• Tejun Heo's avatar
    sysfs: use generic_file_llseek() for sysfs_file_operations · 044e3bc3
    Tejun Heo authored
    13c589d5
    
     ("sysfs: use seq_file when reading regular files")
    converted regular sysfs files to use seq_file.  The commit substituted
    generic_file_llseek() with seq_lseek() for llseek implementation.
    
    Before the change, all regular sysfs files were allowed to seek to any
    position in [0, PAGE_SIZE] as the file size is always PAGE_SIZE and
    generic_file_llseek() allows any seeking inside the range under file
    size; however, seq_lseek()'s behavior is different.  It traverses the
    output by repeatedly invoking ->show() until it reaches the target
    offset or traversal indicates EOF.  As seq_files are fully dynamic and
    may not end at all, it doesn't support seeking from the end
    (SEEK_END).
    
    Apparently, there are userland tools which uses SEEK_END to discover
    the buffer size to use and the switch to seq_lseek() disturbs them as
    SEEK_END fails with -EINVAL.
    
    The only benefits of using seq_lseek() instead of
    generic_file_llseek() are
    
    * Early failure.  If traversing to certain file position should fail,
      seq_lseek() will report such failures on lseek(2) instead of the
      following read/write operations.
    
    * EOF detection.  While SEEK_END is not supported, SEEK_SET/CUR +
      large offset can be used to detect eof - eof at the time of the seek
      anyway as the file size may change dynamically.
    
    Both aren't necessary for sysfs or prospect kernfs users.  Revert to
    genefic_file_llseek() and preserve the original behavior.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    Reported-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    Link: https://lkml.kernel.org/r/20131031114358.GA5551@osiris
    
    Tested-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    044e3bc3