Messages in this thread | | | Date | Wed, 24 Jul 2019 09:05:33 -0500 | From | Josh Poimboeuf <> | Subject | Re: x86 - clang / objtool status |
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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
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