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    SubjectA Plumber’s Wish List for Linux, updated version 2
    Update: this is the second version, it incorporates the original list,
    adds a couple of new items, and includes references to some useful
    feedback and patches that have already been prepared.

    We’d like to share our current wish list of plumbing layer features we
    are hoping to see implemented in the near future in the Linux kernel and
    associated tools. Some items we can implement on our own, others are not
    our area of expertise, and we will need help getting them implemented.

    Acknowledging that this wish list of ours only gets longer and not
    shorter, even though we have implemented a number of other features on
    our own in the previous years, we are posting this list here, in the
    hope to find some help.

    If you happen to be interested in working on something from this list or
    able to help out, we’d be delighted. Please ping us in case you need
    clarifications or more information on specific items.

    Kay, Lennart, Harald, David in the name of all the other plumbers

    And here is the wish list, in no particular order:

    * support user quota on tmpfs to prevent DoS vulnerabilities
    on /tmp, /dev/shm, /run/user/$USER. This is kinda important. Idea:
    global RLIMIT_TMPFS_QUOTA over all mounted tmpfs file systems. NEW!

    * support fallocate() properly: NEW!
    fallocate(5, 0, 0, 7663616) = -1 EOPNOTSUPP

    * events for renames NEW!

    * allow safe unprivileged access NEW!

    * pass information about the open flags to the file system monitors, in
    order to allow clients to figure out whether other applications opened
    files for writing or just read-only. NEW!

    * allow to find out if a file actually was written to, when closed after
    opening it read-write NEW!

    * (ioctl based?) interface to query and modify the label of a mounted
    FAT volume: A FAT label is implemented as a hidden directory entry in
    the file system, which need to be renamed when changing the file system
    label. This is impossible to do from userspace without remounting. Hence
    we’d like to see a kernel interface that is available on the mounted
    file system mount point itself. Of course, bonus points, if this new
    interface can be implemented for other file systems as well.

    * faster xattrs on ext2/3/4 (i.e. allow userspace to make use of xattr
    without paying the performance penalty for the seeks. Alex Larsson will
    provide you with the measurement data how xattr checking is magnitudes
    slower when trying to implement a simple file list). Suggestion: provide
    a simple flag in struct stat to inform userspace whether it is worth
    looking for xattrs (i.e. think STAT_XATTRS_FOUND or STAT_XATTRS_MAYBE)

    * allow creation of read-only bind mounts in a single mount() call,
    instead of two NEW!

    * Similar, allow configuration of namespace propagation settings for
    mount points in the initial mount() syscall, instead of always requiring
    two (which is racy, and ugly, and stuff). NEW!

    memory management:
    * swappiness control as madvise() for individual memory pages NEW!

    core kernel:
    [PATCH] * hostname change notification:;a=commitdiff;h=70b932563a9514b248cc71a29bd0907bf95b4a5e NEW!

    Reviewed and probably ready-to-merge patch: NEW!

    * allow 64 bit PIDs / use 32 bit pids by default, in order to fix PID
    recycle vulnerabilities NEW!

    * allow changing argv[] of a process without mucking with environ[]:
    Something like setproctitle() or a prctl() would be ideal. Of course it
    is questionable if services like sendmail make use of this, but otoh for
    services which fork but do not immediately exec() another binary being
    able to rename this child processes in ps is of importance.

    driver model:
    * CPU modaliases in /sys/devices/system/cpu/cpuX/modalias:
    useful to allow module auto-loading of e.g. cpufreq drivers and KVM
    modules. Andy Kleen has a patch to create the alias file itself. CPU
    ‘struct sysdev’ needs to be converted to ‘struct device’ and a ‘struct
    bus_type cpu’ needs to be introduced to allow proper CPU coldplug event
    replay at bootup. This is one of the last remaining places where
    automatic hardware-triggered module auto-loading is not available. And
    we’d like to see that fix to make numerous ugly userspace work-arounds
    to achieve the same go away.

    * export ‘struct device_type fb/fbcon’ of ‘struct class graphics’
    Userspace wants to easily distinguish ‘fb’ and ‘fbcon’ from each other
    without the need to match on the device name.

    [PATCH] * expose CAP_LAST_CAP somehow in the running kernel at runtime:
    Userspace needs to know the highest valid capability of the running
    kernel, which right now cannot reliably be retrieved from header files
    only. The fact that this value cannot be detected properly right now
    creates various problems for libraries compiled on newer header files
    which are run on older kernels. They assume capabilities are available
    which actually aren’t. Specifically, libcap-ng claims that all running
    processes retain the higher capabilities in this case due to the
    “inverted” semantics of CapBnd in /proc/$PID/status.
    Dan Ballard

    * module-init-tools: provide a proper from
    Early boot tools, installers, driver install disks want to access
    information about available modules, and match devices to available
    modules to hook up driver overwrites, driver update disks, installer
    tweaks, and to optimize bootup module handling.

    * fork throttling mechanism as basic cgroup functionality that is
    available in all hierarchies independent of the controllers used:
    This is important to implement race-free killing of all members of a
    cgroup, so that cgroup member processes cannot fork faster then a cgroup
    supervisor process could kill them. This needs to be recursive, so that
    not only a cgroup but all its subgroups are covered as well.
    Patches for task_conter from Frederic Weisbecker
    Possibly use the freezer Tejun is looking into.

    * proper cgroup-is-empty notification interface:
    The current call_usermodehelper() interface is an unefficient and an
    ugly hack. Tools would prefer anything more lightweight like a netlink,
    poll() or fanotify interface.

    * allow user xattrs to be set on files in the cgroupfs (and maybe

    * allow making use of the “cpu” cgroup controller by default without
    breaking RT. Right now creating a cgroup in the “cpu” hierarchy that
    shall be able to take advantage of RT is impossible for the generic case
    since it needs an RT budget configured which is from a limited resource
    pool. What we want is the ability to create cgroups in “cpu” whose
    processes get an non-RT weight applied, but for RT take advantage of the
    parent’s RT budget. We want the separation of RT and non-RT budget
    assignment in the “cpu” hierarchy, because right now, you lose RT
    functionality in it unless you assign an RT budget. This issue severely
    limits the usefulness of “cpu” hierarchy on general purpose systems
    right now.

    * Add a timerslack cgroup controller, to allow increasing the timer
    slack of user session cgroups when the machine is idle.
    Patch from: Kirill A. Shutemov

    * simple, reliable and future-proof way to detect whether a specific pid
    is running in a CLONE_NEWUTS/CLONE_NEWPID container, i.e. not in the
    root PID namespace/UTS namespace. Currently, there are available a few
    ugly hacks to detect this (for example a process wanting to know whether
    it is running in a PID namespace could just look for a PID 2 being
    around and named kthreadd which is a kernel thread only visible in the
    root namespace), however all these solutions encode information and
    expectations that better shouldn’t be encoded in a namespace test like
    this. This functionality is needed in particular since the removal of
    the the ns cgroup controller which provided the namespace membership
    information to user code.

    * An auxiliary meta data message for AF_UNIX called SCM_CGROUPS (or
    something like that), i.e. a way to attach sender cgroup membership to
    messages sent via AF_UNIX. This is useful in case services such as
    syslog shall be shared among various containers (or service cgroups),
    and the syslog implementation needs to be able to distinguish the
    sending cgroup in order to separate the logs on disk. Of course stm
    SCM_CREDENTIALS can be used to look up the PID of the sender followed by
    a check in /proc/$PID/cgroup, but that is necessarily racy, and actually
    a very real race in real life.

    * SCM_PROCSTATUS for retrieving sender process information supplying at
    least: comm, exec, cmdline, audit session, audit loginuid.

    All time favourites:
    These items have been requested many times already, and we want to make
    sure they aren’t forgotten. We know they are hard to implement, and we
    don’t know how to get there, but nonetheless, here they are:

    * Oldie But Goldie: some kind of unionfs or union mount. A minimal
    version that supports only read-only filesystems would already be a big
    step forward. NEW!

    * revoke() NEW!

    * Notifications when non-child processes die, in an efficient way
    focussing on explicit PIDs (i.e. not taskstats) in some form (idea:
    poll() for POLLERR on /proc/$PID) NEW!

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