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bpf, arm64: adjust the offset of str/ldr(immediate) to positive number
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The BPF STX/LDX instruction uses offset relative to the FP to address
stack space. Since the BPF_FP locates at the top of the frame, the offset
is usually a negative number. However, arm64 str/ldr immediate instruction
requires that offset be a positive number.  Therefore, this patch tries to
convert the offsets.

The method is to find the negative offset furthest from the FP firstly.
Then add it to the FP, calculate a bottom position, called FPB, and then
adjust the offsets in other STR/LDX instructions relative to FPB.

FPB is saved using the callee-saved register x27 of arm64 which is not
used yet.

Before adjusting the offset, the patch checks every instruction to ensure
that the FP does not change in run-time. If the FP may change, no offset
is adjusted.

For example, for the following bpftrace command:

  bpftrace -e 'kprobe:do_sys_open { printf("opening: %s\n", str(arg1)); }'

Without this patch, jited code(fragment):

   0:   bti     c
   4:   stp     x29, x30, [sp, #-16]!
   8:   mov     x29, sp
   c:   stp     x19, x20, [sp, #-16]!
  10:   stp     x21, x22, [sp, #-16]!
  14:   stp     x25, x26, [sp, #-16]!
  18:   mov     x25, sp
  1c:   mov     x26, #0x0                       // #0
  20:   bti     j
  24:   sub     sp, sp, #0x90
  28:   add     x19, x0, #0x0
  2c:   mov     x0, #0x0                        // #0
  30:   mov     x10, #0xffffffffffffff78        // #-136
  34:   str     x0, [x25, x10]
  38:   mov     x10, #0xffffffffffffff80        // #-128
  3c:   str     x0, [x25, x10]
  40:   mov     x10, #0xffffffffffffff88        // #-120
  44:   str     x0, [x25, x10]
  48:   mov     x10, #0xffffffffffffff90        // #-112
  4c:   str     x0, [x25, x10]
  50:   mov     x10, #0xffffffffffffff98        // #-104
  54:   str     x0, [x25, x10]
  58:   mov     x10, #0xffffffffffffffa0        // #-96
  5c:   str     x0, [x25, x10]
  60:   mov     x10, #0xffffffffffffffa8        // #-88
  64:   str     x0, [x25, x10]
  68:   mov     x10, #0xffffffffffffffb0        // #-80
  6c:   str     x0, [x25, x10]
  70:   mov     x10, #0xffffffffffffffb8        // #-72
  74:   str     x0, [x25, x10]
  78:   mov     x10, #0xffffffffffffffc0        // #-64
  7c:   str     x0, [x25, x10]
  80:   mov     x10, #0xffffffffffffffc8        // #-56
  84:   str     x0, [x25, x10]
  88:   mov     x10, #0xffffffffffffffd0        // #-48
  8c:   str     x0, [x25, x10]
  90:   mov     x10, #0xffffffffffffffd8        // #-40
  94:   str     x0, [x25, x10]
  98:   mov     x10, #0xffffffffffffffe0        // #-32
  9c:   str     x0, [x25, x10]
  a0:   mov     x10, #0xffffffffffffffe8        // #-24
  a4:   str     x0, [x25, x10]
  a8:   mov     x10, #0xfffffffffffffff0        // #-16
  ac:   str     x0, [x25, x10]
  b0:   mov     x10, #0xfffffffffffffff8        // #-8
  b4:   str     x0, [x25, x10]
  b8:   mov     x10, #0x8                       // #8
  bc:   ldr     x2, [x19, x10]
  [...]

With this patch, jited code(fragment):

   0:   bti     c
   4:   stp     x29, x30, [sp, #-16]!
   8:   mov     x29, sp
   c:   stp     x19, x20, [sp, #-16]!
  10:   stp     x21, x22, [sp, #-16]!
  14:   stp     x25, x26, [sp, #-16]!
  18:   stp     x27, x28, [sp, #-16]!
  1c:   mov     x25, sp
  20:   sub     x27, x25, #0x88
  24:   mov     x26, #0x0                       // #0
  28:   bti     j
  2c:   sub     sp, sp, #0x90
  30:   add     x19, x0, #0x0
  34:   mov     x0, #0x0                        // #0
  38:   str     x0, [x27]
  3c:   str     x0, [x27, #8]
  40:   str     x0, [x27, #16]
  44:   str     x0, [x27, #24]
  48:   str     x0, [x27, #32]
  4c:   str     x0, [x27, #40]
  50:   str     x0, [x27, #48]
  54:   str     x0, [x27, #56]
  58:   str     x0, [x27, #64]
  5c:   str     x0, [x27, #72]
  60:   str     x0, [x27, #80]
  64:   str     x0, [x27, #88]
  68:   str     x0, [x27, #96]
  6c:   str     x0, [x27, #104]
  70:   str     x0, [x27, #112]
  74:   str     x0, [x27, #120]
  78:   str     x0, [x27, #128]
  7c:   ldr     x2, [x19, #8]
  [...]

Signed-off-by: Xu Kuohai <[email protected]>
  • Loading branch information
Xu Kuohai authored and Nobody committed Mar 16, 2022
1 parent 4cf70b0 commit 669cf41
Showing 1 changed file with 79 additions and 5 deletions.
84 changes: 79 additions & 5 deletions arch/arm64/net/bpf_jit_comp.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@
#define TMP_REG_2 (MAX_BPF_JIT_REG + 1)
#define TCALL_CNT (MAX_BPF_JIT_REG + 2)
#define TMP_REG_3 (MAX_BPF_JIT_REG + 3)
#define FP_BOTTOM (MAX_BPF_JIT_REG + 4)

#define check_imm(bits, imm) do { \
if ((((imm) > 0) && ((imm) >> (bits))) || \
Expand Down Expand Up @@ -63,6 +64,7 @@ static const int bpf2a64[] = {
[TCALL_CNT] = A64_R(26),
/* temporary register for blinding constants */
[BPF_REG_AX] = A64_R(9),
[FP_BOTTOM] = A64_R(27),
};

struct jit_ctx {
Expand All @@ -73,6 +75,7 @@ struct jit_ctx {
int exentry_idx;
__le32 *image;
u32 stack_size;
int fpb_offset;
};

static inline void emit(const u32 insn, struct jit_ctx *ctx)
Expand Down Expand Up @@ -234,9 +237,9 @@ static noinline bool is_lsi_offset(s16 offset, int scale)

/* Tail call offset to jump into */
#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
#define PROLOGUE_OFFSET 8
#define PROLOGUE_OFFSET 10
#else
#define PROLOGUE_OFFSET 7
#define PROLOGUE_OFFSET 9
#endif

static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
Expand All @@ -248,6 +251,7 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
const u8 r9 = bpf2a64[BPF_REG_9];
const u8 fp = bpf2a64[BPF_REG_FP];
const u8 tcc = bpf2a64[TCALL_CNT];
const u8 fpb = bpf2a64[FP_BOTTOM];
const int idx0 = ctx->idx;
int cur_offset;

Expand Down Expand Up @@ -286,9 +290,11 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
emit(A64_PUSH(r6, r7, A64_SP), ctx);
emit(A64_PUSH(r8, r9, A64_SP), ctx);
emit(A64_PUSH(fp, tcc, A64_SP), ctx);
emit(A64_PUSH(fpb, A64_R(28), A64_SP), ctx);

/* Set up BPF prog stack base register */
emit(A64_MOV(1, fp, A64_SP), ctx);
emit(A64_SUB_I(1, fpb, fp, ctx->fpb_offset), ctx);

if (!ebpf_from_cbpf) {
/* Initialize tail_call_cnt */
Expand Down Expand Up @@ -553,11 +559,13 @@ static void build_epilogue(struct jit_ctx *ctx)
const u8 r8 = bpf2a64[BPF_REG_8];
const u8 r9 = bpf2a64[BPF_REG_9];
const u8 fp = bpf2a64[BPF_REG_FP];
const u8 fpb = bpf2a64[FP_BOTTOM];

/* We're done with BPF stack */
emit(A64_ADD_I(1, A64_SP, A64_SP, ctx->stack_size), ctx);

/* Restore fs (x25) and x26 */
emit(A64_POP(fpb, A64_R(28), A64_SP), ctx);
emit(A64_POP(fp, A64_R(26), A64_SP), ctx);

/* Restore callee-saved register */
Expand Down Expand Up @@ -645,12 +653,14 @@ static int add_exception_handler(const struct bpf_insn *insn,
static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
bool extra_pass)
{
u8 dst = bpf2a64[insn->dst_reg];
u8 src = bpf2a64[insn->src_reg];
s16 off = insn->off;
const u8 fp = bpf2a64[BPF_REG_FP];
const u8 code = insn->code;
const u8 dst = bpf2a64[insn->dst_reg];
const u8 src = bpf2a64[insn->src_reg];
const u8 tmp = bpf2a64[TMP_REG_1];
const u8 tmp2 = bpf2a64[TMP_REG_2];
const s16 off = insn->off;
const u8 fpb = bpf2a64[FP_BOTTOM];
const s32 imm = insn->imm;
const int i = insn - ctx->prog->insnsi;
const bool is64 = BPF_CLASS(code) == BPF_ALU64 ||
Expand Down Expand Up @@ -1012,6 +1022,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
case BPF_LDX | BPF_PROBE_MEM | BPF_W:
case BPF_LDX | BPF_PROBE_MEM | BPF_H:
case BPF_LDX | BPF_PROBE_MEM | BPF_B:
if (ctx->fpb_offset > 0 && src == fp) {
src = fpb;
off += ctx->fpb_offset;
}
switch (BPF_SIZE(code)) {
case BPF_W:
if (is_lsi_offset(off, 2)) {
Expand Down Expand Up @@ -1070,6 +1084,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
case BPF_ST | BPF_MEM | BPF_H:
case BPF_ST | BPF_MEM | BPF_B:
case BPF_ST | BPF_MEM | BPF_DW:
if (ctx->fpb_offset > 0 && dst == fp) {
dst = fpb;
off += ctx->fpb_offset;
}
/* Load imm to a register then store it */
emit_a64_mov_i(1, tmp, imm, ctx);
switch (BPF_SIZE(code)) {
Expand Down Expand Up @@ -1113,6 +1131,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
case BPF_STX | BPF_MEM | BPF_H:
case BPF_STX | BPF_MEM | BPF_B:
case BPF_STX | BPF_MEM | BPF_DW:
if (ctx->fpb_offset > 0 && dst == fp) {
dst = fpb;
off += ctx->fpb_offset;
}
switch (BPF_SIZE(code)) {
case BPF_W:
if (is_lsi_offset(off, 2)) {
Expand Down Expand Up @@ -1167,6 +1189,56 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
return 0;
}

/*
* Return 0 if FP may change at runtime, otherwise find the minimum negative
* offset to FP and converts it to positive number.
*/
static int find_fpb_offset(struct bpf_prog *prog)
{
int i;
int offset = 0;

for (i = 0; i < prog->len; i++) {
const struct bpf_insn *insn = &prog->insnsi[i];

switch (BPF_CLASS(insn->code)) {
case BPF_STX:
case BPF_ST:
if (BPF_MODE(insn->code) == BPF_ATOMIC) {
if ((insn->imm == BPF_XCHG ||
insn->imm == (BPF_ADD | BPF_FETCH) ||
insn->imm == (BPF_AND | BPF_FETCH) ||
insn->imm == (BPF_OR | BPF_FETCH) ||
insn->imm == (BPF_XOR | BPF_FETCH)) &&
insn->src_reg == BPF_REG_FP) {
return 0;
}
}
if (BPF_MODE(insn->code) == BPF_MEM &&
insn->dst_reg == BPF_REG_FP) {
if (insn->off < offset)
offset = insn->off;
}
break;

case BPF_JMP32:
case BPF_JMP:
break;

case BPF_ALU:
case BPF_ALU64:
case BPF_LDX:
case BPF_LD:
default:
if (insn->dst_reg == BPF_REG_FP)
return 0;
}
}

/* safely be converted to a positive 'int', since insn->off is 's16' */
return -offset;
}

static int build_body(struct jit_ctx *ctx, bool extra_pass)
{
const struct bpf_prog *prog = ctx->prog;
Expand Down Expand Up @@ -1288,6 +1360,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
goto out_off;
}

ctx.fpb_offset = find_fpb_offset(prog);

/*
* 1. Initial fake pass to compute ctx->idx and ctx->offset.
*
Expand Down

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