lockdep: Avoid /proc/lockdep & lock_stat infinite output

Both /proc/lockdep and /proc/lock_stat output may loop infinitely.

When a read() requests an amount of data smaller than the amount of data
that the seq_file's foo_show() outputs, the output starts looping and
outputs the "stuck" element's data infinitely.  There may be multiple
sequential calls to foo_start(), foo_next()/foo_show(), and foo_stop()
for a single open with sequential read of the file.  The _start() does not
have to start with the 0th element and _show() might be called multiple
times in a row for the same element for a given open/read of the seq_file.

Also header output should not be happening in _start().  All output should
be in _show(), which SEQ_START_TOKEN is meant to help.  Having output in
_start() may also negatively impact seq_file's seq_read() and traverse()
accounting.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Al Viro <viro@ftp.linux.org.uk>

1 files changed, 42 insertions, 19 deletions

diff --git a/kernel/lockdep_proc.c b/kernel/lockdep_proc.c
index c851b2dcc685..8a135bd163c2 100644
--- a/kernel/lockdep_proc.c
+++ b/kernel/lockdep_proc.c
@@ -25,28 +25,38 @@
 
 static void *l_next(struct seq_file *m, void *v, loff_t *pos)
 {
-        struct lock_class *class = v;
+        struct lock_class *class;
 
         (*pos)++;
 
-        if (class->lock_entry.next != &all_lock_classes)
-                class = list_entry(class->lock_entry.next, struct lock_class,
-                                  lock_entry);
-        else
-                class = NULL;
-        m->private = class;
+        if (v == SEQ_START_TOKEN)
+                class = m->private;
+        else {
+                class = v;
+
+                if (class->lock_entry.next != &all_lock_classes)
+                        class = list_entry(class->lock_entry.next,
+                                           struct lock_class, lock_entry);
+                else
+                        class = NULL;
+        }
 
         return class;
 }
 
 static void *l_start(struct seq_file *m, loff_t *pos)
 {
-        struct lock_class *class = m->private;
+        struct lock_class *class;
+        loff_t i = 0;
 
-        if (&class->lock_entry == all_lock_classes.next)
-                seq_printf(m, "all lock classes:\n");
+        if (*pos == 0)
+                return SEQ_START_TOKEN;
 
-        return class;
+        list_for_each_entry(class, &all_lock_classes, lock_entry) {
+                if (++i == *pos)
+                return class;
+        }
+        return NULL;
 }
 
 static void l_stop(struct seq_file *m, void *v)
@@ -101,10 +111,15 @@ static void print_name(struct seq_file *m, struct lock_class *class)
 static int l_show(struct seq_file *m, void *v)
 {
         unsigned long nr_forward_deps, nr_backward_deps;
-        struct lock_class *class = m->private;
+        struct lock_class *class = v;
         struct lock_list *entry;
         char c1, c2, c3, c4;
 
+        if (v == SEQ_START_TOKEN) {
+                seq_printf(m, "all lock classes:\n");
+                return 0;
+        }
+
         seq_printf(m, "%p", class->key);
 #ifdef CONFIG_DEBUG_LOCKDEP
         seq_printf(m, " OPS:%8ld", class->ops);
@@ -523,10 +538,11 @@ static void *ls_start(struct seq_file *m, loff_t *pos)
 {
         struct lock_stat_seq *data = m->private;
 
-        if (data->iter == data->stats)
-                seq_header(m);
+        if (*pos == 0)
+                return SEQ_START_TOKEN;
 
-        if (data->iter == data->iter_end)
+        data->iter = data->stats + *pos;
+        if (data->iter >= data->iter_end)
                 data->iter = NULL;
 
         return data->iter;
@@ -538,8 +554,13 @@ static void *ls_next(struct seq_file *m, void *v, loff_t *pos)
 
         (*pos)++;
 
-        data->iter = v;
-        data->iter++;
+        if (v == SEQ_START_TOKEN)
+                data->iter = data->stats;
+        else {
+                data->iter = v;
+                data->iter++;
+        }
+
         if (data->iter == data->iter_end)
                 data->iter = NULL;
 
@@ -552,9 +573,11 @@ static void ls_stop(struct seq_file *m, void *v)
 
 static int ls_show(struct seq_file *m, void *v)
 {
-        struct lock_stat_seq *data = m->private;
+        if (v == SEQ_START_TOKEN)
+                seq_header(m);
+        else
+                seq_stats(m, v);
 
-        seq_stats(m, data->iter);
         return 0;
 }