Re: [PATCH 10/13] x86/alternative: Support indirect call replacement

[Juergen Gross]

On 04/10/17 17:58, Josh Poimboeuf wrote:
> Add alternative patching support for replacing an instruction with an
> indirect call.  This will be needed for the paravirt alternatives.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  arch/x86/kernel/alternative.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 3344d3382e91..81c577c7deba 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -410,20 +410,28 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
>  		insnbuf_sz = a->replacementlen;
>  
>  		/*
> -		 * 0xe8 is a relative jump; fix the offset.
> -		 *
> -		 * Instruction length is checked before the opcode to avoid
> -		 * accessing uninitialized bytes for zero-length replacements.
> +		 * Fix the address offsets for call and jump instructions which
> +		 * use PC-relative addressing.
>  		 */
>  		if (a->replacementlen == 5 && *insnbuf == 0xe8) {
> +			/* direct call */
>  			*(s32 *)(insnbuf + 1) += replacement - instr;
> -			DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
> +			DPRINTK("Fix direct CALL offset: 0x%x, CALL 0x%lx",
>  				*(s32 *)(insnbuf + 1),
>  				(unsigned long)instr + *(s32 *)(insnbuf + 1) + 5);
> -		}
>  
> -		if (a->replacementlen && is_jmp(replacement[0]))
> +		} else if (a->replacementlen == 6 && *insnbuf == 0xff &&
> +			   *(insnbuf+1) == 0x15) {
> +			/* indirect call */
> +			*(s32 *)(insnbuf + 2) += replacement - instr;
> +			DPRINTK("Fix indirect CALL offset: 0x%x, CALL *0x%lx",
> +				*(s32 *)(insnbuf + 2),
> +				(unsigned long)instr + *(s32 *)(insnbuf + 2) + 6);
> +
> +		} else if (a->replacementlen && is_jmp(replacement[0])) {

Is this correct? Without your patch this was:

if (*insnbuf == 0xe8) ...
if (is_jmp(replacement[0])) ...

Now you have:

if (*insnbuf == 0xe8) ...
else if (*insnbuf == 0xff15) ...
else if (is_jmp(replacement[0])) ...

So only one or none of the three variants will be executed. In the past
it could be none, one or both.


Juergen