linux-kernel - Re: [PATCH] ftrace: fix function type mismatches

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Message-ID: <20191015090055.789a0aed@gandalf.local.home>
Date:   Tue, 15 Oct 2019 09:00:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: fix function type mismatches

On Mon, 14 Oct 2019 13:00:09 -0700
Sami Tolvanen <samitolvanen@...gle.com> wrote:

> On Fri, Oct 11, 2019 at 11:26 AM Steven Rostedt <rostedt@...dmis.org> wrote:
> > Unfortunately, this breaks function graph tracing.  
> 
> Thank you for pointing this out. I should have realized this solution
> wouldn't work.
> 
> > Is there a way to do an alias or something that can fix whatever you
> > are trying to fix?  
> 
> Unfortunately, an alias doesn't work in this case either, because the
> compiler still looks at the type of the target function. I'll find
> another way to solve the issue.
> 

Would this work for you?

-- Steve

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index dae64600ccbf..33b36b34721b 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -111,15 +111,22 @@
 
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 #ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
+/*
+ * Need to also make ftrace_graph_stub point to ftrace_stub
+ * so that the same stub location may have different protocols
+ * and not mess up with C verifiers.
+ */
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__patchable_function_entries))	\
-			__stop_mcount_loc = .;
+			__stop_mcount_loc = .;			\
+			ftrace_graph_stub = ftrace_stub;
 #else
 #define MCOUNT_REC()	. = ALIGN(8);				\
 			__start_mcount_loc = .;			\
 			KEEP(*(__mcount_loc))			\
-			__stop_mcount_loc = .;
+			__stop_mcount_loc = .;			\
+			ftrace_graph_stub = ftrace_stub;
 #endif
 #else
 #define MCOUNT_REC()
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 7950a0356042..fa3ce10d0405 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -332,9 +332,14 @@ int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace)
 	return 0;
 }
 
+/*
+ * Simply points to ftrace_stub, but with the proper protocol.
+ * Defined by the linker script in linux/vmlinux.lds.h
+ */
+extern void ftrace_graph_stub(struct ftrace_graph_ret *);
+
 /* The callbacks that hook a function */
-trace_func_graph_ret_t ftrace_graph_return =
-			(trace_func_graph_ret_t)ftrace_stub;
+trace_func_graph_ret_t ftrace_graph_return = ftrace_graph_stub;
 trace_func_graph_ent_t ftrace_graph_entry = ftrace_graph_entry_stub;
 static trace_func_graph_ent_t __ftrace_graph_entry = ftrace_graph_entry_stub;
 
@@ -614,7 +619,7 @@ void unregister_ftrace_graph(struct fgraph_ops *gops)
 		goto out;
 
 	ftrace_graph_active--;
-	ftrace_graph_return = (trace_func_graph_ret_t)ftrace_stub;
+	ftrace_graph_return = ftrace_graph_stub;
 	ftrace_graph_entry = ftrace_graph_entry_stub;
 	__ftrace_graph_entry = ftrace_graph_entry_stub;
 	ftrace_shutdown(&graph_ops, FTRACE_STOP_FUNC_RET);