[PATCH 7/7] x86/boot/32: Rewrite test_wp_bit()

[Andy Lutomirski]

This code seems to be very old and has gotten only minor updates.
Nowadays we have a shiny function probe_kernel_write() that does
more or less exactly what we need.  Use it.

While we're at it, remove cpuinfo_x86::wp_works_ok, since we panic
on kernels where wp doesn't work okay.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/processor.h |  2 --
 arch/x86/kernel/cpu/proc.c       |  5 ++---
 arch/x86/kernel/setup.c          |  2 --
 arch/x86/mm/init_32.c            | 44 ++++++++--------------------------------
 arch/x86/xen/enlighten.c         |  1 -
 5 files changed, 11 insertions(+), 43 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index edf42c4ac8c8..2c7695c9c684 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -90,8 +90,6 @@ struct cpuinfo_x86 {
 	__u8			x86_model;
 	__u8			x86_mask;
 #ifdef CONFIG_X86_32
-	char			wp_works_ok;	/* It doesn't on 386's */
-
 	/* Problems on some 486Dx4's and old 386's: */
 	char			rfu;
 	char			pad0;
diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
index 18ca99f2798b..6df621ae62a7 100644
--- a/arch/x86/kernel/cpu/proc.c
+++ b/arch/x86/kernel/cpu/proc.c
@@ -31,14 +31,13 @@ static void show_cpuinfo_misc(struct seq_file *m, struct cpuinfo_x86 *c)
 		   "fpu\t\t: %s\n"
 		   "fpu_exception\t: %s\n"
 		   "cpuid level\t: %d\n"
-		   "wp\t\t: %s\n",
+		   "wp\t\t: yes\n",
 		   static_cpu_has_bug(X86_BUG_FDIV) ? "yes" : "no",
 		   static_cpu_has_bug(X86_BUG_F00F) ? "yes" : "no",
 		   static_cpu_has_bug(X86_BUG_COMA) ? "yes" : "no",
 		   static_cpu_has(X86_FEATURE_FPU) ? "yes" : "no",
 		   static_cpu_has(X86_FEATURE_FPU) ? "yes" : "no",
-		   c->cpuid_level,
-		   c->wp_works_ok ? "yes" : "no");
+		   c->cpuid_level);
 }
 #else
 static void show_cpuinfo_misc(struct seq_file *m, struct cpuinfo_x86 *c)
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 56b1177155db..462b7c69443d 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -175,11 +175,9 @@ static struct resource bss_resource = {
 #ifdef CONFIG_X86_32
 /* cpu data as detected by the assembly code in head.S */
 struct cpuinfo_x86 new_cpu_data = {
-	.wp_works_ok = -1,
 };
 /* common cpu data for all cpus */
 struct cpuinfo_x86 boot_cpu_data __read_mostly = {
-	.wp_works_ok = -1,
 };
 EXPORT_SYMBOL(boot_cpu_data);
 
diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
index 5ed3c141bbd5..097089a5e4d5 100644
--- a/arch/x86/mm/init_32.c
+++ b/arch/x86/mm/init_32.c
@@ -56,8 +56,6 @@
 
 unsigned long highstart_pfn, highend_pfn;
 
-static noinline int do_test_wp_bit(void);
-
 bool __read_mostly __vmalloc_start_set = false;
 
 /*
@@ -726,20 +724,21 @@ void __init paging_init(void)
  */
 static void __init test_wp_bit(void)
 {
+	char z = 0;
+
 	printk(KERN_INFO
   "Checking if this processor honours the WP bit even in supervisor mode...");
 
-	/* Any page-aligned address will do, the test is non-destructive */
-	__set_fixmap(FIX_WP_TEST, __pa(&swapper_pg_dir), PAGE_KERNEL_RO);
-	boot_cpu_data.wp_works_ok = do_test_wp_bit();
-	clear_fixmap(FIX_WP_TEST);
+	__set_fixmap(FIX_WP_TEST, __pa_symbol(empty_zero_page), PAGE_KERNEL_RO);
 
-	if (!boot_cpu_data.wp_works_ok) {
+	if (probe_kernel_write((char *)fix_to_virt(FIX_WP_TEST), &z, 1) == 0) {
 		printk(KERN_CONT "No.\n");
 		panic("Linux doesn't support CPUs with broken WP.");
-	} else {
-		printk(KERN_CONT "Ok.\n");
 	}
+
+	clear_fixmap(FIX_WP_TEST);
+
+	printk(KERN_CONT "Ok.\n");
 }
 
 void __init mem_init(void)
@@ -821,8 +820,7 @@ void __init mem_init(void)
 	BUG_ON(VMALLOC_START				>= VMALLOC_END);
 	BUG_ON((unsigned long)high_memory		> VMALLOC_START);
 
-	if (boot_cpu_data.wp_works_ok < 0)
-		test_wp_bit();
+	test_wp_bit();
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
@@ -850,30 +848,6 @@ int arch_remove_memory(u64 start, u64 size)
 #endif
 #endif
 
-/*
- * This function cannot be __init, since exceptions don't work in that
- * section.  Put this after the callers, so that it cannot be inlined.
- */
-static noinline int do_test_wp_bit(void)
-{
-	char tmp_reg;
-	int flag;
-
-	__asm__ __volatile__(
-		"	movb %0, %1	\n"
-		"1:	movb %1, %0	\n"
-		"	xorl %2, %2	\n"
-		"2:			\n"
-		_ASM_EXTABLE(1b,2b)
-		:"=m" (*(char *)fix_to_virt(FIX_WP_TEST)),
-		 "=q" (tmp_reg),
-		 "=r" (flag)
-		:"2" (1)
-		:"memory");
-
-	return flag;
-}
-
 int kernel_set_to_readonly __read_mostly;
 
 void set_kernel_text_rw(void)
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 4951fcf95143..6efa0cc425a2 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -1595,7 +1595,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	/* set up basic CPUID stuff */
 	cpu_detect(&new_cpu_data);
 	set_cpu_cap(&new_cpu_data, X86_FEATURE_FPU);
-	new_cpu_data.wp_works_ok = 1;
 	new_cpu_data.x86_capability[CPUID_1_EDX] = cpuid_edx(1);
 #endif
 
-- 
2.9.3