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arm64: bpf: implement bpf_tail_call() helper
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Add support for JMP_CALL_X (tail call) introduced by commit 04fd61a
("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                      // torvalds#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                      // torvalds#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                      // torvalds#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                      // torvalds#32
  68:   ldr     x10, [x11,x10]
  6c:   add     x10, x10, #0x20
  70:   br      x10
  74:

Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
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@zlim
zlim committed Jun 7, 2016
1 parent cf3003d commit 3626bb1
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Showing 2 changed files with 98 additions and 10 deletions.
3 changes: 2 additions & 1 deletion arch/arm64/net/bpf_jit.h
View file
Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
/*
* BPF JIT compiler for ARM64
*
* Copyright (C) 2014-2015 Zi Shen Lim <zlim.lnx@gmail.com>
* Copyright (C) 2014-2016 Zi Shen Lim <zlim.lnx@gmail.com>
*
* 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
Expand Down Expand Up @@ -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)

Expand Down
105 changes: 96 additions & 9 deletions arch/arm64/net/bpf_jit_comp.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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>
Expand All @@ -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[] = {
Expand All @@ -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),
};
Expand Down Expand Up @@ -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
Expand All @@ -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
Expand All @@ -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)
Expand Down Expand Up @@ -506,6 +585,11 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
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,
Expand Down Expand Up @@ -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);
Expand Down

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