ftrace: use kretprobe trampoline name to test in output

Impact: ia64+tracing build fix

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.

Reported-by: Abhishek Sagar <sagar.abhishek@gmail.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Abhishek Sagar <sagar.abhishek@gmail.com>
Acked-by: Abhishek Sagar <sagar.abhishek@gmail.com>

kernel/trace/trace.c

@@ -1088,17 +1088,20 @@ static void s_stop(struct seq_file *m, void *p)
 	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 */
 
@@ -1107,10 +1110,13 @@ seq_print_sym_short(struct trace_seq *s, const char *fmt, unsigned long address)
 {
 #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;
 }
@@ -1121,9 +1127,12 @@ seq_print_sym_offset(struct trace_seq *s, const char *fmt,
 {
 #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;
 }
@@ -1377,10 +1386,7 @@ print_lat_fmt(struct trace_iterator *iter, unsigned int trace_idx, int cpu)
 
 		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;
 	}
@@ -1496,12 +1502,9 @@ static enum print_line_t print_trace_fmt(struct trace_iterator *iter)
 			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;
 		}