livepatch: handle ancient compilers with more grace

We are aborting a build in case when gcc doesn't support fentry on x86_64
(regs->ip modification can't really reliably work with mcount).

This however breaks allmodconfig for people with older gccs that don't
support -mfentry.

Turn the build-time failure into runtime failure, resulting in the whole
infrastructure not being initialized if CC_USING_FENTRY is unset.
Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>

arch/x86/include/asm/livepatch.h

@@ -25,9 +25,13 @@
 #include <linux/ftrace.h>
 
 #ifdef CONFIG_LIVE_PATCHING
+static inline int klp_check_compiler_support(void)
+{
 #ifndef CC_USING_FENTRY
-#error Your compiler must support -mfentry for live patching to work
+	return 1;
 #endif
+	return 0;
+}
 extern int klp_write_module_reloc(struct module *mod, unsigned long type,
 				  unsigned long loc, unsigned long value);
 

kernel/livepatch/core.c

@@ -911,6 +911,12 @@ static int klp_init(void)
 {
 	int ret;
 
+	ret = klp_check_compiler_support();
+	if (ret) {
+		pr_info("Your compiler is too old; turning off.\n");
+		return -EINVAL;
+	}
+
 	ret = register_module_notifier(&klp_module_nb);
 	if (ret)
 		return ret;