So you could use this to implement fork without sharing signal handlers and files? Given that the latter two seem like the biggest issue with fork why is - to my knowledge - nobody using this?
Numerous people are, though not via raw assembly. You can call clone() via glibc and pass all those same flags; you can also call unshare() from the child process. Take a look at the manpage for clone(), which discusses the difference between the raw syscall and the glibc wrapper. The former works like fork(), so if you use it to spawn a thread rather than a process, you need an assembly wrapper to handle running on a new stack and similar after it returns; the glibc wrapper handles that for you, calling a function pointer you provide:
/* Prototype for the glibc wrapper function */
#include <sched.h>
int clone(int (*fn)(void *), void *child_stack,
int flags, void *arg, ...
/* pid_t *ptid, struct user_desc *tls, pid_t *ctid */ );
/* Prototype for the raw system call */
long clone(unsigned long flags, void *child_stack,
void *ptid, void *ctid,
struct pt_regs *regs);
clone() of a separate process with the various namespace flags (such as creating new network or filesystem namespaces) forms the basis of all Linux container solutions (including Docker and Rocket). clone() of a thread with unusual namespace flags is less common, but not unheard-of.
At least for files, it's also common to just open your files with O_CLOEXEC these days, which (if you're consistent about doing it) protects you in case one of your app's libraries forks, and allows you to avoid dropping down to clone().
> So you could use this to implement fork without sharing signal handlers and files?
I think the terminology is a bit confusing here. You can share signal handlers and files, that is, changes in the child are reflected in the parent and vice versa, or you can inherit signal handlers and files: the child starts with a copy of the parent's signal handlers and file descriptor table. Inheritance is the default behavior from fork(). There is no option to reset signal handlers or start with a clean file descriptor table; you have to reset everything yourself.
