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Subject[PATCH 4/6] tracing: Change f_start() to take event_mutex and verify i_private != NULL
trace_format_open() and trace_format_seq_ops are racy, nothing
protects ftrace_event_call from trace_remove_event_call().

Change f_start() to take event_mutex and verify i_private != NULL,
change f_stop() to drop this lock.

This fixes nothing, but now we can change debugfs_remove("format")
callers to nullify ->i_private and fix the the problem.

Note: the usage of event_mutex is sub-optimal but simple, we can
change this later.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/trace/trace_events.c | 13 +++++++++----
1 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 0f081c0..4b43111 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -838,7 +838,7 @@ enum {

static void *f_next(struct seq_file *m, void *v, loff_t *pos)
{
- struct ftrace_event_call *call = m->private;
+ struct ftrace_event_call *call = event_file_data(m->private);
struct list_head *common_head = &ftrace_common_fields;
struct list_head *head = trace_get_fields(call);
struct list_head *node = v;
@@ -870,7 +870,7 @@ static void *f_next(struct seq_file *m, void *v, loff_t *pos)

static int f_show(struct seq_file *m, void *v)
{
- struct ftrace_event_call *call = m->private;
+ struct ftrace_event_call *call = event_file_data(m->private);
struct ftrace_event_field *field;
const char *array_descriptor;

@@ -923,6 +923,11 @@ static void *f_start(struct seq_file *m, loff_t *pos)
void *p = (void *)FORMAT_HEADER;
loff_t l = 0;

+ /* ->stop() is called even if ->start() fails */
+ mutex_lock(&event_mutex);
+ if (!event_file_data(m->private))
+ return ERR_PTR(-ENODEV);
+
while (l < *pos && p)
p = f_next(m, p, &l);

@@ -931,6 +936,7 @@ static void *f_start(struct seq_file *m, loff_t *pos)

static void f_stop(struct seq_file *m, void *p)
{
+ mutex_unlock(&event_mutex);
}

static const struct seq_operations trace_format_seq_ops = {
@@ -942,7 +948,6 @@ static const struct seq_operations trace_format_seq_ops = {

static int trace_format_open(struct inode *inode, struct file *file)
{
- struct ftrace_event_call *call = inode->i_private;
struct seq_file *m;
int ret;

@@ -951,7 +956,7 @@ static int trace_format_open(struct inode *inode, struct file *file)
return ret;

m = file->private_data;
- m->private = call;
+ m->private = file;

return 0;
}
--
1.5.5.1


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