Subject: Re: [PATCH] arm64: bpf: fix JIT stack setup

From: "Shi, Yang" <[email-masked]>
Date: Fri, 06 Nov 2015 21:55:30 -0800

Please ignore this one, forgot to cc to linux-arm-kernel list.

Sorry for the inconvenience.

Yang

On 11/6/2015 9:34 PM, Yang Shi wrote:
> ARM64 JIT used FP (x29) as eBPF fp register, but FP is subjected to
> change during function call so it may cause the BPF prog stack base address
> change too. Whenever, it pointed to the bottom of BPF prog stack instead of
> the top.
>
> So, when copying data via bpf_probe_read, it will be copied to (SP - offset),
> then it may overwrite the saved FP/LR.
>
> Use x25 to replace FP as BPF stack base register (fp). Since x25 is callee
> saved register, so it will keep intact during function call.
> It is initialized in BPF prog prologue when BPF prog is started to run
> everytime. When BPF prog exits, it could be just tossed.
>
> Other than this the BPf prog stack base need to be setup before function
> call stack.
>
> So, the BPF stack layout looks like:
>
>                                   high
>           original A64_SP =>   0:+-----+ BPF prologue
>                                  |     | FP/LR and callee saved registers
>           BPF fp register => +64:+-----+
>                                  |     |
>                                  | ... | BPF prog stack
>                                  |     |
>                                  |     |
>           current A64_SP =>      +-----+
>                                  |     |
>                                  | ... | Function call stack
>                                  |     |
>                                  +-----+
>                                    low
>
> Signed-off-by: Yang Shi <[email-masked]>
> CC: Zi Shen Lim <[email-masked]>
> CC: Xi Wang <[email-masked]>
> ---
>   arch/arm64/net/bpf_jit_comp.c | 38 +++++++++++++++++++++++++++++++-------
>   1 file changed, 31 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index a44e529..6809647 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -50,7 +50,7 @@ static const int bpf2a64[] = {
>   	[BPF_REG_8] = A64_R(21),
>   	[BPF_REG_9] = A64_R(22),
>   	/* read-only frame pointer to access stack */
> -	[BPF_REG_FP] = A64_FP,
> +	[BPF_REG_FP] = A64_R(25),
>   	/* temporary register for internal BPF JIT */
>   	[TMP_REG_1] = A64_R(23),
>   	[TMP_REG_2] = A64_R(24),
> @@ -155,18 +155,42 @@ static void build_prologue(struct jit_ctx *ctx)
>   	stack_size += 4; /* extra for skb_copy_bits buffer */
>   	stack_size = STACK_ALIGN(stack_size);
>
> +	/*
> +	 * BPF prog stack layout
> +	 *
> +	 *                         high
> +	 * original A64_SP =>   0:+-----+ BPF prologue
> +	 *                        |     | FP/LR and callee saved registers
> +	 * BPF fp register => +64:+-----+
> +	 *                        |     |
> +         *                        | ... | BPF prog stack
> +	 *                        |     |
> +	 *                        |     |
> +	 * current A64_SP =>      +-----+
> +	 *                        |     |
> +	 *                        | ... | Function call stack
> +	 *                        |     |
> +	 *                        +-----+
> +	 *                          low
> +	 *
> +	 */
> +
> +	/* Save FP and LR registers to stay align with ARM64 AAPCS */
> +	emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
> +
>   	/* Save callee-saved register */
>   	emit(A64_PUSH(r6, r7, A64_SP), ctx);
>   	emit(A64_PUSH(r8, r9, A64_SP), ctx);
>   	if (ctx->tmp_used)
>   		emit(A64_PUSH(tmp1, tmp2, A64_SP), ctx);
>
> -	/* Set up BPF stack */
> -	emit(A64_SUB_I(1, A64_SP, A64_SP, stack_size), ctx);
> -
> -	/* Set up frame pointer */
> +	/* Set up BPF prog stack base register (x25) */
>   	emit(A64_MOV(1, fp, A64_SP), ctx);
>
> +	/* Set up function call stack */
> +	emit(A64_SUB_I(1, A64_SP, A64_SP, stack_size), ctx);
> +	emit(A64_MOV(1, A64_FP, A64_SP), ctx);
> +
>   	/* Clear registers A and X */
>   	emit_a64_mov_i64(ra, 0, ctx);
>   	emit_a64_mov_i64(rx, 0, ctx);
> @@ -196,8 +220,8 @@ static void build_epilogue(struct jit_ctx *ctx)
>   	emit(A64_POP(r8, r9, A64_SP), ctx);
>   	emit(A64_POP(r6, r7, A64_SP), ctx);
>
> -	/* Restore frame pointer */
> -	emit(A64_MOV(1, fp, A64_SP), ctx);
> +	/* Restore FP/LR registers */
> +	emit(A64_POP(A64_FP, A64_LR, A64_SP), ctx);
>
>   	/* Set return value */
>   	emit(A64_MOV(1, A64_R(0), r0), ctx);
>