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    SubjectRe: [PATCH 0/2] exec: refactor how call_usermodehelper works, and update the sense of the core_pipe recursion check (v3)
    Ok, version 3, taking olegs next set of notes into account.  Really just made
    the subprocess_info struct visible from the header file (not opaque), and
    modified the cleanup and init routines such that they accept a subprocess_info
    parameter.


    Hey all-
    So, about 6 months ago, I made a set of changes to how the
    core-dump-to-a-pipe feature in the kernel works. We had reports of several
    races, including some reports of apps bypassing our recursion check so that a
    process that was forked as part of a core_pattern setup could infinitely crash
    and refork until the system crashed.

    We fixes those by improving our recursion checks. The new check
    basically refuses to fork a process if its core limit is zero, which works well.

    Unfortunately, I've been getting grief from maintainer of user space
    programs that are inserted as the forked process of core_pattern. They contend
    that in order for their programs (such as abrt and apport) to work, all the
    running processes in a system must have their core limits set to a non-zero
    value, to which I say 'yes'. I did this by design, and think thats the right
    way to do things.

    But I've been asked to ease this burden on user space enough times that
    I thought I would take a look at it. The first suggestion was to make the
    recursion check fail on a non-zero 'special' number, like one. That way the
    core collector process could set its core size ulimit to 1, and enable the
    kernel's recursion detection. This isn't a bad idea on the surface, but I don't
    like it since its opt-in, in that if a program like abrt or apport has a bug and
    fails to set such a core limit, we're left with a recursively crashing system
    again.

    So I've come up with this. What I've done is modify the
    call_usermodehelper api such that an extra parameter is added, a function
    pointer which will be called by the user helper task, after it forks, but before
    it exec's the required process. This will give the caller the opportunity to
    get a call back in the processes context, allowing it to do whatever it needs to
    to the process in the kernel prior to exec-ing the user space code. In the case
    of do_coredump, this callback is ues to set the core ulimit of the helper
    process to 1. This elimnates the opt-in problem that I had above, as it allows
    the ulimit for core sizes to be set to the value of 1, which is what the
    recursion check looks for in do_coredump.

    This patch has been tested successfully by some of the Abrt maintainers
    in fedora, with good results. Patch applies to the latest -mm tree as of this
    AM.

    Neil

    Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
    Tested-by: Jiri Moskovcak <jmoskovc@redhat.com>
    CC: Ingo Molnar <mingo@redhat.com>
    CC: drbd-dev@lists.linbit.com
    CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
    CC: Thomas Sailer <t.sailer@alumni.ethz.ch>
    CC: Adam Belay <abelay@mit.edu>
    CC: Greg Kroah-Hartman <gregkh@suse.de>
    CC: Michal Januszewski <spock@gentoo.org>
    CC: Al Viro <viro@zeniv.linux.org.uk>
    CC: Neil Brown <neilb@suse.de>
    CC: Mark Fasheh <mfasheh@suse.com>
    CC: Paul Menage <menage@google.com>
    CC: Stephen Hemminger <shemminger@linux-foundation.org>
    CC: Kentaro Takeda <takedakn@nttdata.co.jp>


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