• Christian Brauner's avatar
    fork: add clone3 · 7f192e3c
    Christian Brauner authored
    This adds the clone3 system call.
    As mentioned several times already (cf. [7], [8]) here's the promised
    patchset for clone3().
    We recently merged the CLONE_PIDFD patchset (cf. [1]). It took the last
    free flag from clone().
    Independent of the CLONE_PIDFD patchset a time namespace has been discussed
    at Linux Plumber Conference last year and has been sent out and reviewed
    (cf. [5]). It is expected that it will go upstream in the not too distant
    future. However, it relies on the addition of the CLONE_NEWTIME flag to
    clone(). The only other good candidate - CLONE_DETACHED - is currently not
    recyclable as we have identified at least two large or widely used
    codebases that currently pass this flag (cf. [2], [3], and [4]). Given that
    CLONE_PIDFD grabbed the last clone() flag the time namespace is effectively
    blocked. clone3() has the advantage that it will unblock this patchset
    again. In general, clone3() is extensible and allows for the implementation
    of new features.
    The idea is to keep clone3() very simple and close to the original clone(),
    specifically, to keep on supporting old clone()-based workloads.
    We know there have been various creative proposals how a new process
    creation syscall or even api is supposed to look like. Some people even
    going so far as to argue that the traditional fork()+exec() split should be
    abandoned in favor of an in-kernel version of spawn(). Independent of
    whether or not we personally think spawn() is a good idea this patchset has
    and does not want to have anything to do with this.
    One stance we take is that there's no real good alternative to
    clone()+exec() and we need and want to support this model going forward;
    independent of spawn().
    The following requirements guided clone3():
    - bump the number of available flags
    - move arguments that are currently passed as separate arguments
      in clone() into a dedicated struct clone_args
      - choose a struct layout that is easy to handle on 32 and on 64 bit
      - choose a struct layout that is extensible
      - give new flags that currently need to abuse another flag's dedicated
        return argument in clone() their own dedicated return argument
        (e.g. CLONE_PIDFD)
      - use a separate kernel internal struct kernel_clone_args that is
        properly typed according to current kernel conventions in fork.c and is
        different from  the uapi struct clone_args
    - port _do_fork() to use kernel_clone_args so that all process creation
      syscalls such as fork(), vfork(), clone(), and clone3() behave identical
      (Arnd suggested, that we can probably also port do_fork() itself in a
       separate patchset.)
    - ease of transition for userspace from clone() to clone3()
      This very much means that we do *not* remove functionality that userspace
      currently relies on as the latter is a good way of creating a syscall
      that won't be adopted.
    - do not try to be clever or complex: keep clone3() as dumb as possible
    In accordance with Linus suggestions (cf. [11]), clone3() has the following
    /* uapi */
    struct clone_args {
            __aligned_u64 flags;
            __aligned_u64 pidfd;
            __aligned_u64 child_tid;
            __aligned_u64 parent_tid;
            __aligned_u64 exit_signal;
            __aligned_u64 stack;
            __aligned_u64 stack_size;
            __aligned_u64 tls;
    /* kernel internal */
    struct kernel_clone_args {
            u64 flags;
            int __user *pidfd;
            int __user *child_tid;
            int __user *parent_tid;
            int exit_signal;
            unsigned long stack;
            unsigned long stack_size;
            unsigned long tls;
    long sys_clone3(struct clone_args __user *uargs, size_t size)
    clone3() cleanly supports all of the supported flags from clone() and thus
    all legacy workloads.
    The advantage of sticking close to the old clone() is the low cost for
    userspace to switch to this new api. Quite a lot of userspace apis (e.g.
    pthreads) are based on the clone() syscall. With the new clone3() syscall
    supporting all of the old workloads and opening up the ability to add new
    features should make switching to it for userspace more appealing. In
    essence, glibc can just write a simple wrapper to switch from clone() to
    There has been some interest in this patchset already. We have received a
    patch from the CRIU corner for clone3() that would set the PID/TID of a
    restored process without /proc/sys/kernel/ns_last_pid to eliminate a race.
    /* User visible differences to legacy clone() */
    - CLONE_DETACHED will cause EINVAL with clone3()
    - CSIGNAL is deprecated
      It is superseeded by a dedicated "exit_signal" argument in struct
      clone_args freeing up space for additional flags.
      This is based on a suggestion from Andrei and Linus (cf. [9] and [10])
    /* References */
    [1]: b3e58382
    [2]: https://dxr.mozilla.org/mozilla-central/source/security/sandbox/linux/SandboxFilter.cpp#343
    [3]: https://git.musl-libc.org/cgit/musl/tree/src/thread/pthread_create.c#n233
    [4]: https://sources.debian.org/src/blcr/0.8.5-2.3/cr_module/cr_dump_self.c/?hl=740#L740
    [5]: https://lore.kernel.org/lkml/20190425161416.26600-1-dima@arista.com/
    [6]: https://lore.kernel.org/lkml/20190425161416.26600-2-dima@arista.com/
    [7]: https://lore.kernel.org/lkml/CAHrFyr5HxpGXA2YrKza-oB-GGwJCqwPfyhD-Y5wbktWZdt0sGQ@mail.gmail.com/
    [8]: https://lore.kernel.org/lkml/20190524102756.qjsjxukuq2f4t6bo@brauner.io/
    [9]: https://lore.kernel.org/lkml/20190529222414.GA6492@gmail.com/
    [10]: https://lore.kernel.org/lkml/CAHk-=whQP-Ykxi=zSYaV9iXsHsENa+2fdj-zYKwyeyed63Lsfw@mail.gmail.com/
    [11]: https://lore.kernel.org/lkml/CAHk-=wieuV4hGwznPsX-8E0G2FKhx3NjZ9X3dTKh5zKd+iqOBw@mail.gmail.com/
    Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarChristian Brauner <christian@brauner.io>
    Acked-by: default avatarArnd Bergmann <arnd@arndb.de>
    Acked-by: default avatarSerge Hallyn <serge@hallyn.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Pavel Emelyanov <xemul@virtuozzo.com>
    Cc: Jann Horn <jannh@google.com>
    Cc: David Howells <dhowells@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Adrian Reber <adrian@lisas.de>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Andrei Vagin <avagin@gmail.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Florian Weimer <fweimer@redhat.com>
    Cc: linux-api@vger.kernel.org
syscalls.h 54.7 KB