[PATCH v2 3/4] powerpc/kprobes_on_ftrace: Skip livepatch_handler() for jprobes

[Michael Ellerman]

"Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> writes:

> diff --git a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> index fa0921410fa4..e6837e85ec28 100644
> --- a/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> +++ b/arch/powerpc/kernel/trace/ftrace_64_mprofile.S
> @@ -99,13 +99,37 @@ ftrace_call:
>  	bl	ftrace_stub
>  	nop
>  
> -	/* Load ctr with the possibly modified NIP */
> -	ld	r3, _NIP(r1)
> -	mtctr	r3
> +	/* Load the possibly modified NIP */
> +	ld	r15, _NIP(r1)
> +
>  #ifdef CONFIG_LIVEPATCH
> -	cmpd	r14,r3		/* has NIP been altered? */
> +	cmpd	r14, r15		/* has NIP been altered? */
> +#endif
> +
> +#if defined(CONFIG_LIVEPATCH) && defined(CONFIG_KPROBES_ON_FTRACE)
> +	beq	1f
> +
> +	/* Check if there is an active kprobe on us */
> +	subi	r3, r14, 4
> +	bl	is_current_kprobe_addr
> +	nop
> +
> +	/*
> +	 * If r3 == 1, then this is a kprobe/jprobe.
> +	 * else, this is livepatched function.
> +	 *
> +	 * The subsequent conditional branch for livepatch_handler
> +	 * will use the result of this compare. For kprobe/jprobe,
> +	 * we just need to branch to the new NIP, so nothing special
> +	 * is needed.
> +	 */
> +	cmpdi	r3, 1
> +1:
>  #endif

I added some more comments. Hopefully I got them right :D

 #if defined(CONFIG_LIVEPATCH) && defined(CONFIG_KPROBES_ON_FTRACE)
+       /* NIP has not been altered, skip over further checks */
        beq     1f
 
        /* Check if there is an active kprobe on us */
@@ -118,10 +119,11 @@ ftrace_call:
         * If r3 == 1, then this is a kprobe/jprobe.
         * else, this is livepatched function.
         *
-        * The subsequent conditional branch for livepatch_handler
-        * will use the result of this compare. For kprobe/jprobe,
-        * we just need to branch to the new NIP, so nothing special
-        * is needed.
+        * The conditional branch for livepatch_handler below will use the
+        * result of this comparison. For kprobe/jprobe, we just need to branch to
+        * the new NIP, not call livepatch_handler. The branch below is bne, so we
+        * want CR0[EQ] to be true if this is a kprobe/jprobe. Which means we want
+        * CR0[EQ] = (r3 == 1).
         */
        cmpdi   r3, 1
 1:
@@ -147,7 +149,10 @@ ftrace_call:
        addi r1, r1, SWITCH_FRAME_SIZE
 
 #ifdef CONFIG_LIVEPATCH
-        /* Based on the cmpd above, if the NIP was altered handle livepatch */
+        /*
+        * Based on the cmpd or cmpdi above, if the NIP was altered and we're
+        * not on a kprobe/jprobe, then handle livepatch.
+        */
        bne-    livepatch_handler
 #endif


cheers