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Subject[for-next][PATCH 23/33] fgraph: Fix function type mismatches of ftrace_graph_return using ftrace_stub
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

The C compiler is allowing more checks to make sure that function pointers
are assigned to the correct prototype function. Unfortunately, the function
graph tracer uses a special name with its assigned ftrace_graph_return
function pointer that maps to a stub function used by the function tracer
(ftrace_stub). The ftrace_graph_return variable is compared to the
ftrace_stub in some archs to know if the function graph tracer is enabled or
not. This means we can not just simply create a new function stub that
compares it without modifying all the archs.

Instead, have the linker script create a function_graph_stub that maps to
ftrace_stub, and this way we can define the prototype for it to match the
prototype of ftrace_graph_return, and make the compiler checks all happy!

Link: http://lkml.kernel.org/r/20191015090055.789a0aed@gandalf.local.home

Cc: linux-sh@vger.kernel.org
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
arch/sh/boot/compressed/misc.c | 5 +++++
include/asm-generic/vmlinux.lds.h | 17 ++++++++++++++---
kernel/trace/fgraph.c | 11 ++++++++---
3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/arch/sh/boot/compressed/misc.c b/arch/sh/boot/compressed/misc.c
index c15cac9251b9..e69ec12cbbe6 100644
--- a/arch/sh/boot/compressed/misc.c
+++ b/arch/sh/boot/compressed/misc.c
@@ -111,6 +111,11 @@ void __stack_chk_fail(void)
error("stack-protector: Kernel stack is corrupted\n");
}

+/* Needed because vmlinux.lds.h references this */
+void ftrace_stub(void)
+{
+}
+
#ifdef CONFIG_SUPERH64
#define stackalign 8
#else
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index dae64600ccbf..0f358be551cd 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -111,18 +111,29 @@

#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()
+# ifdef CONFIG_FUNCTION_TRACER
+# define MCOUNT_REC() ftrace_graph_stub = ftrace_stub;
+# else
+# define MCOUNT_REC()
+# endif
#endif

#ifdef CONFIG_TRACE_BRANCH_PROFILING
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);
--
2.23.0

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