linux-kernel - [PATCH] ftrace: use kretprobe trampoline name to test in output

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Date:	Fri, 31 Oct 2008 15:44:07 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Abhishek Sagar <sagar.abhishek@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>, ananth@...ibm.com,
	jkenisto@...ibm.com, mhiramat@...hat.com
Subject: [PATCH] ftrace: use kretprobe trampoline name to test in output


[
  Ingo,

  Can you get this ready for 2.6.28. Without this fix, tracing
  breaks the ia64 build.
]

When a function is kprobed, the return address is set to the
kprobe_trampoline, or something similar. This caused the output
of the trace to look confusing when the parent seemed to be this
"kprobe_trampoline" function.

To fix this, Abhishek Sagar added a test of the instruction pointer
of the parent to see if it matched the kprobe_trampoline. If it
did, the output would print a "[unknown/kretprobe'd]" instead.

Unfortunately, not all archs do this the same way, and the trampoline
function may not be exported, which causes failures in builds.

This patch will compare the name instead of the pointer to see
if it matches. This prevents us from depending on a function from
being exported, and should work on all archs. The worst that can
happen is that an arch might use a different name and then we
go back to the confusing output. At least the arch will still build.

Signed-off-by: Steven Rostedt <srostedt@...hat.com>
Tested-by: Abhishek Sagar <sagar.abhishek@...il.com>
Acked-by: Abhishek Sagar <sagar.abhishek@...il.com>
---
 kernel/trace/trace.c |   39 +++++++++++++++++++++------------------
 1 file changed, 21 insertions(+), 18 deletions(-)

Index: linux-tip.git/kernel/trace/trace.c
===================================================================
--- linux-tip.git.orig/kernel/trace/trace.c	2008-10-31 10:41:29.000000000 -0400
+++ linux-tip.git/kernel/trace/trace.c	2008-10-31 11:25:21.000000000 -0400
@@ -1104,17 +1104,20 @@ static void s_stop(struct seq_file *m, v
 	mutex_unlock(&trace_types_lock);
 }
 
-#define KRETPROBE_MSG "[unknown/kretprobe'd]"
-
 #ifdef CONFIG_KRETPROBES
-static inline int kretprobed(unsigned long addr)
+static inline const char *kretprobed(const char *name)
 {
-	return addr == (unsigned long)kretprobe_trampoline;
+	static const char tramp_name[] = "kretprobe_trampoline";
+	int size = sizeof(tramp_name);
+
+	if (strncmp(tramp_name, name, size) == 0)
+		return "[unknown/kretprobe'd]";
+	return name;
 }
 #else
-static inline int kretprobed(unsigned long addr)
+static inline const char *kretprobed(const char *name)
 {
-	return 0;
+	return name;
 }
 #endif /* CONFIG_KRETPROBES */
 
@@ -1123,10 +1126,13 @@ seq_print_sym_short(struct trace_seq *s,
 {
 #ifdef CONFIG_KALLSYMS
 	char str[KSYM_SYMBOL_LEN];
+	const char *name;
 
 	kallsyms_lookup(address, NULL, NULL, NULL, str);
 
-	return trace_seq_printf(s, fmt, str);
+	name = kretprobed(str);
+
+	return trace_seq_printf(s, fmt, name);
 #endif
 	return 1;
 }
@@ -1137,9 +1143,12 @@ seq_print_sym_offset(struct trace_seq *s
 {
 #ifdef CONFIG_KALLSYMS
 	char str[KSYM_SYMBOL_LEN];
+	const char *name;
 
 	sprint_symbol(str, address);
-	return trace_seq_printf(s, fmt, str);
+	name = kretprobed(str);
+
+	return trace_seq_printf(s, fmt, name);
 #endif
 	return 1;
 }
@@ -1392,10 +1401,7 @@ print_lat_fmt(struct trace_iterator *ite
 
 		seq_print_ip_sym(s, field->ip, sym_flags);
 		trace_seq_puts(s, " (");
-		if (kretprobed(field->parent_ip))
-			trace_seq_puts(s, KRETPROBE_MSG);
-		else
-			seq_print_ip_sym(s, field->parent_ip, sym_flags);
+		seq_print_ip_sym(s, field->parent_ip, sym_flags);
 		trace_seq_puts(s, ")\n");
 		break;
 	}
@@ -1511,12 +1517,9 @@ static enum print_line_t print_trace_fmt
 			ret = trace_seq_printf(s, " <-");
 			if (!ret)
 				return TRACE_TYPE_PARTIAL_LINE;
-			if (kretprobed(field->parent_ip))
-				ret = trace_seq_puts(s, KRETPROBE_MSG);
-			else
-				ret = seq_print_ip_sym(s,
-						       field->parent_ip,
-						       sym_flags);
+			ret = seq_print_ip_sym(s,
+					       field->parent_ip,
+					       sym_flags);
 			if (!ret)
 				return TRACE_TYPE_PARTIAL_LINE;
 		}


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