lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Thu, 22 Nov 2018 14:40:50 +0900
From:   Eiichi Tsukata <>
To:     Alexander Viro <>
Cc:, Chris Mason <>,
        Josef Bacik <>,
        David Sterba <>,
        "Theodore Ts'o" <>,
        Andreas Dilger <>,
        Jaegeuk Kim <>, Chao Yu <>,
        Miklos Szeredi <>,
        Bob Peterson <>,
        Andreas Gruenbacher <>,,,,,,
Subject: Re: [PATCH v1 0/4] fs: fix race between llseek SEEK_END and write

2018年11月21日(水) 13:54 Al Viro <>:
> 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).


The lock I added in NFS was nothing but slow down lseek() because a file size is
updated atomically. Even `spin_lock(&inode->i_lock)` is unnecessary.

I'll fix the commit message which only refers to specific local file
systems that use
generic_perform_write() and remove unnecessary locks in some
distributed file systems
(e.g. nfs, cifs, or more) by replacing generic_file_llseek() with
so that `tail` don't have to wait for avian carriers.

Powered by blists - more mailing lists