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Subject: Re: [PATCH] docs: procs -- Describe /proc/<pid>/map_files entry
From: Kees Cook <keescook@chromium.org>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Calvin Owens <calvinowens@fb.com>,
        Alexey Dobriyan <adobriyan@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Peter Feiner <pfeiner@google.com>,
        Pavel Emelyanov <xemul@openvz.org>
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On Tue, Jan 27, 2015 at 1:20 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> On Tue, Jan 27, 2015 at 11:50:49AM -0800, Kees Cook wrote:
>> > +
>> > +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?
>
> In criu we use a few sources of information (ie we scan not only
> map_files, but have to use /proc/pid/smaps as well which has
> offset for mapping). So at the end we have all picture under
> our hands.
>
>> 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.
>
> Opening them goes same way as open of /proc/pid/fd/ entries as
> far as I can tell. This should be enough, or I miss something
> obvious here? Otherwise opening /proc/pid/fd/ should use
> PTRACE_ATTACH instead of PTRACE_MODE_READ (as in proc_fd_access_allowed).

I wouldn't object to enhancing the check to ATTACH from READ, but I
worry what would break on the existing interface.

-Kees

-- 
Kees Cook
Chrome OS Security