generic: fix detect_memory_region() function

This patch fixes the broken detect_memory_region() function on
6.6 kernel.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>

target/linux/generic/pending-6.6/350-mips-kernel-fix-detect_memory_region-function.patch

@@ -0,0 +1,56 @@
+From: Shiji Yang <yangshiji66@outlook.com>
+Date: Wed, 13 Mar 2024 20:28:37 +0800
+Subject: [PATCH] mips: kernel: fix detect_memory_region() function
+
+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@outlook.com>
+---
+ arch/mips/kernel/setup.c | 16 +++++++++++-----
+ 1 file changed, 11 insertions(+), 5 deletions(-)
+
+--- 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 */
+@@ -90,7 +92,7 @@ static struct resource bss_resource = {
+ 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;
+@@ -99,12 +101,16 @@ 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",