Messages in this thread | | | Date | Thu, 20 Sep 2012 07:59:09 +0530 | From | Raghavendra K T <> | Subject | Re: 3.6rc6 slab corruption. |
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On 09/20/2012 03:19 AM, David Rientjes wrote: > On Wed, 19 Sep 2012, David Rientjes wrote: > >>> From 0806b133b5b28081adf23d0d04a99636ed3b861b Mon Sep 17 00:00:00 2001 >>> From: Konrad Rzeszutek Wilk<konrad.wilk@oracle.com> >>> 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<davej@redhat.com> >>> Suggested-by: Linus Torvalds<torvalds@linux-foundation.org> >> >> Tested-by: David Rientjes<rientjes@google.com> >> > > 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); > >
Only problem, I find is histogram data expands dynamically (because it changes). I think having static allocation of 352 bytes as suggested Linus is a good idea.
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