Re: [RFC][PATCH] vfs: always protect diretory file->fpos with inodemutex

From: Li Zefan
Date: Tue Feb 19 2013 - 20:49:53 EST

On 2013/2/19 20:59, Jan Kara wrote:
> On Tue 19-02-13 19:47:30, Li Zefan wrote:
>> On 2013/2/19 17:19, Jan Kara wrote:
>>> On Tue 19-02-13 09:22:40, Li Zefan wrote:
>>>> There's a long long-standing bug...As long as I don't know when it dates
>>>> from.
>>>> I've written and attached a simple program to reproduce this bug, and it can
>>>> immediately trigger the bug in my box. It uses two threads, one keeps calling
>>>> read(), and the other calling readdir(), both on the same directory fd.
>>> So the fact that read() or even write() to fd opened O_RDONLY has *any*
>>> effect on f_pos looks really unexpected to me. I think we really should
>>> have there:
>>> if (ret >= 0)
>>> file_pos_write(...);
>> I thought about this. The problem is then we have to check every fop->write()
>> to see if any of them can return -errno with file->f_pos changed and fix them,
>> though it's do-able.
> But returning error and advancing f_pos would be a bug - specification
> says write() returns the number of bytes written or -1 and f_pos should be
> advanced by the number of bytes written.

Oh, I had an illusion that vfs saves f_pos and calls write() and restore f_pos
if write() fails.

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