Re: [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write

Al Viro
2018-11-21T04:54:41Z

On Wed, Nov 21, 2018 at 11:43:56AM +0900, Eiichi Tsukata wrote:
> Some file systems (including ext4, xfs, ramfs ...) have the following
> problem as I've described in the commit message of the 1/4 patch.
>
> The commit ef3d0fd27e90 ("vfs: do (nearly) lockless generic_file_llseek")
> removed almost all locks in llseek() including SEEK_END. It based on the
> idea that write() updates size atomically. But in fact, write() can be
> divided into two or more parts in generic_perform_write() when pos
> straddles over the PAGE_SIZE, which results in updating size multiple
> times in one write(). It means that llseek() can see the size being
> updated during write().

And? Who has ever promised anything that insane? write(2) can take an arbitrary
amount of time; another process doing lseek() on independently opened descriptor
is *not* going to wait for that (e.g. page-in of the buffer being written, which
just happens to be mmapped from a file on NFS over RFC1149 link).