fprobe: Release rethook after the ftrace_ops is unregistered

JIRA: https://issues.redhat.com/browse/RHEL-26131

commit 5f81018
Author: Jiri Olsa <jolsa@kernel.org>
Date:   Thu Jun 15 13:52:36 2023 +0200

    fprobe: Release rethook after the ftrace_ops is unregistered

    While running bpf selftests it's possible to get following fault:

      general protection fault, probably for non-canonical address \
      0x6b6b6b6b6b6b6b6b: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
      ...
      Call Trace:
       <TASK>
       fprobe_handler+0xc1/0x270
       ? __pfx_bpf_testmod_init+0x10/0x10
       ? __pfx_bpf_testmod_init+0x10/0x10
       ? bpf_fentry_test1+0x5/0x10
       ? bpf_fentry_test1+0x5/0x10
       ? bpf_testmod_init+0x22/0x80
       ? do_one_initcall+0x63/0x2e0
       ? rcu_is_watching+0xd/0x40
       ? kmalloc_trace+0xaf/0xc0
       ? do_init_module+0x60/0x250
       ? __do_sys_finit_module+0xac/0x120
       ? do_syscall_64+0x37/0x90
       ? entry_SYSCALL_64_after_hwframe+0x72/0xdc
       </TASK>

    In unregister_fprobe function we can't release fp->rethook while it's
    possible there are some of its users still running on another cpu.

    Moving rethook_free call after fp->ops is unregistered with
    unregister_ftrace_function call.

    Link: https://lore.kernel.org/all/20230615115236.3476617-1-jolsa@kernel.org/

    Fixes: 5b0ab78 ("fprobe: Add exit_handler support")
    Cc: stable@vger.kernel.org
    Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    Signed-off-by: Jiri Olsa <jolsa@kernel.org>
    Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
    Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Signed-off-by: Viktor Malik <vmalik@redhat.com>

Author: viktormalik

kernel/trace/fprobe.c

@@ -349,19 +349,13 @@ int unregister_fprobe(struct fprobe *fp)
 		    fp->ops.saved_func != fprobe_kprobe_handler))
 		return -EINVAL;
 
-	/*
-	 * rethook_free() starts disabling the rethook, but the rethook handlers
-	 * may be running on other processors at this point. To make sure that all
-	 * current running handlers are finished, call unregister_ftrace_function()
-	 * after this.
-	 */
-	if (fp->rethook)
-		rethook_free(fp->rethook);
-
 	ret = unregister_ftrace_function(&fp->ops);
 	if (ret < 0)
 		return ret;
 
+	if (fp->rethook)
+		rethook_free(fp->rethook);
+
 	ftrace_free_filter(&fp->ops);
 
 	return ret;