Re: [PATCH bpf-next v2 2/2] bpf: Add arm64 JIT support for bpf_addr_space_cast instruction.

[Xu Kuohai]

On 3/21/2024 11:31 PM, Puranjay Mohan wrote:
> LLVM generates bpf_addr_space_cast instruction while translating
> pointers between native (zero) address space and
> __attribute__((address_space(N))). The addr_space=1 is reserved as
> bpf_arena address space.
>
> rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
> converted to normal 32-bit move: wX = wY
>
> rY = addr_space_cast(rX, 1, 0) has to be converted by JIT:
>
> Here I explain using symbolic language what the JIT is supposed to do:
> We have:
> 	src = [src_upper32][src_lower32] // 64 bit src kernel pointer
> 	uvm = [uvm_upper32][uvm_lower32] // 64 bit user_vm_start
>
> The JIT has to make the dst reg like following
> 	dst = [uvm_upper32][src_lower32] // if src_lower32 != 0
> 	dst = [00000000000][00000000000] // if src_lower32 == 0
>
> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
>   arch/arm64/net/bpf_jit.h                     |  1 +
>   arch/arm64/net/bpf_jit_comp.c                | 35 ++++++++++++++++++++
>   tools/testing/selftests/bpf/DENYLIST.aarch64 |  2 --
>   3 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
> index 23b1b34db088..813c3c428fde 100644
> --- a/arch/arm64/net/bpf_jit.h
> +++ b/arch/arm64/net/bpf_jit.h
> @@ -238,6 +238,7 @@
>   #define A64_LSLV(sf, Rd, Rn, Rm) A64_DATA2(sf, Rd, Rn, Rm, LSLV)
>   #define A64_LSRV(sf, Rd, Rn, Rm) A64_DATA2(sf, Rd, Rn, Rm, LSRV)
>   #define A64_ASRV(sf, Rd, Rn, Rm) A64_DATA2(sf, Rd, Rn, Rm, ASRV)
> +#define A64_RORV(sf, Rd, Rn, Rm) A64_DATA2(sf, Rd, Rn, Rm, RORV)
>   
>   /* Data-processing (3 source) */
>   /* Rd = Ra + Rn * Rm */
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index b9b5febe64f0..37c94ebd06b2 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -82,6 +82,7 @@ struct jit_ctx {
>   	__le32 *ro_image;
>   	u32 stack_size;
>   	int fpb_offset;
> +	u64 user_vm_start;
>   };
>   
>   struct bpf_plt {
> @@ -868,6 +869,34 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
>   	/* dst = src */
>   	case BPF_ALU | BPF_MOV | BPF_X:

is it legal to encode BPF_ADDR_SPACE_CAST with BPF_ALU?

>   	case BPF_ALU64 | BPF_MOV | BPF_X:
> +		if (insn->off == BPF_ADDR_SPACE_CAST &&
> +		    insn->imm == 1U << 16) {
> +			/* Zero out tmp2 */
> +			emit(A64_EOR(1, tmp2, tmp2, tmp2), ctx);
> +
> +			/* Move lo_32_bits(src) to dst */
> +			if (dst != src)
> +				emit(A64_MOV(0, dst, src), ctx);
> +
> +			/* Logical shift left by 32 bits */
> +			emit(A64_LSL(1, dst, dst, 32), ctx);
> +
> +			/* Get upper 32 bits of user_vm_start in tmp */
> +			emit_a64_mov_i(0, tmp, ctx->user_vm_start >> 32, ctx);
> +
> +			/* dst |= up_32_bits(user_vm_start) */
> +			emit(A64_ORR(1, dst, dst, tmp), ctx);
> +
> +			/* Rotate by 32 bits to get final result */
> +			emit_a64_mov_i(0, tmp, 32, ctx);
> +			emit(A64_RORV(1, dst, dst, tmp), ctx);
> +
> +			/* If lo_32_bits(dst) == 0, set dst = tmp2(0) */
> +			emit(A64_CBZ(0, dst, 2), ctx);
> +			emit(A64_MOV(1, tmp2, dst), ctx);
> +			emit(A64_MOV(1, dst, tmp2), ctx);

seems we could simplify it to:

emit_a64_mov_i(0, dst, ctx->user_vm_start >> 32, ctx);
emit(A64_LSL(1, dst, dst, 32), ctx);
emit(A64_MOV(0, tmp, src), ctx); // 32-bit mov clears the upper 32 bits
emit(A64_CBZ(1, tmp, 2), ctx);
emit(A64_ORR(1, tmp, dst, tmp), ctx);
emit(A64_MOV(1, dst, tmp), ctx);

> +		break;

not aligned

> +		}
>   		switch (insn->off) {
>   		case 0:
>   			emit(A64_MOV(is64, dst, src), ctx);
> @@ -1690,6 +1719,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>   	}
>   
>   	ctx.fpb_offset = find_fpb_offset(prog);
> +	ctx.user_vm_start = bpf_arena_get_user_vm_start(prog->aux->arena);
>   
>   	/*
>   	 * 1. Initial fake pass to compute ctx->idx and ctx->offset.
> @@ -2511,6 +2541,11 @@ bool bpf_jit_supports_exceptions(void)
>   	return true;
>   }
>   
> +bool bpf_jit_supports_arena(void)
> +{
> +	return true;
> +}
> +
>   void bpf_jit_free(struct bpf_prog *prog)
>   {
>   	if (prog->jited) {
> diff --git a/tools/testing/selftests/bpf/DENYLIST.aarch64 b/tools/testing/selftests/bpf/DENYLIST.aarch64
> index d8ade15e2789..0445ac38bc07 100644
> --- a/tools/testing/selftests/bpf/DENYLIST.aarch64
> +++ b/tools/testing/selftests/bpf/DENYLIST.aarch64
> @@ -10,5 +10,3 @@ fill_link_info/kprobe_multi_link_info            # bpf_program__attach_kprobe_mu
>   fill_link_info/kretprobe_multi_link_info         # bpf_program__attach_kprobe_multi_opts unexpected error: -95
>   fill_link_info/kprobe_multi_invalid_ubuff        # bpf_program__attach_kprobe_multi_opts unexpected error: -95
>   missed/kprobe_recursion                          # missed_kprobe_recursion__attach unexpected error: -95 (errno 95)
> -verifier_arena                                   # JIT does not support arena
> -arena_htab                                       # JIT does not support arena