linux-kernel - [QUESTION] ftrace_test_recursion

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Date:   Wed, 1 Nov 2023 18:32:14 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Florent Revest <revest@...omium.org>
Cc:     linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [QUESTION] ftrace_test_recursion_trylock behaviour

hi,
I'm doing some testing on top of fprobes and noticed that the
ftrace_test_recursion_trylock allows caller from the same context
going through twice.

The change below adds extra fprobe on stack_trace_print, which is
called within the sample_entry_handler and I can see it being executed
with following trace output:

           <...>-457     [003] ...1.    32.352554: sample_entry_handler: Enter <kernel_clone+0x0/0x380> ip = 0xffffffff81177420
           <...>-457     [003] ...2.    32.352578: sample_entry_handler_extra: Enter <stack_trace_print+0x0/0x60> ip = 0xffffffff8127ae70

IOW nested ftrace_test_recursion_trylock call in the same context succeeded.

It seems the reason is the TRACE_CTX_TRANSITION bit logic.

Just making sure it's intentional.. we have kprobe_multi code on top of
fprobe with another re-entry logic and that might behave differently based
on ftrace_test_recursion_trylock logic.

thanks,
jirka


---
diff --git a/samples/fprobe/fprobe_example.c b/samples/fprobe/fprobe_example.c
index 64e715e7ed11..531272af0682 100644
--- a/samples/fprobe/fprobe_example.c
+++ b/samples/fprobe/fprobe_example.c
@@ -23,6 +23,9 @@
 static struct fprobe sample_probe;
 static unsigned long nhit;
 
+static struct fprobe sample_probe_extra;
+static char *symbol_extra = "stack_trace_print";
+
 static char symbol[MAX_SYMBOL_LEN] = "kernel_clone";
 module_param_string(symbol, symbol, sizeof(symbol), 0644);
 MODULE_PARM_DESC(symbol, "Probed symbol(s), given by comma separated symbols or a wildcard pattern.");
@@ -48,6 +51,15 @@ static void show_backtrace(void)
 	stack_trace_print(stacks, len, 24);
 }
 
+static int sample_entry_handler_extra(struct fprobe *fp, unsigned long ip,
+				      unsigned long ret_ip,
+				      struct pt_regs *regs, void *data)
+{
+	if (use_trace)
+		trace_printk("Enter <%pS> ip = 0x%p\n", (void *)ip, (void *)ip);
+	return 0;
+}
+
 static int sample_entry_handler(struct fprobe *fp, unsigned long ip,
 				unsigned long ret_ip,
 				struct pt_regs *regs, void *data)
@@ -96,6 +108,8 @@ static int __init fprobe_init(void)
 	sample_probe.entry_handler = sample_entry_handler;
 	sample_probe.exit_handler = sample_exit_handler;
 
+	sample_probe_extra.entry_handler = sample_entry_handler_extra;
+
 	if (strchr(symbol, '*')) {
 		/* filter based fprobe */
 		ret = register_fprobe(&sample_probe, symbol,
@@ -137,12 +151,19 @@ static int __init fprobe_init(void)
 	else
 		pr_info("Planted fprobe at %s\n", symbol);
 
+	ret = register_fprobe_syms(&sample_probe_extra, (const char **) &symbol_extra, 1);
+	if (ret < 0)
+		pr_err("register_fprobe extra failed, returned %d\n", ret);
+	else
+		pr_info("Planted extra fprobe at %s\n", symbol_extra);
+
 	return ret;
 }
 
 static void __exit fprobe_exit(void)
 {
 	unregister_fprobe(&sample_probe);
+	unregister_fprobe(&sample_probe_extra);
 
 	pr_info("fprobe at %s unregistered. %ld times hit, %ld times missed\n",
 		symbol, nhit, sample_probe.nmissed);