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

[Rich Felker <dalias@libc.org>]

On Tue, Oct 13, 2020 at 06:24:45PM +0300, Alexey Izbyshev wrote:
> On 2020-10-13 05:47, Markus Wichmann wrote:
> >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.
> >
> What do you mean by "do it properly"? Unless you mean doing
> syscalls, it seems that I'd have the same issues with clone() (with
> CLONE_VFORK, since I'm trying to avoid copying of page tables) as I
> do with vfork(). Namely, I'd still have to care about signals, and I
> wouldn't be able to safely call setxid() (and, frankly, anything
> else from a C library if we want a solution that's, while being
> Linux-specific, still portable across C libraries).

Indeed, it's not safe to call libc functions from a CLONE_VM context.
We might want to make some sort of contract about a subset that are
safe to call, but right now there really isn't such a set. AS-safe
functions might be close, and indeed after the __synccall change
set*id should in theory work from a clone() context too.

Really, unless you're trying to support NOMMU, "do it properly" means
just forgetting about CLONE_VM if posix_spawn doesn't meet your needs
and using plain fork+...+exec.

Rich