Re: [PATCH] ftrace: fix function type mismatches

[Steven Rostedt]

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

> On Fri, Oct 11, 2019 at 11:26 AM Steven Rostedt <rost...@goodmis.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);