Date:	Wed, 7 Mar 2012 08:52:30 +1300
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-man@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] prctl: Add PR_SET_MM option description

On Wed, Mar 7, 2012 at 7:22 AM, Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> On Wed, Mar 07, 2012 at 07:00:14AM +1300, Michael Kerrisk (man-pages) wrote:
>> Hi Cyrill,
>>
>> Just a couple of comments for the moment.
>>
>> On Thu, Mar 1, 2012 at 1:23 AM, Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
>> > Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
>> > CC: Tejun Heo <tj@...nel.org>
>> > CC: Pavel Emelyanov <xemul@...allels.com>
>> > ---
>> >  man2/prctl.2 |  104 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> >  1 files changed, 104 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/man2/prctl.2 b/man2/prctl.2
>> > index effad2a..4d6244f 100644
>> > --- a/man2/prctl.2
>> > +++ b/man2/prctl.2
>> > @@ -378,6 +378,110 @@ Return the current per-process machine check kill policy.
>> >  All unused
>> >  .BR prctl ()
>> >  arguments must be zero.
>> > +.TP
>> > +.BR PR_SET_MM " (since Linux 3.3)"
>> > +Allows a user to modify certain kernel memory map descriptor fields
>> > +of the calling process.
>> > +Usually these fields are set by the kernel and dynamic loader (see
>> > +.BR ld.so (8)
>> > +for more information) and a regular application should not use this feature.
>> > +Still there are cases such as self-modifying programs, where a program might
>> > +find it useful to change its own memory map.
>>
>> By the way, do you have a *simple* program that demonstrates some
>> usage of R_SET_MM?
>
> Hi Michael,
>
> well, at moment we've only crtools itself which uses this facility,
> so if we need complete standalone example I need to think about it.
>
>>
>> > +The kernel must be built with
>> > +.BR CONFIG_CHECKPOINT_RESTORE
>> > +option turned on, otherwise this feature will not be accessible
>> > +from a user space level.
>> > +The calling process must have
>> > +.BR CAP_SYS_ADMIN
>> > +(see
>> > +.BR capabilities (7)
>> > +for details) capability granted.
>>
>> As we discussed earlier (offlist), there are probably better choices
>> than the hugely overloaded CAP_SYS_ADMIN (see
>> http://man7.org/linux/man-pages/man7/capabilities.7.html). And if the
>> capability governing PR_SET_MM is to change, then it would be good to
>> do so before 3.3 is released. What are the plans on this point?
>>
>
> Yeah, I thought about changing it to CAP_SYS_RESOURCE here.
> And I'll post a patch. The problem at moment that there another
> two snippets needed for prctl -- ability to set new /proc/pid/exe
> symlink and to obtaine clear-tid-address. So there is a discussion
> now about symlink change. Once we finish with it -- i'll post
> update for capability.
>
> If you prefer to have it done earlier -- no problem, I'll cook
> a patch today instead on top of everything we've already
> merged into linux-next. What would you prefer?

It would make sense if the capability requirements were finalized
before 3.3 is released. Changing them after 3.3 creates (at least a
little) pain for userspace.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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