x86/boot/32: Rewrite test_wp_bit()

This code seems to be very old and has gotten only minor updates.
It's overcomplicated and has a bunch of comments that are, at best,
of purely historical interest.  Nowadays we have a shiny function
probe_kernel_write() that does more or less exactly what we need.
Use it.

I switched the page that we test from swapper_pg_dir to
empty_zero_page because writing zero to empty_zero_page is more
obviously safe than writing to the paging structures.  (It's
extremely unlikely that any of this would cause problems in practice
because the write will fail on any supported CPU.)
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/0b9e64ab0236de30e7572213cea77bf95ae2e990.1490831211.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

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;
 
 /*
@@ -726,22 +724,21 @@ void __init paging_init(void)
  */
 static void __init test_wp_bit(void)
 {
-	int wp_works_ok;
+	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);
-	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 (!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)
@@ -851,30 +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)