ftrace: selftest: remove broken trace_direct_tramp

The ftrace selftest code has a trace_direct_tramp() function which it
uses as a direct call trampoline. This happens to work on x86, since the
direct call's return address is in the usual place, and can be returned
to via a RET, but in general the calling convention for direct calls is
different from regular function calls, and requires a trampoline written
in assembly.

On s390, regular function calls place the return address in %r14, and an
ftrace patch-site in an instrumented function places the trampoline's
return address (which is within the instrumented function) in %r0,
preserving the original %r14 value in-place. As a regular C function
will return to the address in %r14, using a C function as the trampoline
results in the trampoline returning to the caller of the instrumented
function, skipping the body of the instrumented function.

Note that the s390 issue is not detcted by the ftrace selftest code, as
the instrumented function is trivial, and returning back into the caller
happens to be equivalent.

On arm64, regular function calls place the return address in x30, and
an ftrace patch-site in an instrumented function saves this into r9
and places the trampoline's return address (within the instrumented
function) in x30. A regular C function will return to the address in
x30, but will not restore x9 into x30. Consequently, using a C function
as the trampoline results in returning to the trampoline's return
address having corrupted x30, such that when the instrumented function
returns, it will return back into itself.

To avoid future issues in this area, remove the trace_direct_tramp()
function, and require that each architecture with direct calls provides
a stub trampoline, named ftrace_stub_direct_tramp. This can be written
to handle the architecture's trampoline calling convention, and in
future could be used elsewhere (e.g. in the ftrace ops sample, to
measure the overhead of direct calls), so we may as well always build it
in.

Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Mark Rutland <[email protected]>
Cc: Li Huafei <[email protected]>
Cc: Xu Kuohai <[email protected]>
Signed-off-by: Florent Revest <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

arch/s390/kernel/mcount.S

@@ -32,6 +32,11 @@ ENTRY(ftrace_stub)
 	BR_EX	%r14
 ENDPROC(ftrace_stub)
 
+SYM_CODE_START(ftrace_stub_direct_tramp)
+	lgr	%r1, %r0
+	BR_EX	%r1
+SYM_CODE_END(ftrace_stub_direct_tramp)
+
 	.macro	ftrace_regs_entry, allregs=0
 	stg	%r14,(__SF_GPRS+8*8)(%r15)	# save traced function caller
 

arch/x86/kernel/ftrace_32.S

@@ -163,6 +163,11 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
 	jmp	.Lftrace_ret
 SYM_CODE_END(ftrace_regs_caller)
 
+SYM_FUNC_START(ftrace_stub_direct_tramp)
+	CALL_DEPTH_ACCOUNT
+	RET
+SYM_FUNC_END(ftrace_stub_direct_tramp)
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 SYM_CODE_START(ftrace_graph_caller)
 	pushl	%eax

arch/x86/kernel/ftrace_64.S

@@ -309,6 +309,10 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 SYM_FUNC_END(ftrace_regs_caller)
 STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller)
 
+SYM_FUNC_START(ftrace_stub_direct_tramp)
+	CALL_DEPTH_ACCOUNT
+	RET
+SYM_FUNC_END(ftrace_stub_direct_tramp)
 
 #else /* ! CONFIG_DYNAMIC_FTRACE */
 

include/linux/ftrace.h

@@ -413,6 +413,8 @@ int unregister_ftrace_direct(struct ftrace_ops *ops, unsigned long addr,
 int modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr);
 int modify_ftrace_direct_nolock(struct ftrace_ops *ops, unsigned long addr);
 
+void ftrace_stub_direct_tramp(void);
+
 #else
 struct ftrace_ops;
 # define ftrace_direct_func_count 0

kernel/trace/trace_selftest.c

@@ -786,14 +786,6 @@ static struct fgraph_ops fgraph_ops __initdata  = {
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 static struct ftrace_ops direct;
-#ifndef CALL_DEPTH_ACCOUNT
-#define CALL_DEPTH_ACCOUNT ""
-#endif
-
-noinline __noclone static void trace_direct_tramp(void)
-{
-	asm(CALL_DEPTH_ACCOUNT);
-}
 #endif
 
 /*
@@ -873,7 +865,7 @@ trace_selftest_startup_function_graph(struct tracer *trace,
 	 */
 	ftrace_set_filter_ip(&direct, (unsigned long)DYN_FTRACE_TEST_NAME, 0, 0);
 	ret = register_ftrace_direct(&direct,
-				     (unsigned long)trace_direct_tramp);
+				     (unsigned long)ftrace_stub_direct_tramp);
 	if (ret)
 		goto out;
 
@@ -894,7 +886,7 @@ trace_selftest_startup_function_graph(struct tracer *trace,
 	unregister_ftrace_graph(&fgraph_ops);
 
 	ret = unregister_ftrace_direct(&direct,
-				       (unsigned long) trace_direct_tramp,
+				       (unsigned long)ftrace_stub_direct_tramp,
 				       true);
 	if (ret)
 		goto out;