arm64/bpf: use movn/movk/movk sequence to generate kernel addresses

Ard Biesheuvel · borkmann · commit cc2b8ed13695 · 2018-11-30T10:23:25.000+01:00
On arm64, all executable code is guaranteed to reside in the vmalloc
space (or the module space), and so jump targets will only use 48
bits at most, and the remaining bits are guaranteed to be 0x1.

This means we can generate an immediate jump address using a sequence
of one MOVN (move wide negated) and two MOVK instructions, where the
first one sets the lower 16 bits but also sets all top bits to 0x1.

Signed-off-by: Ard Biesheuvel &lt;ard.biesheuvel@linaro.org&gt;
Acked-by: Will Deacon &lt;will.deacon@arm.com&gt;
Acked-by: Daniel Borkmann &lt;daniel@iogearbox.net&gt;
Signed-off-by: Daniel Borkmann &lt;daniel@iogearbox.net&gt;
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
@@ -134,19 +134,18 @@ static inline void emit_a64_mov_i64(const int reg, const u64 val,
 }
 
 /*
- * This is an unoptimized 64 immediate emission used for BPF to BPF call
- * addresses. It will always do a full 64 bit decomposition as otherwise
- * more complexity in the last extra pass is required since we previously
- * reserved 4 instructions for the address.
+ * Kernel addresses in the vmalloc space use at most 48 bits, and the
+ * remaining bits are guaranteed to be 0x1. So we can compose the address
+ * with a fixed length movn/movk/movk sequence.
  */
 static inline void emit_addr_mov_i64(const int reg, const u64 val,
 				     struct jit_ctx *ctx)
 {
 	u64 tmp = val;
 	int shift = 0;
 
-	emit(A64_MOVZ(1, reg, tmp & 0xffff, shift), ctx);
-	for (;shift < 48;) {
+	emit(A64_MOVN(1, reg, ~tmp & 0xffff, shift), ctx);
+	while (shift < 32) {
 		tmp >>= 16;
 		shift += 16;
 		emit(A64_MOVK(1, reg, tmp & 0xffff, shift), ctx);
@@ -634,11 +633,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 					    &func_addr, &func_addr_fixed);
 		if (ret < 0)
 			return ret;
-		if (func_addr_fixed)
-			/* We can use optimized emission here. */
-			emit_a64_mov_i64(tmp, func_addr, ctx);
-		else
-			emit_addr_mov_i64(tmp, func_addr, ctx);
+		emit_addr_mov_i64(tmp, func_addr, ctx);
 		emit(A64_BLR(tmp), ctx);
 		emit(A64_MOV(1, r0, A64_R(0)), ctx);
 		break;
