[PATCH 3/3] arm64: enforce x1|x2|x3 == 0 upon kernel entry as per boot protocol

[ard.biesheuvel@linaro.org (Ard Biesheuvel)]

According to the arm64 boot protocol, registers x1 to x3 should be
zero upon kernel entry, and non-zero values are reserved for future
use. This future use is going to be problematic if we never enforce
the current rules, so start enforcing them now, by emitting a warning
if non-zero values are detected.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 arch/arm64/kernel/head.S  |  8 ++++++++
 arch/arm64/kernel/setup.c | 13 +++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 1651c0fd50e6..fe5354eae069 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -231,6 +231,10 @@ section_table:
 #endif
 
 ENTRY(stext)
+	adr	x8, boot_regs			// record the contents of
+	stp	x0, x1, [x8]			// x0 .. x3 at kernel entry
+	stp	x2, x3, [x8, #16]
+
 	mov	x21, x0				// x21=FDT
 	bl	el2_setup			// Drop to EL1, w20=cpu_boot_mode
 	adrp	x24, KERNEL_START - TEXT_OFFSET	// x24=PHYS_OFFSET
@@ -251,6 +255,10 @@ ENTRY(stext)
 	b	__cpu_setup			// initialise processor
 ENDPROC(stext)
 
+	.align	3
+ENTRY(boot_regs)
+	.skip	4 * 8				// x0 .. x3
+
 /*
  * Determine validity of the x21 FDT pointer.
  * The dtb must be 8-byte aligned and live in the first 512M of memory.
diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
index 6c5fb5aff325..2b81d0a907ce 100644
--- a/arch/arm64/kernel/setup.c
+++ b/arch/arm64/kernel/setup.c
@@ -532,3 +532,16 @@ const struct seq_operations cpuinfo_op = {
 	.stop	= c_stop,
 	.show	= c_show
 };
+
+static int verify_boot_protocol(void)
+{
+	extern u64 boot_regs[];
+
+	if (boot_regs[1] || boot_regs[2] || boot_regs[3]) {
+		pr_err("WARNING: boot protocol violation detected (x1 == %llx, x2 == %llx, x3 == %llx)\n",
+			boot_regs[1], boot_regs[2], boot_regs[3]);
+		pr_err("WARNING: your bootloader may fail to load newer kernels\n");
+	}
+	return 0;
+}
+late_initcall(verify_boot_protocol);
-- 
1.8.3.2