• Linus Torvalds's avatar
    vfs: atomic f_pos accesses as per POSIX · 9c225f26
    Linus Torvalds authored
    Our write() system call has always been atomic in the sense that you get
    the expected thread-safe contiguous write, but we haven't actually
    guaranteed that concurrent writes are serialized wrt f_pos accesses, so
    threads (or processes) that share a file descriptor and use "write()"
    concurrently would quite likely overwrite each others data.
    
    This violates POSIX.1-2008/SUSv4 Section XSI 2.9.7 that says:
    
     "2.9.7 Thread Interactions with Regular File Operations
    
      All of the following functions shall be atomic with respect to each
      other in the effects specified in POSIX.1-2008 when they operate on
      regular files or symbolic links: [...]"
    
    and one of the effects is the file position update.
    
    This unprotected file position behavior is not new behavior, and nobody
    has ever cared.  Until now.  Yongzhi Pan reported unexpected behavior to
    Michael Kerrisk that was due to this.
    
    This resolves the issue with a f_pos-specific lock that is taken by
    read/write/lseek on file descriptors that may be shared across threads
    or processes.
    Reported-by: default avatarYongzhi Pan <panyongzhi@gmail.com>
    Reported-by: default avatarMichael Kerrisk <mtk.manpages@gmail.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    9c225f26
open.c 25.9 KB