Messages in this thread | | | Subject | Re: [PATCH v2 1/2] livepatch: introduce shadow variable API | From | Joe Lawrence <> | Date | Thu, 20 Jul 2017 11:48:41 -0400 |
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On 07/20/2017 10:45 AM, Miroslav Benes wrote: > >>>>> + * >>>>> + * Note: allocates @new_size space for shadow variable data and copies >>>>> + * @new_size bytes from @new_data into the shadow varaible's own @new_data >>>>> + * space. If @new_data is NULL, @new_size is still allocated, but no >>>>> + * copy is performed. >>>> >>>> I must say I'm not entirely happy with this. I don't know if this is what >>>> Petr had in mind (I'm sure he'll get to the patch set soon). Calling >>>> memcpy instead of a simple assignment in v1 seems worse. >>> >>> This change was a bit of a experiment on my part in reaction to >>> adding klp_shadow_get_or_attach(). >>> >>> I like the simplicity of v1's pointer assignment -- in fact, moving all >>> allocation responsiblity (klp_shadow meta-data and data[] area) out to >>> the caller is doable, though implementing klp_shadow_get_or_attach() and >>> and klp_shadow_detach_all() complicates matters, for example, adding an >>> alloc/release callback. I originally attempted this for v2, but turned >>> back when the API and implementation grew complicated. If the memcpy >>> and gfp_flag restrictions are too ugly, I can try revisting that >>> approach. Ideas welcome :) >> >> Well, I didn't like callbacks either :). And no, I do not have a better >> idea. I still need to think about it. > > Done and I agree that memcpy approach is not so bad after all :). So I'm > fine with it.
I looked at it again this morning and a "pass-your-own" allocation API always comes back to adding callbacks and other complications :( In the end, most callers will be shadowing pointers and not entire structures, so I think the copy isn't too bad.
On a related note, if we keep the allocations and memcpy, how about I shift around the attach/get calls like so:
__klp_shadow_attach set shadow variable member values memcpy add to hash
klp_shadow_attach alloc new shadow var lock call __klp_shadow_attach with new alloc unlock
klp_shadow_get_or_attach be optimistic, call klp_shadow_get (if found, return it) be pessimistic, alloc new shadow var lock call klp_shadow_get again if unlikely found kfree unneeded alloc else call __klp_shadow_attach with new alloc unlock return whichever shadow var we used
This way both calls can accept gfp_flags that may sleep, with the only downside that klp_shadow_get_or_attach may allocate an unnecessary shadow variable in the unlikely case that it's found on the second klp_shadow_get attempt (under the lock). No more clunky "bool lock" flag either. :)
-- Joe
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