linux-kernel - Re: [PATCH 4/4] ARM: net fix emit_udiv() for BPF_ALU | BPF_DIV

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Date:	Mon, 4 May 2015 18:57:09 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Nicolas Schichan <nschichan@...ebox.fr>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Daniel Borkmann <dborkman@...hat.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] ARM: net fix emit_udiv() for BPF_ALU | BPF_DIV |
 BPF_K intruction.

On Mon, May 04, 2015 at 06:16:30PM +0200, Nicolas Schichan wrote:
> On 05/01/2015 07:37 PM, Russell King - ARM Linux wrote:
> > On Wed, Apr 29, 2015 at 03:37:37PM +0200, Nicolas Schichan wrote:
> [...]
> >> diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
> >> index b5f470d..ffaf311 100644
> >> --- a/arch/arm/net/bpf_jit_32.c
> >> +++ b/arch/arm/net/bpf_jit_32.c
> >> @@ -449,10 +449,10 @@ static inline void emit_udiv(u8 rd, u8 rm, u8 rn, struct jit_ctx *ctx)
> >>  		return;
> >>  	}
> >>  #endif
> >> -	if (rm != ARM_R0)
> >> -		emit(ARM_MOV_R(ARM_R0, rm), ctx);
> >>  	if (rn != ARM_R1)
> >>  		emit(ARM_MOV_R(ARM_R1, rn), ctx);
> >> +	if (rm != ARM_R0)
> >> +		emit(ARM_MOV_R(ARM_R0, rm), ctx);
> > 
> > I don't think you've thought enough about this.  What if rm is ARM_R1?
> > What if rn = ARM_R0 and rm = ARM_R1?
> >
> > How about:
> > 
> > 	if (rn == ARM_R0 && rm == ARM_R1) {
> > 		emit(ARM_MOV_R(ARM_R3, rn), ctx); // r3 <- r0(rn)
> > 		emit(ARM_MOV_R(ARM_R0, rm), ctx); // r0 <- r1(rm)
> > 		emit(ARM_MOV_R(ARM_R1, ARM_R3), ctx); // r1 <- r3
> > 	} else if (rn == ARM_R0) {
> > 		emit(ARM_MOV_R(ARM_R1, rn), ctx); // r1 <- rn
> > 		if (rm != ARM_R0)
> > 			emit(ARM_MOV_R(ARM_R0, rm), ctx); // r0 <- rm
> > 	} else {
> > 		if (rm != ARM_R0)
> > 			emit(ARM_MOV_R(ARM_R0, rm), ctx); // r0 <- rm
> > 		if (rn != ARM_R1)
> > 			emit(ARM_MOV_R(ARM_R1, rn), ctx); // r1 <- rn
> > 	}
> > 
> 
> Hello Russell,
> 
> In the current JIT, emit_udiv() is only being called with:
> 
> - rm = ARM_R4 (r_A) and rn = ARM_R0 (r_scrach) for BPF_ALU | BPF_DIV | BPF_K
> 
> - rm = ARM_R4 (r_A) and rn = ARM_R5 (r_X) for BPF_ALU | BPF_DIV | BPF_X
> 
> so it should not cause any issue in the current code state.
> 
> But yes, I'll rework the patch to avoid any other nasty surprises should the
> code change.

Maybe then add a comment detailing the current conditions that this is
coded for so that if you're not around when the code changes, others are
aware of the issue.

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