Re: RFC: on adding new CLONE_* flags [WAS Re: [PATCH 0/4] clone: add CLONE_PIDFD]

[Linus Torvalds]

On Tue, Apr 30, 2019 at 9:19 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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