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fgraph: Fix function type mismatches of ftrace_graph_return using ftr…
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…ace_stub

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>
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Steven Rostedt (VMware) committed Nov 14, 2019
1 parent 953ae45 commit b83b43f
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Showing 3 changed files with 27 additions and 6 deletions.
5 changes: 5 additions & 0 deletions arch/sh/boot/compressed/misc.c
Original file line number Diff line number Diff line change
Expand Up @@ -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
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17 changes: 14 additions & 3 deletions include/asm-generic/vmlinux.lds.h
Original file line number Diff line number Diff line change
Expand Up @@ -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
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11 changes: 8 additions & 3 deletions kernel/trace/fgraph.c
Original file line number Diff line number Diff line change
Expand Up @@ -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;

Expand Down Expand Up @@ -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);
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