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

[Josh Poimboeuf]

On Mon, Sep 23, 2019 at 03:33:52PM +0200, Peter Zijlstra wrote:
> 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.

I like your idea better, makes the fix more localized and shrinks the
uaccess window.  Something like so (not even compile tested):

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c 
b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index b5f6937369ea..406af374f728 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -2847,6 +2847,11 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void 
*data,
                        u64_to_user_ptr(args->buffers_ptr);
                unsigned int i;
 
+               for (i = 0; i < args->buffer_count; i++) {
+                       exec2_list[i].offset =
+                               gen8_canonical_addr(exec2_list[i].offset & 
PIN_OFFSET_MASK);
+               }
+
                /* Copy the new buffer offsets back to the user's exec list. */
                /*
                 * Note: count * sizeof(*user_exec_list) does not overflow,
@@ -2862,8 +2867,6 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void 
*data,
                        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);