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    SubjectRe: [PATCH 1/2] prctl: Add PR_SET_MM option description
    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@openvz.org> wrote:
    > > Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
    > > CC: Tejun Heo <tj@kernel.org>
    > > CC: Pavel Emelyanov <xemul@parallels.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?


    Cyrill
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