linux-kernel - Re: [PATCH v2] ARM: net fix emit_udiv() for BPF_ALU | BPF_DIV

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Date:	Sun, 10 May 2015 13:31:26 +0200
From:	Mircea Gherzan <mgherzan@...il.com>
To:	Nicolas Schichan <nschichan@...ebox.fr>,
	Russell King <linux@....linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>
CC:	Alexei Starovoitov <ast@...mgrid.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] ARM: net fix emit_udiv() for BPF_ALU | BPF_DIV | BPF_K
 intruction.

On 05/06/2015 06:31 PM, Nicolas Schichan wrote:
> In that case, emit_udiv() will be called with rn == ARM_R0 (r_scratch)
> and loading rm first into ARM_R0 will result in jit_udiv() function
> being called the same dividend and divisor. Fix that by loading rn
> first into ARM_R1 and then rm into ARM_R0.
>
> Signed-off-by: Nicolas Schichan <nschichan@...ebox.fr>
> Cc: <stable@...r.kernel.org> # v3.13+
> Fixes: aee636c4809f (bpf: do not use reciprocal divide)
> ---
>   arch/arm/net/bpf_jit_32.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> This patch was part of a serie adding support to JITed seccomp filters
> to ARM but as it is an unrelated fix, it is more appropriate to send
> it separately.
>
> Changes from previous version: add a comment clarifying how
> emit_udiv() is called when translating BPF_ALU | BPF_DIV | BPF_K and
> BPF_ALU | BPF_DIV | BPF_X with respect to the rm and rn parameters.
>
> diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
> index e1268f9..f412b53 100644
> --- a/arch/arm/net/bpf_jit_32.c
> +++ b/arch/arm/net/bpf_jit_32.c
> @@ -449,10 +449,21 @@ 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);
> +
> +	/*
> +	 * For BPF_ALU | BPF_DIV | BPF_K instructions, rm is ARM_R4
> +	 * (r_A) and rn is ARM_R0 (r_scratch) so load rn first into
> +	 * ARM_R1 to avoid accidentally overwriting ARM_R0 with rm
> +	 * before using it as a source for ARM_R1.
> +	 *
> +	 * For BPF_ALU | BPF_DIV | BPF_X rm is ARM_R4 (r_A) and rn is
> +	 * ARM_R5 (r_X) so there is no particular register overlap
> +	 * issues.
> +	 */
>   	if (rn != ARM_R1)
>   		emit(ARM_MOV_R(ARM_R1, rn), ctx);
> +	if (rm != ARM_R0)
> +		emit(ARM_MOV_R(ARM_R0, rm), ctx);
>
>   	ctx->seen |= SEEN_CALL;
>   	emit_mov_i(ARM_R3, (u32)jit_udiv, ctx);
>

Acked-by: Mircea Gherzan <mgherzan@...il.com>
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