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

From: Li Zefan
Date: Fri Mar 22 2013 - 01:49:19 EST

On 2013/3/21 12:48, Ming Lei wrote:
> 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.

Yes, it can...As I said, it's irrelevant, because it's vfs that changes

SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)
struct fd f = fdget(fd);
ssize_t ret = -EBADF;

if (f.file) {
loff_t pos = file_pos_read(f.file); <--- read f_pos
ret = vfs_read(f.file, buf, count, &pos); <--- return -EISDIR
file_pos_write(f.file, pos); <--- write f_pos
return ret;

>> 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.

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