Subject: Re: [v3 PATCH 3/3] ppc32/kprobe: don't emulate store when kprobe stwu r1

From: "tiejun.chen" <[email-masked]>
Date: Sun, 3 Jun 2012 14:59:19 +0800

On 06/03/2012 01:07 PM, Tiejun Chen wrote:
> We don't do the real store operation for kprobing 'stwu Rx,(y)R1'
> since this may corrupt the exception frame, now we will do this
> operation safely in exception return code after migrate current
> exception frame below the kprobed function stack.
> 
> So we only update gpr[1] here and trigger a thread flag to mask
> this.
> 
> Note we should make sure if we trigger kernel stack over flow.
> 
> Signed-off-by: Tiejun Chen <[email-masked]>
> ---
>  arch/powerpc/lib/sstep.c |   37 +++++++++++++++++++++++++++++++++++--
>  1 files changed, 35 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 9a52349..a4ce463 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -566,7 +566,7 @@ int __kprobes emulate_step(struct pt_regs *regs, unsigned int instr)
>  	unsigned long int ea;
>  	unsigned int cr, mb, me, sh;
>  	int err;
> -	unsigned long old_ra;
> +	unsigned long old_ra, val3, r1;
>  	long ival;
>  
>  	opcode = instr >> 26;
> @@ -1486,11 +1486,44 @@ int __kprobes emulate_step(struct pt_regs *regs, unsigned int instr)
>  		goto ldst_done;
>  
>  	case 36:	/* stw */
> -	case 37:	/* stwu */
>  		val = regs->gpr[rd];
>  		err = write_mem(val, dform_ea(instr, regs), 4, regs);
>  		goto ldst_done;
>  
> +	case 37:	/* stwu */
> +		__asm__ __volatile__("mr %0,1" : "=r" (r1) :);

I'll remove this line, please see below.

> +
> +		val = regs->gpr[rd];
> +		val3 = dform_ea(instr, regs);
> +		/*
> +		 * For PPC32 we always use stwu to change stack point with r1. So
> +		 * this emulated store may corrupt the exception frame, now we
> +		 * have to provide the exception frame trampoline, which is pushed
> +		 * below the kprobed function stack. So we only update gpr[1] but
> +		 * don't emulate the real store operation. We will do real store
> +		 * operation safely in exception return code by checking this flag.
> +		 */
> +		if ((ra == 1) && !(regs->msr & MSR_PR) && (val3 >= r1)) {
> +			/*
> +			 * Check if we will touch kernel sack overflow
> +			 */
> +			if (r1 - STACK_INT_FRAME_SIZE <= current->thread.ksp_limit) {

OOPS. This line should be:
			
		if (val3 - STACK_INT_FRAME_SIZE <= current->thread.ksp_limit) {

Tiejun

> +				printk(KERN_CRIT "Can't kprobe this since Kernel stack overflow.\n");
> +				err = -EINVAL;
> +				break;
> +			}
> +
> +			/*
> +			 * Check if we already set since that means we'll
> +			 * lose the previous value.
> +			 */
> +			WARN_ON(test_thread_flag(TIF_EMULATE_STACK_STORE));
> +			set_thread_flag(TIF_EMULATE_STACK_STORE);
> +			err = 0;
> +		} else
> +			err = write_mem(val, val3, 4, regs);
> +		goto ldst_done;
> +
>  	case 38:	/* stb */
>  	case 39:	/* stbu */
>  		val = regs->gpr[rd];