On Thu, 14 Apr 2022 12:22:16 -0400
Xu Kuohai <xukuohai@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
> ---
> 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();
This doesn't look like it belongs in this patch.
-- Steve
> +
> ret = register_ftrace_graph(&fgraph_ops);
> if (ret) {
> warn_failed_init_tracer(trace, ret);
