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>

2 files changed, 11 insertions, 1 deletions

diff --git a/arch/x86/include/asm/livepatch.h b/arch/x86/include/asm/livepatch.h
index b5608d7757fd..26e58134c8cb 100644
--- a/arch/x86/include/asm/livepatch.h
+++ b/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);
 
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 6f6387912da7..ce42d3b930dc 100644
--- a/kernel/livepatch/core.c
+++ b/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;