Re: [PATCH 1/2] sysfs: fix race between readdir and lseek

From: Ming Lei
Date: Thu Mar 21 2013 - 00:48:43 EST

On Thu, Mar 21, 2013 at 11:28 AM, Li Zefan <lizefan@xxxxxxxxxx> wrote:
> On 2013/3/21 11:17, Ming Lei wrote:
>> On Thu, Mar 21, 2013 at 10:41 AM, Li Zefan <lizefan@xxxxxxxxxx> wrote:
>>> In fact the same race exists between readdir() and read()/write()...
>> Fortunately, no read()/write() are implemented on sysfs directory, :-)
> That's irrelevant...

As far as sysfs is concerned, the filp->f_ops can't be changed in
read/write path.

> See my report:

Yes, I know there might be some mess after the commit ef3d0fd2
(vfs: do (nearly) lockless generic_file_llseek).

Also looks it has been stated in Documentation/filesystems/Locking:

->llseek() locking has moved from llseek to the individual llseek
implementations. If your fs is not using generic_file_llseek, you
need to acquire and release the appropriate locks in your ->llseek().
For many filesystems, it is probably safe to acquire the inode
mutex or just to use i_size_read() instead.
Note: this does not protect the file->f_pos against concurrent modifications
since this is something the userspace has to take care about.

Ming Lei
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