Re: [PATCH bpf-next v3 5/7] bpf, arm64: Support to poke bpf prog

Jakub Sitnicki
2022-05-10T11:45:41Z

Thanks for incorporating the attach to BPF progs bits into the series.

I have a couple minor comments. Please see below.

On Sun, Apr 24, 2022 at 11:40 AM -04, Xu Kuohai wrote:
> 1. Set up the bpf prog entry in the same way as fentry to support
> trampoline. Now bpf prog entry looks like this:
>
> bti c // if BTI enabled
> mov x9, x30 // save lr
> nop // to be replaced with jump instruction
> paciasp // if PAC enabled
>
> 2. Update bpf_arch_text_poke() to poke bpf prog. If the instruction
> to be poked is bpf prog's first instruction, skip to the nop
> instruction in the prog entry.
>
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> ---
> arch/arm64/net/bpf_jit.h | 1 +
> arch/arm64/net/bpf_jit_comp.c | 41 +++++++++++++++++++++++++++--------
> 2 files changed, 33 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
> index 194c95ccc1cf..1c4b0075a3e2 100644
> --- a/arch/arm64/net/bpf_jit.h
> +++ b/arch/arm64/net/bpf_jit.h
> @@ -270,6 +270,7 @@
> #define A64_BTI_C A64_HINT(AARCH64_INSN_HINT_BTIC)
> #define A64_BTI_J A64_HINT(AARCH64_INSN_HINT_BTIJ)
> #define A64_BTI_JC A64_HINT(AARCH64_INSN_HINT_BTIJC)
> +#define A64_NOP A64_HINT(AARCH64_INSN_HINT_NOP)
>
> /* DMB */
> #define A64_DMB_ISH aarch64_insn_gen_dmb(AARCH64_INSN_MB_ISH)
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 3f9bdfec54c4..293bdefc5d0c 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -237,14 +237,23 @@ static bool is_lsi_offset(int offset, int scale)
> return true;
> }
>
> -/* Tail call offset to jump into */
> -#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) || \
> - IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)
> -#define PROLOGUE_OFFSET 9
> +#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
> +#define BTI_INSNS 1
> +#else
> +#define BTI_INSNS 0
> +#endif
> +
> +#if IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)
> +#define PAC_INSNS 1
> #else
> -#define PROLOGUE_OFFSET 8
> +#define PAC_INSNS 0
> #endif

Above can be folded into:

#define BTI_INSNS (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) ? 1 : 0)
#define PAC_INSNS (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) ? 1 : 0)

>
> +/* Tail call offset to jump into */
> +#define PROLOGUE_OFFSET (BTI_INSNS + 2 + PAC_INSNS + 8)
> +/* Offset of nop instruction in bpf prog entry to be poked */
> +#define POKE_OFFSET (BTI_INSNS + 1)
> +
> static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
> {
> const struct bpf_prog *prog = ctx->prog;
> @@ -281,12 +290,15 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
> *
> */
>
> + if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> + emit(A64_BTI_C, ctx);

I'm no arm64 expert, but this looks like a fix for BTI.

Currently we never emit BTI because ARM64_BTI_KERNEL depends on
ARM64_PTR_AUTH_KERNEL, while BTI must be the first instruction for the
jump target [1]. Am I following correctly?

[1] https://lwn.net/Articles/804982/

> +
> + emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
> + emit(A64_NOP, ctx);
> +
> /* Sign lr */
> if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL))
> emit(A64_PACIASP, ctx);
> - /* BTI landing pad */
> - else if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> - emit(A64_BTI_C, ctx);
>
> /* Save FP and LR registers to stay align with ARM64 AAPCS */
> emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
> @@ -1552,9 +1564,11 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
> u32 old_insn;
> u32 new_insn;
> u32 replaced;
> + unsigned long offset = ~0UL;
> enum aarch64_insn_branch_type branch_type;
> + char namebuf[KSYM_NAME_LEN];
>
> - if (!is_bpf_text_address((long)ip))
> + if (!__bpf_address_lookup((unsigned long)ip, NULL, &offset, namebuf))
> /* Only poking bpf text is supported. Since kernel function
> * entry is set up by ftrace, we reply on ftrace to poke kernel
> * functions. For kernel funcitons, bpf_arch_text_poke() is only
> @@ -1565,6 +1579,15 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
> */
> return -EINVAL;
>
> + /* bpf entry */
> + if (offset == 0UL)
> + /* skip to the nop instruction in bpf prog entry:
> + * bti c // if BTI enabled
> + * mov x9, x30
> + * nop
> + */
> + ip = (u32 *)ip + POKE_OFFSET;

This is very much personal preference, however, I find the use pointer
arithmetic too clever here. Would go for a more verbose:

offset = POKE_OFFSET * AARCH64_INSN_SIZE;
ip = (void *)((unsigned long)ip + offset);

> +
> if (poke_type == BPF_MOD_CALL)
> branch_type = AARCH64_INSN_BRANCH_LINK;
> else

I think it'd make more sense to merge this patch with patch 4 (the
preceding one).

Initial implementation of of bpf_arch_text_poke() from patch 4 is not
fully functional, as it will always fail for bpf_arch_text_poke(ip,
BPF_MOD_CALL, ...) calls. At least, I find it a bit confusing.

Otherwise than that:

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>