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    SubjectRe: [PATCH RESEND] implement uid and gid mount options for ext2, ext3 and ext4
    On 2012-05-10 Thu 16:42 +0200, Ludwig Nussel wrote:
    > …
    > When using 'real' file systems on removable storage devices such as
    > hard disks or usb sticks people quickly face the problem that their
    > Linux users have different uids on different machines. Therefore one
    > cannot modify or even read files created on a different machine
    > without running chown as root or storing everything with mode 777.
    > Simple file systems such as vfat don't have that problem as they
    > don't store file ownership information and one can pass the uid
    > files should belong to as mount option.
    > The following two patches (for 3.4.0-rc4) implement the uid (and
    > gid) mount option for ext2, ext3 and ext4 to make them actually
    > useful on removable media. If a file system is mounted with the uid
    > option all files appear to be owned by the specified uid. Only newly
    > created files actually end up with that uid as owner on disk though.
    > Ownership of existing files cannot be changed permanently if the uid
    > option was specified.
    > Acked-by: Rob Landley <>
    > Signed-off-by: Ludwig Nussel <>
    > ---
    > Documentation/filesystems/ext2.txt | 9 ++++++
    > Documentation/filesystems/ext3.txt | 9 ++++++
    > Documentation/filesystems/ext4.txt | 9 ++++++
    > fs/ext2/ext2.h | 8 +++++
    > fs/ext2/inode.c | 42 ++++++++++++++++++++------
    > fs/ext2/super.c | 57 +++++++++++++++++++++++++++++++++++-
    > fs/ext3/ext3.h | 8 +++++
    > fs/ext3/inode.c | 50 ++++++++++++++++++++++---------
    > fs/ext3/super.c | 57 +++++++++++++++++++++++++++++++++++-
    > fs/ext4/ext4.h | 4 ++
    > fs/ext4/inode.c | 50 ++++++++++++++++++++++---------
    > fs/ext4/super.c | 49 ++++++++++++++++++++++++++++++-
    > 12 files changed, 311 insertions(+), 41 deletions(-)
    > …

    In short:
    Problem solving at its root is more efficient than at “end of pipe”.

    IMHO this is an example of “end of pipe“ thinking with following downsides:
    * Maintainers point of view:
    * Introduces new problems: Breaking holes in the access restrictions
    provided by the Linux kernel at will of unprivileged users would render
    the kernel unusable for reliable operation in multiuser environments.
    * Adds code complexity and risk of bugs.
    * Adds future maintainance load.
    * Users point of view:
    * Editing /etc/fstab or using mount commands with options not in
    /etc/fstab require root privileges anyway, at least on sane systems.
    * Adds usage complexity (new vs. old files, on disk vs. pretended UIDs …).
    * Adds risk of usage errors.

    IMHO the “right thing to do” is to solve the problem at its root:
    My habit is, whenever I use {group,user}add commands:
    * In advance I create a list of all current and future users (user, GID, UID)
    common to all systems that might exchange files. The list is designed to
    have “headroom” for future additions.
    * I always consult this list and use options --gid $userGID --uid $userUID to
    {group,user}add commands.
    * Exchanging files with an unforeseen system is an exception, which requires
    root privileges anyway,

    * Decent migration of files to other systems via backups, external storage …
    * No NEW wholes in the access restrictions provided by the Linux kernel.
    * No NEW kernel code possibly introducing bugs.
    * No need to learn new mount options.
    * No NEW risks of usage errors.

    * If UIDs differ on machines FORESEEN for file exchange, this is an
    administrator error, not a kernel deficit.
    * File exchange with an UNFORESEEN system requires root privileges anyway.

    Roland Eggner
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