On Tue, Oct 29, 2019 at 12:27:07PM +0100, Christian Brauner wrote: > On Mon, Oct 28, 2019 at 08:09:13PM +0100, Jann Horn wrote: > > On Mon, Oct 28, 2019 at 6:21 PM Christian Brauner > > <christian.brauner@xxxxxxxxxx> wrote: > > > On Mon, Oct 28, 2019 at 04:12:09PM +0100, Jann Horn wrote: > > > > On Fri, Oct 25, 2019 at 6:59 PM Michael Kerrisk (man-pages) > > > > <mtk.manpages@xxxxxxxxx> wrote: > > > > > I've made a first shot at adding documentation for clone3(). You can > > > > > see the diff here: > > > > > https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=faa0e55ae9e490d71c826546bbdef954a1800969 > > [...] > > > > You might want to note somewhere that its flags can't be > > > > seccomp-filtered because they're stored in memory, making it > > > > inappropriate to use in heavily sandboxed processes. > > > > > > Hm, I don't think that belongs on the clone manpage. Granted that > > > process creation is an important syscall but so are a bunch of others > > > that aren't filterable because of pointer arguments. > > > We can probably mention on the seccomp manpage that seccomp can't filter > > > on pointer arguments and then provide a list of examples. If you setup a > > > seccomp filter and don't know that you can't filter syscalls with > > > pointer args that seems pretty bad to begin with. > > > > Fair enough. > > > > [...] > > > One thing I never liked about clone() was that userspace had to know > > > about stack direction. And there is a lot of ugly code in userspace that > > > has nasty clone() wrappers like: > > [...] > > > where stack + stack_size is addition on a void pointer which usually > > > clang and gcc are not very happy about. > > > I wanted to bring this up on the mailing list soon: If possible, I don't > > > want userspace to need to know about stack direction and just have stack > > > point to the beginning and then have the kernel do the + stack_size > > > after the copy_clone_args_from_user() if the arch needs it. For example, > > > by having a dumb helder similar to copy_thread_tls()/coyp_thread() that > > > either does the + stack_size or not. Right now, clone3() is supported on > > > parisc and afaict, the stack grows upwards for it. I'm not sure if there > > > are obvious reasons why that won't work or it would be a bad idea... > > > > That would mean adding a new clone flag that redefines how those > > parameters work and describing the current behavior in the manpage as > > the behavior without the flag (which doesn't exist on 5.3), right? > > I would break API and if someone reports breakage we'll revert and go > the more complicated route you outlined (see [1]). @Jann, I think the following patch might even be enough?... @Florian, do you have an opinion about always passing the stack from the lowest address with clone3()? >From 72b2a5711fd37e34e87df1b29b2e1885bb28cf75 Mon Sep 17 00:00:00 2001 From: Christian Brauner <christian.brauner@xxxxxxxxxx> Date: Tue, 29 Oct 2019 13:55:39 +0100 Subject: [PATCH] fork: stack direction Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> --- kernel/fork.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/fork.c b/kernel/fork.c index bcdf53125210..22dc72071a6d 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2584,6 +2584,13 @@ static bool clone3_args_valid(const struct kernel_clone_args *kargs) return true; } +static inline void clone3_prepare_stack(struct kernel_clone_args *kargs) +{ +#if !defined(CONFIG_STACK_GROWSUP) && !defined(CONFIG_IA64) + kargs->stack += kargs->stack_size; +#endif +} + /** * clone3 - create a new process with specific properties * @uargs: argument structure @@ -2605,6 +2612,8 @@ SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size) if (err) return err; + clone3_prepare_stack(&kargs); + if (!clone3_args_valid(&kargs)) return -EINVAL; -- 2.23.0
