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

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Message-ID: <CACVXFVMS644BpX7voMHS-xXoRs9-G4y96=ubk_NAj6KDTy1ENQ@mail.gmail.com>
Date:	Thu, 21 Mar 2013 12:48:36 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] sysfs: fix race between readdir and lseek

On Thu, Mar 21, 2013 at 11:28 AM, Li Zefan <lizefan@...wei.com> wrote:
> On 2013/3/21 11:17, Ming Lei wrote:
>> On Thu, Mar 21, 2013 at 10:41 AM, Li Zefan <lizefan@...wei.com> 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:
>
> https://patchwork.kernel.org/patch/2160771/

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.


Thanks,
--
Ming Lei
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