bpf, arm64: adjust the offset of str/ldr(immediate) to positive number

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@huawei.com>

Author: Xu Kuohai

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.
 	 *