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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 30 Apr 2019 09:26:57 -0700
Message-ID: <CAHk-=wgxqoH=Cztd7W67pYJyZwPPNtY5P7K66u2QTDEpEYHVAw@mail.gmail.com>
Subject: Re: RFC: on adding new CLONE_* flags [WAS Re: [PATCH 0/4] clone: add CLONE_PIDFD]
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Jann Horn <jannh@google.com>, Kevin Easton <kevin@guarana.org>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Enrico Weigelt, metux IT consult" <lkml@metux.net>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Howells <dhowells@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Daniel Colascione <dancol@google.com>
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On Tue, Apr 30, 2019 at 9:19 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Of course, if you *don't* need the exact vfork() semantics, clone
> itself actually very much supports a callback model with s separate
> stack. You can basically do this:
>
>  - allocate new stack for the child
>  - in trivial asm wrapper, do:
>     - push the callback address on the child stack
>     - clone(CLONE_VFORK|CLONE_VM|CLONE_SIGCHLD, chld_stack, NULL, NULL,NULL)
>     - "ret"
>  - free new stack
>
> where the "ret" in the child will just go to the callback, while the
> parent (eventually) just returns from the trivial wrapper and frees
> the new stack (which by definition is no longer used, since the child
> has exited or execve'd.

In fact, Florian, maybe this is the solution to your "I want to use
vfork for posix_spawn(), but I don't know if I can trust it" problem.

Just use clone() directly. On WSL it will presumably just fail, and
you can then fall back on doing the slow stupid
fork+pipes-to-communicate.

On valgrind, I don't know what will happen. Maybe it will just do an
unchecked posix_spawn() because valgrind doesn't catch it?

                  Linus