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    SubjectRe: [PATCH 0/4] coredump: core dump masking support v4
    Hi,
    Thank you for your kind comments.

    I'm still discussing the answer with my senior colleagues, so please
    wait a few days. I think I can reply at the beginning of next week.

    Best regards,
    --
    Hidehiro Kawai
    Hitachi, Ltd., Systems Development Laboratory


    Andrew Morton wrote:

    > On Fri, 02 Mar 2007 13:41:30 +0900
    > "Kawai, Hidehiro" <hidehiro.kawai.ez@hitachi.com> wrote:
    >
    >
    >>This patch series is version 4 of the core dump masking feature,
    >>which provides a per-process flag not to dump anonymous shared
    >>memory segments.
    >
    >
    > First up, please convince us that this problem cannot be solved in
    > userspace. Note that we now support dumping core over a pipe to a
    > userspace application which can perform filtering such as this (see
    > Documentation/sysctl/kernel.txt:core_pattern).
    >
    >
    > Assuming that your argument is successful...
    >
    > - The unpleasing trylock in proc_coredump_omit_anon_shared_write() is
    > there, I believe, to handle the case where a coredump is presently in
    > progress. We don't want to change the filtering rule while the dump is
    > happening.
    >
    > What I suggest you do instead is to take a copy of
    > mm->coredump_omit_anon_shared into a local variable with one single read
    > per coredump. Pass that local down into all the callees which need to
    > see it. That way, no locking is needed.
    >
    > - These games we're playing with the atomicity of the bitfields in the
    > mm_struct need to go away.
    >
    > First up, please prepare a standalone patch which removes
    > mm_struct.dumpable and adds `unsigned long mm_struct.flags'. Include a
    > comment telling people that they must use atomic bitops (set_bit, clear_bit) on
    > mm_struct.flags.
    >
    > Reimplement the current three-value dumpable silliness using two or
    > three separate flags in mm_struct.flags. Of course, this design means
    > that there will be tiny timing windows where the value of these two or
    > three flags have intermediate, invalid states. Please take care of those
    > little windows and document how you did so. I expect a suitable approach
    > would be to set and clear the flags in a suitable order, so that if a
    > race _does_ happen, the results are benign.
    >
    > - Once that is done, you're ready to think about your new functionality.
    > Start out with
    >
    > #define MM_FLAG_COREDUMP_OMIT_ANON_SHARED (1 << 3)
    >
    > or whatever, and it all becomes easy.
    >
    > - Finally, the code as you have it here is very specific to your specific
    > requirement: don't dump shared memory segments.
    >
    > But if we're going to implement in-kernel core-dump filtering of this
    > nature, we should design it extensibly, even if we don't actually
    > implement those extensions at this time.
    >
    > Because other people might (reasonably) wish to omit anonymous memory,
    > or private mappings, or file-backed VMAs, or whatever.
    >
    > So maybe /proc/pid/coredump_omit_anon_shared should become
    > /proc/pid/core_dumpfilter, which is a carefully documented bitmask.
    >
    >


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