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    SubjectRe: [PATCH 1/2] prctl: Add PR_SET_MM option description
    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?

    > +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?

    Cheers,

    Michael


    > +The value in
    > +.I arg2
    > +is one of the options below, while
    > +.I arg3
    > +provides a new value for this option.
    > +
    > +.BR PR_SET_MM_START_CODE
    > +to set the address above which program text can run.
    > +The corresponding memory area must be readable and executable,
    > +but not writable or shareable (see
    > +.BR mprotect (2)
    > +and
    > +.BR mmap (2)
    > +for more information).
    > +
    > +.BR PR_SET_MM_END_CODE
    > +to set the address below which program text can run.
    > +The corresponding memory area must be readable and executable,
    > +but not writable or shareable.
    > +
    > +.BR PR_SET_MM_START_DATA
    > +to set the address above which program data+bss is placed.
    > +The corresponding memory area must be readable and writable,
    > +but not executable or shareable.
    > +
    > +.B PR_SET_MM_END_DATA
    > +to set the address below which program data+bss is placed.
    > +The corresponding memory area must be readable and writable,
    > +but not executable or shareable.
    > +
    > +.BR PR_SET_MM_START_STACK
    > +to set the start address of the stack.
    > +The corresponding memory area must be readable and writable.
    > +
    > +.BR PR_SET_MM_START_BRK
    > +to set the address above which program heap can be expanded with
    > +.BR brk (2)
    > +call.
    > +The address must not be greater than ending address of
    > +the current program data segment, neither it may exceed
    > +resource limit for data (see
    > +.BR setrlimit (2)
    > +for more information).
    > +
    > +.BR PR_SET_MM_BRK
    > +to set the current
    > +.BR brk (2)
    > +value.
    > +The requirements for address are the same as for
    > +.BR PR_SET_MM_START_BRK
    > +option.
    > +
    > +.BR PR_SET_MM_ARG_START
    > +to set the address above which program command line is placed.
    > +
    > +.BR PR_SET_MM_ARG_END
    > +to set the address below which program command line is placed.
    > +
    > +.BR PR_SET_MM_ENV_START
    > +to set the address above which program environment is placed.
    > +
    > +.BR PR_SET_MM_ENV_END
    > +to set the address below which program environment is placed.
    > +
    > +The address passed with
    > +.BR PR_SET_MM_ARG_START ,
    > +.BR PR_SET_MM_ARG_END ,
    > +.BR PR_SET_MM_ENV_START ,
    > +.BR PR_SET_MM_ENV_END ,
    > +should belong to a process stack area, thus corresponding memory area
    > +must be readable, writable and (depending on the kernel
    > +configuration) has
    > +.BR MAP_GROWSDOWN
    > +attribute set (see
    > +.BR mmap (2)
    > +for details).
    > +
    > +.BR PR_SET_MM_AUXV
    > +to set a new auxiliary vector.
    > +The
    > +.I arg3
    > +argument should provide the address of the vector.
    > +The
    > +.I arg4
    > +is the size of the vector.
    > +.\"
    >  .SH "RETURN VALUE"
    >  On success,
    >  .BR PR_GET_DUMPABLE ,
    > --
    > 1.7.7.6
    >



    --
    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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