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 <xukuohai@xxxxxxxxxx>
---
arch/arm64/net/bpf_jit_comp.c | 84 ++++++++++++++++++++++++++++++++---
1 file changed, 79 insertions(+), 5 deletions(-)
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index e56274ce001a..9e1fd7a1e08f 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -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))) || \
@@ -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 {
@@ -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)
@@ -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)
@@ -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;
@@ -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 */
@@ -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 */
@@ -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 ||
@@ -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)) {
@@ -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)) {
@@ -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)) {
@@ -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;
@@ -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.
*
--
2.30.2
