On Wed, 19 Sep 2012, David Rientjes wrote:
> > From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001
> > From: Konrad Rzeszutek Wilk <[email protected]>
> > Date: Wed, 19 Sep 2012 11:23:01 -0400
> > Subject: [PATCH 1/2] debugfs: Add lock for u32_array_read
> >
> > Dave Jones spotted that the u32_array_read was doing something funny:
> >
> > =============================================================================
> > BUG kmalloc-64 (Not tainted): Redzone overwritten
> > -----------------------------------------------------------------------------
> >
> > INFO: 0xffff88001f4b4970-0xffff88001f4b4977. First byte 0xbb instead of 0xcc
> > INFO: Allocated in u32_array_read+0xd1/0x110 age=0 cpu=6 pid=32767
> > __slab_alloc+0x516/0x5a5
> > __kmalloc+0x213/0x2c0
> > u32_array_read+0xd1/0x110
> > .. snip..
> > INFO: Freed in u32_array_read+0x99/0x110 age=0 cpu=0 pid=32749
> > __slab_free+0x3f/0x3bf
> > kfree+0x2d5/0x310
> > u32_array_read+0x99/0x110
> >
> > Linus tracked it down and found out that "debugfs is racy for that case
> > [read calls in parallel on the debugfs]. At least the file->private_data
> > accesses are, for the case of that "u32_array" case.
> >
> > In fact it is racy in ... the whole "file->private_data" access ..
> > If you have multiple readers on the same file, the whole
> >
> > if (file->private_data) {
> > kfree(file->private_data);
> > file->private_data = NULL;
> > }
> >
> > file->private_data = format_array_alloc("%u", data->array,
> >
> > data->elements);
> >
> > thing is just a disaster waiting to happen." He suggested
> > putting a lock which this patch does.
> >
>
> Since these are non-seekable files, it must also race to find *ppos == 0.
>
> > The consequence of this is that it will trigger more spinlock usage,
> > as this particular debugfs is used to provide a histogram of spinlock
> > contention. But memory corruption is a worst offender then that.
> >
> > Reported-by: Dave Jones <[email protected]>
> > Suggested-by: Linus Torvalds <[email protected]>
>
> Tested-by: David Rientjes <[email protected]>
>
An alternative to this, though, might be to never test for *ppos == 0 in
u32_array_read() and do the format_array_alloc() in u32_array_open() to
initialize file->private_data. If that allocation fails, just return
-ENOMEM. Then you never need to add a mutex in the read path.
Any reason we can't do this?
---
fs/debugfs/file.c | 33 +++++++++++----------------------
1 file changed, 11 insertions(+), 22 deletions(-)
diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -526,12 +526,6 @@ struct array_data {
u32 elements;
};
-static int u32_array_open(struct inode *inode, struct file *file)
-{
- file->private_data = NULL;
- return nonseekable_open(inode, file);
-}
-
static size_t format_array(char *buf, size_t bufsize, const char *fmt,
u32 *array, u32 array_size)
{
@@ -573,26 +567,21 @@ static char *format_array_alloc(const char *fmt, u32
*array,
return ret;
}
-static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
- loff_t *ppos)
+static int u32_array_open(struct inode *inode, struct file *file)
{
- struct inode *inode = file->f_path.dentry->d_inode;
struct array_data *data = inode->i_private;
- size_t size;
- if (*ppos == 0) {
- if (file->private_data) {
- kfree(file->private_data);
- file->private_data = NULL;
- }
-
- file->private_data = format_array_alloc("%u", data->array,
- data->elements);
- }
+ file->private_data = format_array_alloc("%u", data->array,
+ data->elements);
+ if (!file->private_data)
+ return -ENOMEM;
+ return nonseekable_open(inode, file);
+}
- size = 0;
- if (file->private_data)
- size = strlen(file->private_data);
+static ssize_t u32_array_read(struct file *file, char __user *buf, size_t len,
+ loff_t *ppos)
+{
+ size_t size = strlen(file->private_data);
return simple_read_from_buffer(buf, len, ppos,
file->private_data, size);
--
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