On Wed, Jul 24, 2019 at 03:35:16PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 24, 2019 at 07:55:25AM -0500, Josh Poimboeuf wrote:
> > On Wed, Jul 24, 2019 at 09:47:32AM +0200, Peter Zijlstra wrote:
> > > On Tue, Jul 23, 2019 at 09:43:24PM -0500, Josh Poimboeuf wrote:
> > > > On Thu, Jul 18, 2019 at 10:40:09PM +0200, Thomas Gleixner wrote:
> > > >
> > > > > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:
> > > > > .altinstr_replacement+0x86: redundant UACCESS disable
> > > >
> > > > Looking at this one, I think I agree with objtool.
> > > >
> > > > PeterZ, Linus, I know y'all discussed this code a few months ago.
> > > >
> > > > __copy_from_user() already does a CLAC in its error path. So isn't the
> > > > user_access_end() redundant for the __copy_from_user() error path?
> > >
> > > Hmm, is this a result of your c705cecc8431 ("objtool: Track original
> > > function across branches") ?
> > >
> > > I'm thinking it might've 'overlooked' the CLAC in the error path before
> > > (because it didn't have a related function) and now it sees it and
> > > worries about it.
> > >
> > > Then again, I'm not seeing this warning on my GCC builds; so what's
> > > happening?
> >
> > According to the github issue[1] my patch doesn't fix the warning with
> > Clang. So questions remain:
>
> I was thinking your patch resulted in the warning due to the exception
> code gaining a ->func.
I had the same thought.
> But then that doesn't make sense either, because all that lives in
> copy_user_64.S which is a completely different translation unit.
Hm? __copy_from_user() uses raw_copy_from_user() to do the STAC/CLAC in
a header file for the __builtin_constant_p() case.
> > a) what is objtool actually warning about?
>
> CLAC with AC already clear. Either we do double CLAC at the end, or we
> do CLAC without having done STAC first.
>
> The issue isn't BAD(tm), as AC clear is the safe state, but it typically
> indicates confused code flow.
But as I said my patch didn't fix the Clang warning. Or is there
another redundant UACCESS disable you know about?
> > b) why doesn't objtool detect the case I found?
>
> With GCC you mean? Yes, that is really really weird.
With both compilers...
> Let me go stare at objdump output for this file (which doesn't build
> with:
>
> make O=defconfig-build/ drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o
> )
--
Josh
