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

[Dave Hansen <dave.hansen@intel.com>]

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?

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.