[PATCH bpf-next v4 2/6] ftrace: Fix deadloop caused by direct call in ftrace selftest

[Steven Rostedt]

On Tue, 17 May 2022 03:18:34 -0400
Xu Kuohai <xukuohai at huawei.com> 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 rewrite a dedicated trace_direct_tramp() for arm64.
> 
> Reported-by: Li Huafei <lihuafei1 at huawei.com>
> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
> ---

Acked-by: Steven Rostedt (Google) <rostedt at goodmis.org>

-- Steve