Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

[Peter Zijlstra]

On Thu, Feb 28, 2019 at 08:01:11PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 28, 2019 at 10:29:25AM -0800, Linus Torvalds wrote:
> > On Thu, Feb 28, 2019 at 10:02 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > Weird, that jump is from C, not from a .fixup table. objtool _should_
> > > see that and complain if there is a AC=1 path that reaches RET.
> > 
> > No, unsafe_put_user() actually does the "asm goto" thing, so the jump
> > is literally hidden as an exception entry. And apparently objtool
> > doesn't follow exceptions (which *normally* doesn't matter for code
> > liveness analysis since they normally jump back to right after the
> > excepting instruction, but maybe it misses some exception handling
> > code because of it?).
> > 
> > You may have looked at unsafe_get_user(), which does indeed make the
> > branch as C code, because gcc currently does not allow outputs from
> > "asm goto" statements (which "get" obviously needs).
> 
> Indeed I did. But it looks like objtool actually does parse .fixup. What
> appears to go wrong is the 'visited' marker for backward jumps.
> 
> If we've been there with AC=0 first, and then backjump with AC=1, things
> go missing.
> 
> I've also now confused myself on how it branches from alternatives. It
> looks like it now considers paths that take the STAC alternative, and
> exit through the NOP alternative (which should be CLAC) and then hit
> RET with AC=1.
> 
> I'll get this sorted, eventually..

Ha!

Original file:

CC      drivers/gpu/drm/i915/i915_gem_execbuffer.o
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x3c: redundant UACCESS disable
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x66: redundant UACCESS disable

With the dodgy patch:

CC      drivers/gpu/drm/i915/i915_gem_execbuffer.o
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: eb_relocate_slow()+0x1f9: call to kvfree() with UACCESS enabled
drivers/gpu/drm/i915/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x315: call to kvfree() with UACCESS enabled


Let me do an allmodconfig build to see how much pain is caused by that
redundant CLAC warning.