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List:       linux-virtualization
Subject:    Re: [PATCH v9 10/11] x86/paravirt: Adapt assembly for PIE support
From:       Peter Zijlstra <peterz () infradead ! org>
Date:       2019-07-31 12:53:06
Message-ID: 20190731125306.GU31381 () hirez ! programming ! kicks-ass ! net
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On Tue, Jul 30, 2019 at 12:12:54PM -0700, Thomas Garnier wrote:
> if PIE is enabled, switch the paravirt assembly constraints to be
> compatible. The %c/i constrains generate smaller code so is kept by
> default.
> 
> Position Independent Executable (PIE) support will allow to extend the
> KASLR randomization range below 0xffffffff80000000.
> 
> Signed-off-by: Thomas Garnier <thgarnie@chromium.org>
> Acked-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/include/asm/paravirt_types.h | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
> index 70b654f3ffe5..fd7dc37d0010 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -338,9 +338,25 @@ extern struct paravirt_patch_template pv_ops;
>  #define PARAVIRT_PATCH(x)					\
>  	(offsetof(struct paravirt_patch_template, x) / sizeof(void *))
>  
> +#ifdef CONFIG_X86_PIE
> +#define paravirt_opptr_call "a"
> +#define paravirt_opptr_type "p"
> +
> +/*
> + * Alternative patching requires a maximum of 7 bytes but the relative call is
> + * only 6 bytes. If PIE is enabled, add an additional nop to the call
> + * instruction to ensure patching is possible.
> + */
> +#define PARAVIRT_CALL_POST  "nop;"

I'm confused; where does the 7 come from? The relative call is 6 bytes,
a normal call is 5 bytes (which is what we normally replace them with),
and the longest 'native' sequence we seem to have is also 6 bytes
(.cpu_usergs_sysret64).

> +#else
> +#define paravirt_opptr_call "c"
> +#define paravirt_opptr_type "i"
> +#define PARAVIRT_CALL_POST  ""
> +#endif
> +
>  #define paravirt_type(op)				\
>  	[paravirt_typenum] "i" (PARAVIRT_PATCH(op)),	\
> -	[paravirt_opptr] "i" (&(pv_ops.op))
> +	[paravirt_opptr] paravirt_opptr_type (&(pv_ops.op))
>  #define paravirt_clobber(clobber)		\
>  	[paravirt_clobber] "i" (clobber)
>  
> @@ -379,9 +395,10 @@ int paravirt_disable_iospace(void);
>   * offset into the paravirt_patch_template structure, and can therefore be
>   * freely converted back into a structure offset.
>   */
> -#define PARAVIRT_CALL					\
> -	ANNOTATE_RETPOLINE_SAFE				\
> -	"call *%c[paravirt_opptr];"
> +#define PARAVIRT_CALL						\
> +	ANNOTATE_RETPOLINE_SAFE					\
> +	"call *%" paravirt_opptr_call "[paravirt_opptr];"	\
> +	PARAVIRT_CALL_POST
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