On Mon, Jan 27, 2020 at 06:11:45PM -0800, Palmer Dabbelt wrote:

 
+	/* Handle BPF_REG_0, which may be in the wrong place because the ARM64
+	 * ABI doesn't match the BPF ABI for function calls. */
+	if (ctx->reg0_in_reg1) {
+		/* If we're writing BPF_REG_0 then we don't need to do any
+		 * extra work to get the registers back in their correct
+		 * locations. */
+		if (insn->dst_reg == BPF_REG_0)
+			ctx->reg0_in_reg1 = false;
+
+		/* If we're writing to BPF_REG_1 then we need to save BPF_REG_0
+		 * into the correct location if it's still alive, as otherwise
+		 * it will be clobbered. */
+		if (insn->dst_reg == BPF_REG_1) {
+			if (!dead_register(ctx, off + 1, BPF_REG_0))
+				emit(A64_MOV(1, A64_R(7), A64_R(0)), ctx);
+			ctx->reg0_in_reg1 = false;
+		}
+	}

I'm not sure this is correct, since it processes insns as a linear code, but
there could be jumps in the middle. The logic should be following the control
flow of the program. The verifier is a better place to do such analysis.
I don't see how JITs can do it on their own.

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