[PATCH] mips: setup: fix detect_memory_region() function

[Shiji Yang]

1. Do not use memcmp() on unallocated memory, as the new introduced
   fortify dynamic object size check[1] will report unexpected result.
2. Use a fixed pattern instead of a random function pointer as the
   magic value.
3. Flip magic value and double check it.

[1] 439a1bcac648 ("fortify: Use __builtin_dynamic_object_size() when available")
Signed-off-by: Shiji Yang <yangshiji66@xxxxxxxxxxx>
---
 arch/mips/kernel/setup.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index 9c30de151..354458fa9 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -46,6 +46,8 @@
 #include <asm/prom.h>
 #include <asm/fw/fw.h>
 
+#define MIPS_MEM_TEST_PATTERN		0xaa5555aa
+
 #ifdef CONFIG_MIPS_ELF_APPENDED_DTB
 char __section(".appended_dtb") __appended_dtb[0x100000];
 #endif /* CONFIG_MIPS_ELF_APPENDED_DTB */
@@ -86,7 +88,7 @@ static struct resource bss_resource = { .name = "Kernel bss", };
 unsigned long __kaslr_offset __ro_after_init;
 EXPORT_SYMBOL(__kaslr_offset);
 
-static void *detect_magic __initdata = detect_memory_region;
+static u32 detect_magic __initdata;
 
 #ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
 unsigned long ARCH_PFN_OFFSET;
@@ -95,12 +97,16 @@ EXPORT_SYMBOL(ARCH_PFN_OFFSET);
 
 void __init detect_memory_region(phys_addr_t start, phys_addr_t sz_min, phys_addr_t sz_max)
 {
-	void *dm = &detect_magic;
+	void *dm = (void *)KSEG1ADDR(&detect_magic);
 	phys_addr_t size;
 
 	for (size = sz_min; size < sz_max; size <<= 1) {
-		if (!memcmp(dm, dm + size, sizeof(detect_magic)))
-			break;
+		__raw_writel(MIPS_MEM_TEST_PATTERN, dm);
+		if (__raw_readl(dm) == __raw_readl(dm + size)) {
+			__raw_writel(~MIPS_MEM_TEST_PATTERN, dm);
+			if (__raw_readl(dm) == __raw_readl(dm + size))
+				break;
+		}
 	}
 
 	pr_debug("Memory: %lluMB of RAM detected at 0x%llx (min: %lluMB, max: %lluMB)\n",
-- 
2.44.0