Re: [PATCH v2 10/12] powerpc/entry32: Blacklist exception entry points for kprobe.

From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Christophe Leroy <christophe.leroy@c-s.fr>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Paul Mackerras <paulus@samba.org>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 10/12] powerpc/entry32: Blacklist exception entry points for kprobe.
Date: Tue, 31 Mar 2020 22:35:22 +0530	[thread overview]
Message-ID: <1585673757.0rkxmweypy.naveen@linux.ibm.com> (raw)
In-Reply-To: <f61ac599855e674ebb592464d0ea32a3ba9c6644.1585670437.git.christophe.leroy@c-s.fr>

Christophe Leroy wrote:
> kprobe does not handle events happening in real mode.
> 
> As exception entry points are running with MMU disabled,
> blacklist them.
> 
> The handling of TLF_NAPPING and TLF_SLEEPING is moved before the
> CONFIG_TRACE_IRQFLAGS which contains 'reenable_mmu' because from there
> kprobe will be possible as the kernel will run with MMU enabled.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
> v2: Moved TLF_NAPPING and TLF_SLEEPING handling
> ---
>  arch/powerpc/kernel/entry_32.S | 37 ++++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index 94f78c03cb79..215aa3a6d4f7 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -51,6 +51,7 @@ mcheck_transfer_to_handler:
>  	mfspr	r0,SPRN_DSRR1
>  	stw	r0,_DSRR1(r11)
>  	/* fall through */
> +_ASM_NOKPROBE_SYMBOL(mcheck_transfer_to_handler)
> 
>  	.globl	debug_transfer_to_handler
>  debug_transfer_to_handler:
> @@ -59,6 +60,7 @@ debug_transfer_to_handler:
>  	mfspr	r0,SPRN_CSRR1
>  	stw	r0,_CSRR1(r11)
>  	/* fall through */
> +_ASM_NOKPROBE_SYMBOL(debug_transfer_to_handler)
> 
>  	.globl	crit_transfer_to_handler
>  crit_transfer_to_handler:
> @@ -94,6 +96,7 @@ crit_transfer_to_handler:
>  	rlwinm	r0,r1,0,0,(31 - THREAD_SHIFT)
>  	stw	r0,KSP_LIMIT(r8)
>  	/* fall through */
> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>  #endif
> 
>  #ifdef CONFIG_40x
> @@ -115,6 +118,7 @@ crit_transfer_to_handler:
>  	rlwinm	r0,r1,0,0,(31 - THREAD_SHIFT)
>  	stw	r0,KSP_LIMIT(r8)
>  	/* fall through */
> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>  #endif
> 
>  /*
> @@ -127,6 +131,7 @@ crit_transfer_to_handler:
>  	.globl	transfer_to_handler_full
>  transfer_to_handler_full:
>  	SAVE_NVGPRS(r11)
> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_full)
>  	/* fall through */
> 
>  	.globl	transfer_to_handler
> @@ -227,6 +232,23 @@ transfer_to_handler_cont:
>  	SYNC
>  	RFI				/* jump to handler, enable MMU */
> 
> +#if defined (CONFIG_PPC_BOOK3S_32) || defined(CONFIG_E500)
> +4:	rlwinm	r12,r12,0,~_TLF_NAPPING
> +	stw	r12,TI_LOCAL_FLAGS(r2)
> +	b	power_save_ppc32_restore
> +
> +7:	rlwinm	r12,r12,0,~_TLF_SLEEPING
> +	stw	r12,TI_LOCAL_FLAGS(r2)
> +	lwz	r9,_MSR(r11)		/* if sleeping, clear MSR.EE */
> +	rlwinm	r9,r9,0,~MSR_EE
> +	lwz	r12,_LINK(r11)		/* and return to address in LR */
> +	kuap_restore r11, r2, r3, r4, r5
> +	lwz	r2, GPR2(r11)
> +	b	fast_exception_return
> +#endif
> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler)
> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_cont)
> +

A very minor nit is that the above NOKPROBE annotation actually covers 
the block of code below between the label '1:' till 'reenable_mmu', but 
isn't obvious from the code. Splitting off 'reenable_mmu' would have 
made that clear.

You don't have to fix that though -- a kprobe still won't be allowed 
there and anyone interested should be able to look up this mail chain.

- Naveen