[patch] add /proc/pid/stack to dump task's stack trace

[Ken Chen]

This patch adds the ability to query a task's stack trace via /proc/pid/stack.
It is considered to be more useful than /proc/pid/wchan as it provides full
stack trace instead of single depth.

Signed-off-by: Ken Chen <kenchen@xxxxxxxxxx>

index bcceb99..3202d5c 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -139,6 +139,7 @@ Table 1-1: Process specific entries in /proc
  statm		Process memory status information
  status		Process status in human readable form
  wchan		If CONFIG_KALLSYMS is set, a pre-decoded wchan
+ stack		If CONFIG_KALLSYMS is set, report full stack trace
  smaps		Extension based on maps, the rss size for each mapped file
 ..............................................................................

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 486cf3f..466e519 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -65,6 +65,7 @@
 #include <linux/mm.h>
 #include <linux/rcupdate.h>
 #include <linux/kallsyms.h>
+#include <linux/stacktrace.h>
 #include <linux/resource.h>
 #include <linux/module.h>
 #include <linux/mount.h>
@@ -338,6 +339,35 @@ static int proc_pid_wchan
 	else
 		return sprintf(buffer, "%s", symname);
 }
+
+#define MAX_STACK_TRACE_DEPTH	32
+static int proc_stack_trace(struct task_struct *task, char *buffer)
+{
+	int i, len = 0;
+	unsigned long *entries;
+	struct stack_trace trace;
+
+	entries = kmalloc(sizeof(*entries) * MAX_STACK_TRACE_DEPTH, GFP_KERNEL);
+	if (!entries)
+		goto out;
+
+	trace.nr_entries = 0;
+	trace.max_entries = MAX_STACK_TRACE_DEPTH;
+	trace.entries = entries;
+	trace.skip = 0;
+
+	read_lock(&tasklist_lock);
+	save_stack_trace_tsk(task, &trace);
+	read_unlock(&tasklist_lock);
+
+	for (i = 0; i < trace.nr_entries; i++) {
+		len += sprintf(buffer + len, "[<%p>] %pS\n",
+				(void *) entries[i], (void *) entries[i]);
+	}
+	kfree(entries);
+out:
+	return len;
+}
 #endif /* CONFIG_KALLSYMS */

 #ifdef CONFIG_SCHEDSTATS
@@ -2489,6 +2519,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 	DIR("attr",       S_IRUGO|S_IXUGO, attr_dir),
 #endif
 #ifdef CONFIG_KALLSYMS
+	INF("stack",      S_IRUSR, stack_trace),
 	INF("wchan",      S_IRUGO, pid_wchan),
 #endif
 #ifdef CONFIG_SCHEDSTATS
--
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