Re: [musl] Calling setxid() in a vfork()-child

[Markus Wichmann <nullplan@gmx.net>]

On Mon, Oct 12, 2020 at 11:30:58PM +0300, Alexey Izbyshev wrote:
> Unfortunately, while posix_spawn() + a helper executable would be fine in
> many cases (more so for applications than libraries), addition of a helper
> to an existing library exposing a process creation API that can't be
> implemented in terms of posix_spawn() may be not straightforward. If the
> helper is just an executable file, we'll need to locate it, which may be
> simply impossible when our library is called if, say, switching mount
> namespaces is involved (which may be out of control of our library).
> Solutions may exist (locate and open() at startup or memfd_create(), and
> then execveat()?), but vfork() (+ preventing execution of signal handlers in
> the child) seems so much simpler, and doesn't add the overhead of an extra
> execve() too.
>
> Alexey
>

If dropping privileges is all you want, then posix_spawn() has a flag
for that. And if you are foregoing portability anyway by doing anything
between vfork() and execve(), might as well use clone() and do it
properly.

Yes, locating a helper binary is a bit of a problem. I've been recently
struggling with that myself. While you can make the binary part of the
main application (or library, in your case), using posix_spawn() means
the binary must have a path name, so using tempfile() is out of the
question (unless you use the proc file name, which is, again,
non-portable).

Ciao,
Markus