fork: add clone6
This adds the clone6 system call.
As mentioned several times already (cf. [7], [8]) here's the promised
patchset for clone6().
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
recycable as we have identified at least two large or widely used codebases
that currently pass this flag (cf. [2], [3], and [4]). Given that we
grabbed the last clone() flag we effectively blocked the time namespace
patchset. It just seems right that we unblock it again.
The idea is to keep clone6() 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 us for clone6():
- bump the number of available flags as much as possible while ensuring
that all flag arguments are passed in registers so they remain easily
accessible for seccomp.
- move non-flag arguments that are currently passed as separate arguments
in clone() into a dedicated struct
- 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)
- ease of transition for userspace from clone() to clone6()
- do not try to be clever or complex: keep clone6() as dumb as possible
What we came up with is clone6() which has the following signature:
struct clone6_args {
__s32 pidfd;
__aligned_u64 parent_tidptr;
__aligned_u64 child_tidptr;
__aligned_u64 stack;
__aligned_u64 stack_size;
__aligned_u64 tls;
};
long sys_clone6(struct clone6_args __user *uargs,
unsigned int flags1,
unsigned int flags2,
unsigned int flags3,
unsigned int flags4,
unsigned int flags5);
clone6() cleanly supports all of the supported flags from clone() in
flags1, i.e. flags1 is full and all legacy workloads are supported with
clone6().
With clone6() we have 160 flag values in total which - even for a feature
heavy syscall like clone - should hold quite a while. If they are really
all taken at some point we can simply bite the bullet and start adding
additional flag arguments into struct clone6 itself.
Another 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 clone6() 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
clone6().
There has been some interest in this patchset already. We have received a
patch from the CRIU corner for clone6() that would set the PID/TID of a
restored process without /proc/sys/kernel/ns_last_pid to eliminate a race.
/* References */
[1]: b3e5838252665ee4cfa76b82bdf1198dca81e5be
[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/
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian@brauner.io>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Pavel Emelyanov <xemul@virtuozzo.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
5 files changed
