Subject: Re: linux-next: Tree for Sep 18 (objtool)

From: Peter Zijlstra <[email-masked]>
Date: Mon, 23 Sep 2019 15:33:52 +0200

On Mon, Sep 23, 2019 at 07:49:01AM -0500, Josh Poimboeuf wrote:
> On Mon, Sep 23, 2019 at 11:20:24AM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 18, 2019 at 09:04:21PM -0700, Randy Dunlap wrote:
> > > On 9/18/19 3:10 PM, Mark Brown wrote:
> > > > Hi all,
> > > > 
> > > > Changes since 20190917:
> > > > 
> > > 
> > > on x86_64:
> > > 
> > > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x2fb: call to gen8_canonical_addr() with UACCESS enabled
> > 
> > I'm thinking that comes from:
> > 
> > 				offset = gen8_canonical_addr(offset & ~UPDATE);
> > 				if (unlikely(__put_user(offset, &urelocs[r-stack].presumed_offset))) {
> > 
> > however, per commit 6ae865615fc4 (and 2a418cf3f5f1) the compiler really
> > should not be sticking gen8_canonical_addr() after __uaccess_begin().
> > 
> > /me puzzled...
> 
> I think you're looking at the wrong code.  It has user_access_begin/end
> around it:
> 
> 		if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list)))
> 			goto end;
> 
> 		for (i = 0; i < args->buffer_count; i++) {
> 			if (!(exec2_list[i].offset & UPDATE))
> 				continue;
> 
> 			exec2_list[i].offset =
> 				gen8_canonical_addr(exec2_list[i].offset & PIN_OFFSET_MASK);
> 			unsafe_put_user(exec2_list[i].offset,
> 					&user_exec_list[i].offset,
> 					end_user);
> 		}
> end_user:
> 		user_access_end();
> 

Oh, Duh... Yeah, so the alternative to your solution is to do 2 loops.
Not sure which would be better.