fixed to be suitable?

Andrew,

This is way more useful than /proc/$PID/maps, because this allows you
to reliably obtain an FD to an arbitrary mapped memory segment.

EG:
  $ cp /bin/sleep ~/my-sleep
  $ ~/my-sleep 1000 & kid=$!
  $ rm ~/my-sleep
  $ mkdir ~/mapped_files
  $ for file in /proc/$kid/map_files/*; do cp "$file" ~/mapped_files/; done

This trivially gets me copies of every mapped file, including the deleted
"my-sleep" binary, it also trivially handles lazy-unmounted filesystems,
chroots, namespaces, and all sorts of similar kernel cleverness.

It's exactly the same kind of utility that /proc/$PID/fd/* has, but for
mmap().

With /proc/$kid/maps you simply can't work around some of those
kinds of issues (IE: deleted files, etc).

Cheers,
Kyle Moffett
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