Date: Tue, 13 Oct 2020 12:07:45 -0400 From: Rich Felker <dalias@...c.org> To: Alexey Izbyshev <izbyshev@...ras.ru> Cc: musl@...ts.openwall.com Subject: Re: Calling setxid() in a vfork()-child 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
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