linux-kernel - Re: [PATCH] docs: procs -- Describe /proc/<pid>/map

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Date:	Tue, 27 Jan 2015 11:50:49 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Calvin Owens <calvinowens@...com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Peter Feiner <pfeiner@...gle.com>,
	Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [PATCH] docs: procs -- Describe /proc/<pid>/map_files entry

On Tue, Jan 27, 2015 at 1:41 AM, Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: "Kirill A. Shutemov" <kirill@...temov.name>
> CC: Calvin Owens <calvinowens@...com>
> CC: Alexey Dobriyan <adobriyan@...il.com>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: "Eric W. Biederman" <ebiederm@...ssion.com>
> CC: Al Viro <viro@...iv.linux.org.uk>
> CC: Peter Feiner <pfeiner@...gle.com>
> CC: Pavel Emelyanov <xemul@...nvz.org>
> ---
>
> Gentlemen, could you please take a look once time permit.
> Which questions this text raises so I could add more info
> here (how we use it in criu, ptrace_may_access guards?)
>
>  Documentation/filesystems/proc.txt |   23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> Index: linux-2.6.git/Documentation/filesystems/proc.txt
> ===================================================================
> --- linux-2.6.git.orig/Documentation/filesystems/proc.txt
> +++ linux-2.6.git/Documentation/filesystems/proc.txt
> @@ -42,6 +42,7 @@ Table of Contents
>    3.6  /proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
>    3.7   /proc/<pid>/task/<tid>/children - Information about task children
>    3.8   /proc/<pid>/fdinfo/<fd> - Information about opened file
> +  3.9   /proc/<pid>/map_files - Information about memory mapped files
>
>    4    Configuring procfs
>    4.1  Mount options
> @@ -1763,6 +1764,28 @@ pair provide additional information part
>         with TIMER_ABSTIME option which will be shown in 'settime flags', but 'it_value'
>         still exhibits timer's remaining time.
>
> +3.9    /proc/<pid>/map_files - Information about memory mapped files
> +---------------------------------------------------------------------
> +This directory consists of simbolic links which represent memory mapped files
> +the process is carrying. A typical output is like the following
> +
> +     | lr-------- 1 root root 64 Jan 27 11:24 333c600000-333c620000 -> /usr/lib64/ld-2.18.so
> +     | lr-------- 1 root root 64 Jan 27 11:24 333c81f000-333c820000 -> /usr/lib64/ld-2.18.so
> +     | lr-------- 1 root root 64 Jan 27 11:24 333c820000-333c821000 -> /usr/lib64/ld-2.18.so
> +     | ...
> +     | lr-------- 1 root root 64 Jan 27 11:24 35d0421000-35d0422000 -> /usr/lib64/libselinux.so.1
> +     | lr-------- 1 root root 64 Jan 27 11:24 400000-41a000 -> /usr/bin/ls
> +
> +The name of a link is virtual memory bounds a particular map exhibits, i.e.
> +vm_area_struct::vm_start-vm_area_struct::vm_end.
> +
> +The main purpose of map_files directory is to be able to retrieve a set of
> +memory mapped files in a fast way instead of parsing /proc/<pid>/maps or
> +/proc/<pid>/smaps which contain a way more records. Same time one can open(2)
> +mappings from the listings of two processes and comparing inodes figure out
> +which anonymous memory areas are actually shared.

Thanks for details! I still don't understand how this is used for
checkpoint/restore when the mmap offset isn't shown. Can't a process
map, say 4K of a file, from different offsets, and it would show up
as:

400000-401000 -> /some/file
401000-402000 -> /some/file

but there'd be no way to know how to restore that mapping?

Are these symlinks "regular" symlinks, or are they something more
special that bypasses VFS? If it bypasses VFS, I think adding and open
check with PTRACE_ATTACH is needed, since now you're able to _modify_
the memory space of the target process instead of just reading it.

-Kees

-Kees

-- 
Kees Cook
Chrome OS Security
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