Re: [PATCH v3 1/8] powerpc/uaccess: Add unsafe_copy_from_user

["Christopher M. Riedl" <cmr@codefail.de>]

On Tue Jan 19, 2021 at 6:33 AM CST, Christophe Leroy wrote:
>
>
> Le 19/01/2021 à 03:11, Michael Ellerman a écrit :
> > "Christopher M. Riedl" <cmr@codefail.de> writes:
> >> On Mon Jan 11, 2021 at 7:22 AM CST, Christophe Leroy wrote:
> >>> Le 09/01/2021 à 04:25, Christopher M. Riedl a écrit :
> >>>> Implement raw_copy_from_user_allowed() which assumes that userspace read
> >>>> access is open. Use this new function to implement raw_copy_from_user().
> >>>> Finally, wrap the new function to follow the usual "unsafe_" convention
> >>>> of taking a label argument.
> >>>
> >>> I think there is no point implementing raw_copy_from_user_allowed(), see
> >>> https://github.com/linuxppc/linux/commit/4b842e4e25b1 and
> >>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/8c74fc9ce8131cabb10b3e95dc0e430f396ee83e.1610369143.git.christophe.leroy@csgroup.eu/
> >>>
> >>> You should simply do:
> >>>
> >>> #define unsafe_copy_from_user(d, s, l, e) \
> >>> unsafe_op_wrap(__copy_tofrom_user((__force void __user *)d, s, l), e)
> >>>
> >>
> >> I gave this a try and the signal ops decreased by ~8K. Now, to be
> >> honest, I am not sure what an "acceptable" benchmark number here
> >> actually is - so maybe this is ok? Same loss with both radix and hash:
> >>
> >> 	|                                      | hash   | radix  |
> >> 	| ------------------------------------ | ------ | ------ |
> >> 	| linuxppc/next                        | 118693 | 133296 |
> >> 	| linuxppc/next w/o KUAP+KUEP          | 228911 | 228654 |
> >> 	| unsafe-signal64                      | 200480 | 234067 |
> >> 	| unsafe-signal64 (__copy_tofrom_user) | 192467 | 225119 |
> >>
> >> To put this into perspective, prior to KUAP and uaccess flush, signal
> >> performance in this benchmark was ~290K on hash.
> > 
> > If I'm doing the math right 8K is ~4% of the best number.
> > 
> > It seems like 4% is worth a few lines of code to handle these constant
> > sizes. It's not like we have performance to throw away.
> > 
> > Or, we should chase down where the call sites are that are doing small
> > constant copies with copy_to/from_user() and change them to use
> > get/put_user().
> > 
>
> Christopher, when you say you gave it a try, is I my series or only the
> following ?
>
> #define unsafe_copy_from_user(d, s, l, e) \
> unsafe_op_wrap(__copy_tofrom_user((__force void __user *)d, s, l), e)
>

I only used the above to replace this patch in my series (so none of my
changes implementing raw_copy_from_user_allowed() are included).

>
> Because I see no use of unsafe_copy_from_user() that would explain that.
>
> Christophe