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    SubjectRe: [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@freebox.fr>
    > Cc: <stable@vger.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@gmail.com>


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