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SubjectRe: [PATCH v2 1/2] livepatch: introduce shadow variable API
On Thu, 20 Jul 2017, Joe Lawrence wrote:

> 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.

I agree.

> 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. :)

Yes, this looks good.

Thanks,
Miroslav

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