[RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool

[Peter Zijlstra]

On Fri, Jun 26, 2020 at 01:29:31PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 25, 2020 at 03:40:42PM -0700, Sami Tolvanen wrote:

> > Anyway, since objtool is run before recordmcount, I just left this unchanged
> > for testing and ignored the recordmcount warnings about __mcount_loc already
> > existing. Something is a bit off still though, I see this at boot:
> > 
> >   ------------[ ftrace bug ]------------
> >   ftrace failed to modify
> >   [<ffffffff81000660>] __tracepoint_iter_initcall_level+0x0/0x40
> >    actual:   0f:1f:44:00:00
> >   Initializing ftrace call sites
> >   ftrace record flags: 0
> >    (0)
> >    expected tramp: ffffffff81056500
> >   ------------[ cut here ]------------
> > 
> > Otherwise, this looks pretty good.
> 
> Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
> finding the CALL -- because objtool already made it a NOP...
> 
> Weird, I thought recordmcount would also write NOPs, it certainly has
> code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
> but I'd rather Steve explain this before I wreck things further.

Something like so would ignore whatever text is there and rewrite it
with ideal_nop.

---
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index c84d28e90a58..98a6a93d7615 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -109,9 +109,11 @@ static int __ref
 ftrace_modify_code_direct(unsigned long ip, const char *old_code,
 			  const char *new_code)
 {
-	int ret = ftrace_verify_code(ip, old_code);
-	if (ret)
-		return ret;
+	if (old_code) {
+		int ret = ftrace_verify_code(ip, old_code);
+		if (ret)
+			return ret;
+	}
 
 	/* replace the text with the new text */
 	if (ftrace_poke_late)
@@ -124,9 +126,8 @@ ftrace_modify_code_direct(unsigned long ip, const char *old_code,
 int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long addr)
 {
 	unsigned long ip = rec->ip;
-	const char *new, *old;
+	const char *new;
 
-	old = ftrace_call_replace(ip, addr);
 	new = ftrace_nop_replace();
 
 	/*
@@ -138,7 +139,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long ad
 	 * just modify the code directly.
 	 */
 	if (addr == MCOUNT_ADDR)
-		return ftrace_modify_code_direct(ip, old, new);
+		return ftrace_modify_code_direct(ip, NULL, new);
 
 	/*
 	 * x86 overrides ftrace_replace_code -- this function will never be used