After direct call is enabled for arm64, ftrace selftest enters a dead loop:
<trace_selftest_dynamic_test_func>: 00 bti c 01 mov x9, x30 <trace_direct_tramp>: 02 bl <trace_direct_tramp> ----------> ret | lr/x30 is 03, return to 03 | 03 mov w0, #0x0 <-----------------------------| | | | dead loop! | | | 04 ret ---- lr/x30 is still 03, go back to 03 ----|
The reason is that when the direct caller trace_direct_tramp() returns to the patched function trace_selftest_dynamic_test_func(), lr is still the address after the instrumented instruction in the patched function, so when the patched function exits, it returns to itself!
To fix this issue, we need to restore lr before trace_direct_tramp() exits, so make trace_direct_tramp() a weak symbol and rewrite it for arm64.
To detect this issue directly, call DYN_FTRACE_TEST_NAME() before register_ftrace_graph().
Reported-by: Li Huafei lihuafei1@huawei.com Signed-off-by: Xu Kuohai xukuohai@huawei.com --- arch/arm64/kernel/entry-ftrace.S | 10 ++++++++++ kernel/trace/trace_selftest.c | 4 +++- 2 files changed, 13 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S index dfe62c55e3a2..e58eb06ec9b2 100644 --- a/arch/arm64/kernel/entry-ftrace.S +++ b/arch/arm64/kernel/entry-ftrace.S @@ -357,3 +357,13 @@ SYM_CODE_START(return_to_handler) ret SYM_CODE_END(return_to_handler) #endif /* CONFIG_FUNCTION_GRAPH_TRACER */ + +#ifdef CONFIG_FTRACE_SELFTEST +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS +SYM_FUNC_START(trace_direct_tramp) + mov x10, x30 + mov x30, x9 + ret x10 +SYM_FUNC_END(trace_direct_tramp) +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */ +#endif /* CONFIG_FTRACE_SELFTEST */ diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c index abcadbe933bb..38b0d5c9a1e0 100644 --- a/kernel/trace/trace_selftest.c +++ b/kernel/trace/trace_selftest.c @@ -785,7 +785,7 @@ static struct fgraph_ops fgraph_ops __initdata = { };
#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS -noinline __noclone static void trace_direct_tramp(void) { } +void __weak trace_direct_tramp(void) { } #endif
/* @@ -868,6 +868,8 @@ trace_selftest_startup_function_graph(struct tracer *trace, if (ret) goto out;
+ DYN_FTRACE_TEST_NAME(); + ret = register_ftrace_graph(&fgraph_ops); if (ret) { warn_failed_init_tracer(trace, ret);