ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default

JIRA: https://issues.redhat.com/browse/RHEL-101598

commit a8b9cf6
Author: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Date:   Wed Jan 10 09:13:06 2024 +0900

    ftrace: Fix DIRECT_CALLS to use SAVE_REGS by default

    The commit 60c8971 ("ftrace: Make DIRECT_CALLS work WITH_ARGS
    and !WITH_REGS") changed DIRECT_CALLS to use SAVE_ARGS when there
    are multiple ftrace_ops at the same function, but since the x86 only
    support to jump to direct_call from ftrace_regs_caller, when we set
    the function tracer on the same target function on x86, ftrace-direct
    does not work as below (this actually works on arm64.)

    At first, insmod ftrace-direct.ko to put a direct_call on
    'wake_up_process()'.

     # insmod kernel/samples/ftrace/ftrace-direct.ko
     # less trace
    ...
              <idle>-0       [006] ..s1.   564.686958: my_direct_func: waking up rcu_preempt-17
              <idle>-0       [007] ..s1.   564.687836: my_direct_func: waking up kcompactd0-63
              <idle>-0       [006] ..s1.   564.690926: my_direct_func: waking up rcu_preempt-17
              <idle>-0       [006] ..s1.   564.696872: my_direct_func: waking up rcu_preempt-17
              <idle>-0       [007] ..s1.   565.191982: my_direct_func: waking up kcompactd0-63

    Setup a function filter to the 'wake_up_process' too, and enable it.

     # cd /sys/kernel/tracing/
     # echo wake_up_process > set_ftrace_filter
     # echo function > current_tracer
     # less trace
    ...
              <idle>-0       [006] ..s3.   686.180972: wake_up_process <-call_timer_fn
              <idle>-0       [006] ..s3.   686.186919: wake_up_process <-call_timer_fn
              <idle>-0       [002] ..s3.   686.264049: wake_up_process <-call_timer_fn
              <idle>-0       [002] d.h6.   686.515216: wake_up_process <-kick_pool
              <idle>-0       [002] d.h6.   686.691386: wake_up_process <-kick_pool

    Then, only function tracer is shown on x86.
    But if you enable 'kprobe on ftrace' event (which uses SAVE_REGS flag)
    on the same function, it is shown again.

     # echo 'p wake_up_process' >> dynamic_events
     # echo 1 > events/kprobes/p_wake_up_process_0/enable
     # echo > trace
     # less trace
    ...
              <idle>-0       [006] ..s2.  2710.345919: p_wake_up_process_0: (wake_up_process+0x4/0x20)
              <idle>-0       [006] ..s3.  2710.345923: wake_up_process <-call_timer_fn
              <idle>-0       [006] ..s1.  2710.345928: my_direct_func: waking up rcu_preempt-17
              <idle>-0       [006] ..s2.  2710.349931: p_wake_up_process_0: (wake_up_process+0x4/0x20)
              <idle>-0       [006] ..s3.  2710.349934: wake_up_process <-call_timer_fn
              <idle>-0       [006] ..s1.  2710.349937: my_direct_func: waking up rcu_preempt-17

    To fix this issue, use SAVE_REGS flag for multiple ftrace_ops flag of
    direct_call by default.

    Link: https://lore.kernel.org/linux-trace-kernel/170484558617.178953.1590516949390270842.stgit@devnote2

    Fixes: 60c8971 ("ftrace: Make DIRECT_CALLS work WITH_ARGS and !WITH_REGS")
    Cc: stable@vger.kernel.org
    Cc: Florent Revest <revest@chromium.org>
    Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
    Reviewed-by: Mark Rutland <mark.rutland@arm.com>
    Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]
    Acked-by: Jiri Olsa <jolsa@kernel.org>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/ftrace.c

@@ -5275,7 +5275,17 @@ static LIST_HEAD(ftrace_direct_funcs);
 
 static int register_ftrace_function_nolock(struct ftrace_ops *ops);
 
+/*
+ * If there are multiple ftrace_ops, use SAVE_REGS by default, so that direct
+ * call will be jumped from ftrace_regs_caller. Only if the architecture does
+ * not support ftrace_regs_caller but direct_call, use SAVE_ARGS so that it
+ * jumps from ftrace_caller for multiple ftrace_ops.
+ */
+#ifndef HAVE_DYNAMIC_FTRACE_WITH_REGS
 #define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_ARGS)
+#else
+#define MULTI_FLAGS (FTRACE_OPS_FL_DIRECT | FTRACE_OPS_FL_SAVE_REGS)
+#endif
 
 static int check_direct_multi(struct ftrace_ops *ops)
 {