Re: [PATCH 1/1] x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly

[Chang S. Bae]

On 10/20/2022 9:57 AM, Dave Hansen wrote:
> On 10/18/22 15:13, Chang S. Bae wrote:
> > @@ -1141,10 +1141,14 @@ void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
> >   			 */
> >   			pkru.pkru = pkru_val;
> >   			membuf_write(&to, &pkru, sizeof(pkru));
> > +		} else if (!(header.xfeatures & BIT_ULL(i))) {
> > +			/*
> > +			 * Every extended state component has an all zeros
> > +			 * init state.
> > +			 */
> > +			membuf_zero(&to, xstate_sizes[i]);
> >   		} else {
> > -			copy_feature(header.xfeatures & BIT_ULL(i), &to,
> > -				     __raw_xsave_addr(xsave, i),
> > -				     __raw_xsave_addr(xinit, i),
> > +			membuf_write(&to, __raw_xsave_addr(xsave, i),
> >   				     xstate_sizes[i]);
> >   		}
>
> Just to add a bit more context, this is inside this loop:
>
>          mask = fpstate->user_xfeatures;
>          for_each_extended_xfeature(i, mask) {
>                  if (zerofrom < xstate_offsets[i])
>                          membuf_zero(&to, xstate_offsets[i] - zerofrom);
> 		...
> 	}
>          if (to.left)
>                  membuf_zero(&to, to.left);
>
> In other words, the loop and the surrounding code already know how to
> membuf_zero() any gaps in the middle or the end of the user buffer.
> Would it be simpler to just adjust the 'mask' over which the loop iterates?

Yeah, right!

> I think that would end up being something like:
>
> 	 mask = fpstate->user_xfeatures &
> 		(xsave->xfeatures | xinit->xfeatures);
>
> Logically, that makes sense too.  We're copying out of either 'xsave' or
> 'xinit'.  If a feature isn't in either one of those we can't do the
> copy_feature() on it.

Yes, it is. But, one tricky part here is xinit->xstate_bv is zero. 
Instead, xinit->xcomp_bv appears to be relevant. Also, we want this for 
dynamic features that rely on XSAVES. Then, the change can be something 
like this:

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index e77cabfa802f..3f3286d7e1a8 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1125,6 +1125,15 @@ void __copy_xstate_to_uabi_buf(struct membuf to, 
struct fpstate *fpstate,
         */
        mask = fpstate->user_xfeatures;

+       /*
+        * Dynamic features are not present in init_fpstate since they have
+        * an all zeros init state. When they are in init state, instead of
+        * retrieving them from init_fpstate, remove those from 'mask' to
+        * zero the user buffer.
+        */
+       if (fpu_state_size_dynamic())
+               mask &= (header.xfeatures | xinit->header.xcomp_bv);

Thanks,
Chang