Re: Re: Hangup calling setuid() from vfork() child

[Markus Wichmann <nullplan@gmx.net>]

On Mon, Sep 30, 2019 at 12:57:34PM -0700, Joshua Hudson wrote:
> >It's simpler than that. The (retired) specification for vfork did not
> >allow anything but _exit or execve in the child after vfork, so the
> >issue doesn't arise and it works perfectly fine with threads as long
> >as you follow the requirement.

I remembered that while making dinner (after sending my first response).

>
> I'm reading the man page for vfork and it says what it actually does, that
> is overlay the child process on the memory of the calling process.
>

I don't know about you, but my manpage quite clearly states that vfork()
is equivalent to clone(CLONE_VM | CLONE_VFORK | SIGCHLD), that is:
Parent and child share memory, parent (only the calling thread) is
suspended until child execs or exits, and when it does, the child gets a
SIGCHLD.

If the child process changes anything in memory, that is reflected in
the parent. Basically, the vfork() child is in an invalid state and this
cannot be repaired without damaging the parent.

> posix_spawn can't be used in the originating location, and fork() is
> hogging too much memory.

fork() only "hogs" that memory which either parent or child modify
afterwards. You wish to use vfork(), so I guess the child process won't
go long before either exec or exit(), right? So you might want to enable
memory overcommit.

I don't know about your application, but your options are:

- Decouple the child part into another program proper, and use
  posix_spawn() to call it.
- Use fork() and eat the memory cost.
- Use clone() and eat the non-portability. Note that clone(CLONE_VFORK)
  has identical semantics to vfork(), so no calling setuid() there,
  either.

Ciao,
Markus