Re: [Xen-devel] [PATCH] x86/compat: correct SMEP/SMAP NOPs patching

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, May 25, 2016 at 09:00:49AM -0600, Jan Beulich wrote:
> Correct the number of single byte NOPs we want to be replaced in case
> neither SMEP nor SMAP are available.
> 
> Also simplify the expression adding these NOPs - at that location .
> equals .Lcr4_orig, and removing that part of the expression fixes a
> bogus ".space or fill with negative value, ignored" warning by very old
> gas (which actually is what made me look at those constructs again).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/x86_64/compat/entry.S
> +++ b/xen/arch/x86/x86_64/compat/entry.S
> @@ -175,7 +175,7 @@ compat_bad_hypercall:
>  ENTRY(compat_restore_all_guest)
>          ASSERT_INTERRUPTS_DISABLED
>  .Lcr4_orig:
> -        .skip (.Lcr4_alt_end - .Lcr4_alt) - (. - .Lcr4_orig), 0x90
> +        .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
>  .Lcr4_orig_end:
>          .pushsection .altinstr_replacement, "ax"
>  .Lcr4_alt:
> @@ -200,7 +200,8 @@ ENTRY(compat_restore_all_guest)
>          jne   1b
>  .Lcr4_alt_end:
>          .section .altinstructions, "a"
> -        altinstruction_entry .Lcr4_orig, .Lcr4_orig, X86_FEATURE_ALWAYS, 12, > 0
> +        altinstruction_entry .Lcr4_orig, .Lcr4_orig, X86_FEATURE_ALWAYS, \
> +                             (.Lcr4_orig_end - .Lcr4_orig), 0
>          altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMEP, \
>                               (.Lcr4_orig_end - .Lcr4_orig), \
>                               (.Lcr4_alt_end - .Lcr4_alt)
> 
> 
> 

FWIW:
Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>

And subject to review from Andrew:

Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>


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