[PATCH V2] mips: kernel: fix detect_memory_region() function

[Shiji Yang <yangshiji66@outlook.com>]

The detect_memory_region() has been broken on 6.6 kernel[1]. This
patch fixes it by:
1. Do not use memcmp() on unallocated memory, as the new introduced
   fortify dynamic object size check[2] will return 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.
4. Enable this feature only for 32-bit CPUs. Currently, only ath79 and
   ralink CPUs are using it. And 64-bit CPU doesn't have the KSEG1ADDR
   definition.

[1] https://github.com/openwrt/openwrt/pull/14774#issuecomment-1989356746
[2] commit 439a1bcac648 ("fortify: Use __builtin_dynamic_object_size() when available")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Tested-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
---
 arch/mips/include/asm/bootinfo.h |  2 ++
 arch/mips/kernel/setup.c         | 17 ++++++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/mips/include/asm/bootinfo.h b/arch/mips/include/asm/bootinfo.h
index 2128ba903391..8516c11916a4 100644
--- a/arch/mips/include/asm/bootinfo.h
+++ b/arch/mips/include/asm/bootinfo.h
@@ -93,7 +93,9 @@ const char *get_system_type(void);
 
 extern unsigned long mips_machtype;
 
+#ifndef CONFIG_64BIT
 extern void detect_memory_region(phys_addr_t start, phys_addr_t sz_min,  phys_addr_t sz_max);
+#endif
 
 extern void prom_init(void);
 extern void prom_free_prom_memory(void);
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index 12a1a4ffb602..3a3bc1b7e62e 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -86,21 +86,27 @@ 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;
-
 #ifdef CONFIG_MIPS_AUTO_PFN_OFFSET
 unsigned long ARCH_PFN_OFFSET;
 EXPORT_SYMBOL(ARCH_PFN_OFFSET);
 #endif
 
+#ifndef CONFIG_64BIT
+static u32 detect_magic __initdata;
+#define MIPS_MEM_TEST_PATTERN		0xaa5555aa
+
 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",
@@ -111,6 +117,7 @@ void __init detect_memory_region(phys_addr_t start, phys_addr_t sz_min, phys_add
 
 	memblock_add(start, size);
 }
+#endif /* CONFIG_64BIT */
 
 /*
  * Manage initrd
-- 
2.45.1