ARM: net: bpf: fix LDX instructions

When the source and destination register are identical, our JIT does not
generate correct code, which leads to kernel oopses.

Fix this by (a) generating more efficient code, and (b) making use of
the temporary earlier if we will overwrite the address register.

Fixes: 39c13c20 ("arm: eBPF JIT compiler")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/net/bpf_jit_32.c

@@ -913,33 +913,53 @@ static inline void emit_str_r(const u8 dst, const u8 src, bool dstk,
 }
 
 /* dst = *(size*)(src + off) */
-static inline void emit_ldx_r(const u8 dst, const u8 src, bool dstk,
-			      const s32 off, struct jit_ctx *ctx, const u8 sz){
+static inline void emit_ldx_r(const u8 dst[], const u8 src, bool dstk,
+			      s32 off, struct jit_ctx *ctx, const u8 sz){
 	const u8 *tmp = bpf2a32[TMP_REG_1];
-	u8 rd = dstk ? tmp[1] : dst;
+	const u8 *rd = dstk ? tmp : dst;
 	u8 rm = src;
+	s32 off_max;
 
-	if (off) {
+	if (sz == BPF_H)
+		off_max = 0xff;
+	else
+		off_max = 0xfff;
+
+	if (off < 0 || off > off_max) {
 		emit_a32_mov_i(tmp[0], off, false, ctx);
 		emit(ARM_ADD_R(tmp[0], tmp[0], src), ctx);
 		rm = tmp[0];
+		off = 0;
+	} else if (rd[1] == rm) {
+		emit(ARM_MOV_R(tmp[0], rm), ctx);
+		rm = tmp[0];
 	}
 	switch (sz) {
-	case BPF_W:
-		/* Load a Word */
-		emit(ARM_LDR_I(rd, rm, 0), ctx);
+	case BPF_B:
+		/* Load a Byte */
+		emit(ARM_LDRB_I(rd[1], rm, off), ctx);
+		emit_a32_mov_i(dst[0], 0, dstk, ctx);
 		break;
 	case BPF_H:
 		/* Load a HalfWord */
-		emit(ARM_LDRH_I(rd, rm, 0), ctx);
+		emit(ARM_LDRH_I(rd[1], rm, off), ctx);
+		emit_a32_mov_i(dst[0], 0, dstk, ctx);
 		break;
-	case BPF_B:
-		/* Load a Byte */
-		emit(ARM_LDRB_I(rd, rm, 0), ctx);
+	case BPF_W:
+		/* Load a Word */
+		emit(ARM_LDR_I(rd[1], rm, off), ctx);
+		emit_a32_mov_i(dst[0], 0, dstk, ctx);
+		break;
+	case BPF_DW:
+		/* Load a Double Word */
+		emit(ARM_LDR_I(rd[1], rm, off), ctx);
+		emit(ARM_LDR_I(rd[0], rm, off + 4), ctx);
 		break;
 	}
 	if (dstk)
-		emit(ARM_STR_I(rd, ARM_SP, STACK_VAR(dst)), ctx);
+		emit(ARM_STR_I(rd[1], ARM_SP, STACK_VAR(dst[1])), ctx);
+	if (dstk && sz == BPF_DW)
+		emit(ARM_STR_I(rd[0], ARM_SP, STACK_VAR(dst[0])), ctx);
 }
 
 /* Arithmatic Operation */
@@ -1440,22 +1460,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		rn = sstk ? tmp2[1] : src_lo;
 		if (sstk)
 			emit(ARM_LDR_I(rn, ARM_SP, STACK_VAR(src_lo)), ctx);
-		switch (BPF_SIZE(code)) {
-		case BPF_W:
-			/* Load a Word */
-		case BPF_H:
-			/* Load a Half-Word */
-		case BPF_B:
-			/* Load a Byte */
-			emit_ldx_r(dst_lo, rn, dstk, off, ctx, BPF_SIZE(code));
-			emit_a32_mov_i(dst_hi, 0, dstk, ctx);
-			break;
-		case BPF_DW:
-			/* Load a double word */
-			emit_ldx_r(dst_lo, rn, dstk, off, ctx, BPF_W);
-			emit_ldx_r(dst_hi, rn, dstk, off+4, ctx, BPF_W);
-			break;
-		}
+		emit_ldx_r(dst, rn, dstk, off, ctx, BPF_SIZE(code));
 		break;
 	/* R0 = ntohx(*(size *)(((struct sk_buff *)R6)->data + imm)) */
 	case BPF_LD | BPF_ABS | BPF_W: