linux-kernel - Re: [PATCH] x86-paravirt: prevent gcc from generating the wrong addressing mode

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Date:	Mon, 16 Mar 2009 17:59:34 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86-paravirt: prevent gcc from generating the wrong
	addressing mode

Thanks Jeremy.

Acked-by: Alok N Kataria <akataria@...are.com>

On Mon, 2009-03-16 at 17:24 -0700, Jeremy Fitzhardinge wrote:
> When we generate a call sequence for calling a paravirtualized
> function, we presume that the generated code is "call *0xXXXXX",
> which is a 6 byte opcode; this is larger than a normal
> direct call, and so we can patch a direct call over it.
> 
> At the moment, however we give gcc enough rope to hang us by
> putting the address in a register and generating a two byte
> indirect-via-register call.  Prevent this by explicitly
> dereferencing the function pointer and passing it into the
> asm as a constant.
> 
> This prevents crashes in VMI, as it cannot handle unpatchable
> callsites.
> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> Cc: Alok Kataria <akataria@...are.com>
> 
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index d4fec1f..6922d16 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -395,7 +395,7 @@ extern struct pv_lock_ops pv_lock_ops;
>  
>  #define paravirt_type(op)				\
>  	[paravirt_typenum] "i" (PARAVIRT_PATCH(op)),	\
> -	[paravirt_opptr] "m" (op)
> +	[paravirt_opptr] "i" (&(op))
>  #define paravirt_clobber(clobber)		\
>  	[paravirt_clobber] "i" (clobber)
>  
> @@ -449,7 +449,7 @@ 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	"call *%[paravirt_opptr];"
> +#define PARAVIRT_CALL	"call *%c[paravirt_opptr];"
>  
>  /*
>   * These macros are intended to wrap calls through one of the paravirt
> 
> 

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