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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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