[PATCH net-next v3 2/4] arm64: bpf: implement bpf_tail_call() helper
From: Zi Shen Lim
Date: Thu Jun 09 2016 - 00:21:36 EST
Add support for JMP_CALL_X (tail call) introduced by commit 04fd61ab36ec
("bpf: allow bpf programs to tail-call other bpf programs").
bpf_tail_call() arguments:
ctx - context pointer passed to next program
array - pointer to map which type is BPF_MAP_TYPE_PROG_ARRAY
index - index inside array that selects specific program to run
In this implementation arm64 JIT jumps into callee program after prologue,
so callee program reuses the same stack. For tail_call_cnt, we use the
callee-saved R26 (which was already saved/restored but previously unused
by JIT).
With this patch a tail call generates the following code on arm64:
if (index >= array->map.max_entries)
goto out;
34: mov x10, #0x10 // #16
38: ldr w10, [x1,x10]
3c: cmp w2, w10
40: b.ge 0x0000000000000074
if (tail_call_cnt > MAX_TAIL_CALL_CNT)
goto out;
tail_call_cnt++;
44: mov x10, #0x20 // #32
48: cmp x26, x10
4c: b.gt 0x0000000000000074
50: add x26, x26, #0x1
prog = array->ptrs[index];
if (prog == NULL)
goto out;
54: mov x10, #0x68 // #104
58: ldr x10, [x1,x10]
5c: ldr x11, [x10,x2]
60: cbz x11, 0x0000000000000074
goto *(prog->bpf_func + prologue_size);
64: mov x10, #0x20 // #32
68: ldr x10, [x11,x10]
6c: add x10, x10, #0x20
70: br x10
74:
Signed-off-by: Zi Shen Lim <zlim.lnx@xxxxxxxxx>
---
arch/arm64/net/bpf_jit.h | 3 +-
arch/arm64/net/bpf_jit_comp.c | 105 ++++++++++++++++++++++++++++++++++++++----
2 files changed, 98 insertions(+), 10 deletions(-)
diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
index aee5637..7c16e54 100644
--- a/arch/arm64/net/bpf_jit.h
+++ b/arch/arm64/net/bpf_jit.h
@@ -1,7 +1,7 @@
/*
* BPF JIT compiler for ARM64
*
- * Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
+ * Copyright (C) 2014-2016 Zi Shen Lim <zlim.lnx@xxxxxxxxx>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2 as
@@ -55,6 +55,7 @@
#define A64_BL(imm26) A64_BRANCH((imm26) << 2, LINK)
/* Unconditional branch (register) */
+#define A64_BR(Rn) aarch64_insn_gen_branch_reg(Rn, AARCH64_INSN_BRANCH_NOLINK)
#define A64_BLR(Rn) aarch64_insn_gen_branch_reg(Rn, AARCH64_INSN_BRANCH_LINK)
#define A64_RET(Rn) aarch64_insn_gen_branch_reg(Rn, AARCH64_INSN_BRANCH_RETURN)
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 49ba37e..51abc97 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -18,6 +18,7 @@
#define pr_fmt(fmt) "bpf_jit: " fmt
+#include <linux/bpf.h>
#include <linux/filter.h>
#include <linux/printk.h>
#include <linux/skbuff.h>
@@ -33,6 +34,7 @@ int bpf_jit_enable __read_mostly;
#define TMP_REG_1 (MAX_BPF_JIT_REG + 0)
#define TMP_REG_2 (MAX_BPF_JIT_REG + 1)
+#define TCALL_CNT (MAX_BPF_JIT_REG + 2)
/* Map BPF registers to A64 registers */
static const int bpf2a64[] = {
@@ -54,6 +56,8 @@ static const int bpf2a64[] = {
/* temporary registers for internal BPF JIT */
[TMP_REG_1] = A64_R(10),
[TMP_REG_2] = A64_R(11),
+ /* tail_call_cnt */
+ [TCALL_CNT] = A64_R(26),
/* temporary register for blinding constants */
[BPF_REG_AX] = A64_R(9),
};
@@ -146,13 +150,18 @@ static inline int epilogue_offset(const struct jit_ctx *ctx)
#define STACK_SIZE STACK_ALIGN(_STACK_SIZE)
-static void build_prologue(struct jit_ctx *ctx)
+#define PROLOGUE_OFFSET 8
+
+static int build_prologue(struct jit_ctx *ctx)
{
const u8 r6 = bpf2a64[BPF_REG_6];
const u8 r7 = bpf2a64[BPF_REG_7];
const u8 r8 = bpf2a64[BPF_REG_8];
const u8 r9 = bpf2a64[BPF_REG_9];
const u8 fp = bpf2a64[BPF_REG_FP];
+ const u8 tcc = bpf2a64[TCALL_CNT];
+ const int idx0 = ctx->idx;
+ int cur_offset;
/*
* BPF prog stack layout
@@ -162,8 +171,6 @@ static void build_prologue(struct jit_ctx *ctx)
* |FP/LR|
* current A64_FP => -16:+-----+
* | ... | callee saved registers
- * +-----+
- * | | x25/x26
* BPF fp register => -64:+-----+ <= (BPF_FP)
* | |
* | ... | BPF prog stack
@@ -183,18 +190,90 @@ static void build_prologue(struct jit_ctx *ctx)
emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
emit(A64_MOV(1, A64_FP, A64_SP), ctx);
- /* Save callee-saved register */
+ /* Save callee-saved registers */
emit(A64_PUSH(r6, r7, A64_SP), ctx);
emit(A64_PUSH(r8, r9, A64_SP), ctx);
+ emit(A64_PUSH(fp, tcc, A64_SP), ctx);
- /* Save fp (x25) and x26. SP requires 16 bytes alignment */
- emit(A64_PUSH(fp, A64_R(26), A64_SP), ctx);
-
- /* Set up BPF prog stack base register (x25) */
+ /* Set up BPF prog stack base register */
emit(A64_MOV(1, fp, A64_SP), ctx);
+ /* Initialize tail_call_cnt */
+ emit(A64_MOVZ(1, tcc, 0, 0), ctx);
+
/* Set up function call stack */
emit(A64_SUB_I(1, A64_SP, A64_SP, STACK_SIZE), ctx);
+
+ cur_offset = ctx->idx - idx0;
+ if (cur_offset != PROLOGUE_OFFSET) {
+ pr_err_once("PROLOGUE_OFFSET = %d, expected %d!\n",
+ cur_offset, PROLOGUE_OFFSET);
+ return -1;
+ }
+ return 0;
+}
+
+static int out_offset = -1; /* initialized on the first pass of build_body() */
+static int emit_bpf_tail_call(struct jit_ctx *ctx)
+{
+ /* bpf_tail_call(void *prog_ctx, struct bpf_array *array, u64 index) */
+ const u8 r2 = bpf2a64[BPF_REG_2];
+ const u8 r3 = bpf2a64[BPF_REG_3];
+
+ const u8 tmp = bpf2a64[TMP_REG_1];
+ const u8 prg = bpf2a64[TMP_REG_2];
+ const u8 tcc = bpf2a64[TCALL_CNT];
+ const int idx0 = ctx->idx;
+#define cur_offset (ctx->idx - idx0)
+#define jmp_offset (out_offset - (cur_offset))
+ size_t off;
+
+ /* if (index >= array->map.max_entries)
+ * goto out;
+ */
+ off = offsetof(struct bpf_array, map.max_entries);
+ emit_a64_mov_i64(tmp, off, ctx);
+ emit(A64_LDR32(tmp, r2, tmp), ctx);
+ emit(A64_CMP(0, r3, tmp), ctx);
+ emit(A64_B_(A64_COND_GE, jmp_offset), ctx);
+
+ /* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+ * goto out;
+ * tail_call_cnt++;
+ */
+ emit_a64_mov_i64(tmp, MAX_TAIL_CALL_CNT, ctx);
+ emit(A64_CMP(1, tcc, tmp), ctx);
+ emit(A64_B_(A64_COND_GT, jmp_offset), ctx);
+ emit(A64_ADD_I(1, tcc, tcc, 1), ctx);
+
+ /* prog = array->ptrs[index];
+ * if (prog == NULL)
+ * goto out;
+ */
+ off = offsetof(struct bpf_array, ptrs);
+ emit_a64_mov_i64(tmp, off, ctx);
+ emit(A64_LDR64(tmp, r2, tmp), ctx);
+ emit(A64_LDR64(prg, tmp, r3), ctx);
+ emit(A64_CBZ(1, prg, jmp_offset), ctx);
+
+ /* goto *(prog->bpf_func + prologue_size); */
+ off = offsetof(struct bpf_prog, bpf_func);
+ emit_a64_mov_i64(tmp, off, ctx);
+ emit(A64_LDR64(tmp, prg, tmp), ctx);
+ emit(A64_ADD_I(1, tmp, tmp, sizeof(u32) * PROLOGUE_OFFSET), ctx);
+ emit(A64_BR(tmp), ctx);
+
+ /* out: */
+ if (out_offset == -1)
+ out_offset = cur_offset;
+ if (cur_offset != out_offset) {
+ pr_err_once("tail_call out_offset = %d, expected %d!\n",
+ cur_offset, out_offset);
+ return -1;
+ }
+ return 0;
+#undef cur_offset
+#undef jmp_offset
}
static void build_epilogue(struct jit_ctx *ctx)
@@ -506,6 +585,11 @@ emit_cond_jmp:
emit(A64_POP(A64_FP, A64_LR, A64_SP), ctx);
break;
}
+ /* tail call */
+ case BPF_JMP | BPF_CALL | BPF_X:
+ if (emit_bpf_tail_call(ctx))
+ return -EFAULT;
+ break;
/* function return */
case BPF_JMP | BPF_EXIT:
/* Optimization: when last instruction is EXIT,
@@ -780,7 +864,10 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
goto out_off;
}
- build_prologue(&ctx);
+ if (build_prologue(&ctx)) {
+ prog = orig_prog;
+ goto out_off;
+ }
ctx.epilogue_offset = ctx.idx;
build_epilogue(&ctx);
--
1.9.1