x86: Sanity-check static_cpu_has usage

static_cpu_has may be used only after alternatives have run. Before that
it always returns false if constant folding with __builtin_constant_p()
doesn't happen. And you don't want that.

This patch is the result of me debugging an issue where I overzealously
put static_cpu_has in code which executed before alternatives have run
and had to spend some time with scratching head and cursing at the
monitor.

So add a jump to a warning which screams loudly when we use this
function too early. The alternatives patch that check away in
conjunction with patching the rest of the kernel image.

[ hpa: factored this into its own configuration option.  If we want to
  have an overarching option, it should be an option which selects
  other options, not as a group option in the source code. ]
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1370772454-6106-4-git-send-email-bp@alien8.deSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/Kconfig.debug

@@ -304,4 +304,14 @@ config DEBUG_NMI_SELFTEST
 
 	  If unsure, say N.
 
+config X86_DEBUG_STATIC_CPU_HAS
+	bool "Debug alternatives"
+	depends on DEBUG_KERNEL
+	---help---
+	  This option causes additional code to be generated which
+	  fails if static_cpu_has() is used before alternatives have
+	  run.
+
+	  If unsure, say N.
+
 endmenu

arch/x86/include/asm/cpufeature.h

@@ -356,15 +356,35 @@ extern const char * const x86_power_flags[32];
 #endif /* CONFIG_X86_64 */
 
 #if __GNUC__ >= 4
+extern void warn_pre_alternatives(void);
+
 /*
  * Static testing of CPU features.  Used the same as boot_cpu_has().
  * These are only valid after alternatives have run, but will statically
  * patch the target code for additional performance.
- *
  */
 static __always_inline __pure bool __static_cpu_has(u16 bit)
 {
 #if __GNUC__ > 4 || __GNUC_MINOR__ >= 5
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+		/*
+		 * Catch too early usage of this before alternatives
+		 * have run.
+		 */
+		asm goto("1: jmp %l[t_warn]\n"
+			 "2:\n"
+			 ".section .altinstructions,\"a\"\n"
+			 " .long 1b - .\n"
+			 " .long 0\n"		/* no replacement */
+			 " .word %P0\n"		/* 1: do replace */
+			 " .byte 2b - 1b\n"	/* source len */
+			 " .byte 0\n"		/* replacement len */
+			 ".previous\n"
+			 /* skipping size check since replacement size = 0 */
+			 : : "i" (X86_FEATURE_ALWAYS) : : t_warn);
+#endif
+
 		asm goto("1: jmp %l[t_no]\n"
 			 "2:\n"
 			 ".section .altinstructions,\"a\"\n"
@@ -379,7 +399,13 @@ static __always_inline __pure bool __static_cpu_has(u16 bit)
 		return true;
 	t_no:
 		return false;
-#else
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+	t_warn:
+		warn_pre_alternatives();
+		return false;
+#endif
+#else /* GCC_VERSION >= 40500 */
 		u8 flag;
 		/* Open-coded due to __stringify() in ALTERNATIVE() */
 		asm volatile("1: movb $0,%0\n"

arch/x86/kernel/cpu/common.c

@@ -1364,3 +1364,11 @@ void __cpuinit cpu_init(void)
 	fpu_init();
 }
 #endif
+
+#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS
+void warn_pre_alternatives(void)
+{
+	WARN(1, "You're using static_cpu_has before alternatives have run!\n");
+}
+EXPORT_SYMBOL_GPL(warn_pre_alternatives);
+#endif