On Wed, 14 Jan 2015, Masami Hiramatsu wrote:

quoted

diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index f45acad..29fa417 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h

@@ -4,8 +4,10 @@
 #ifdef CONFIG_FUNCTION_TRACER
 #ifdef CC_USING_FENTRY
 # define MCOUNT_ADDR		((long)(__fentry__))
+# define arch_ftrace_ipmodify_compiler_support(void) ({ 1; })
 #else
 # define MCOUNT_ADDR		((long)(mcount))
+#define arch_ftrace_ipmodify_compiler_support(void) ({ 0; })

Hmm, can we just define ARCH_FTRACE_SUPPORT_IPMODIFY here?

Yup, that's more or less what Steven suggested as well. I'll send v2.

quoted

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 929a733..11370fd 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c

@@ -1809,6 +1809,11 @@ static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
 	if (!(ops->flags & FTRACE_OPS_FL_IPMODIFY))
 		return 0;
 
+	if (!arch_ftrace_ipmodify_compiler_support()) {
+		WARN(1, "Your compiler doesn't support features necessary for IPMODIFY");
+		return 0;
+	}

Actually, if ftrace doesn't support IPMODIFY, I would like to just drop
the entire code for CONFIG_KPROBES_ON_FTRACE(this is a hidden config),
instead of checking at runtime.

So, there are 2 ifdefs of code in kernel/kprobes.c for 
CONFIG_KPROBES_ON_FTRACE, those should also check 
ARCH_FTRACE_SUPPORT_IPMODIFY too.

Makes sense. Will send that as a followup patch once the way how to do 
this in ftrace is settled.

Thanks,

-- 
Jiri Kosina
SUSE Labs