clone (nr 56)

Linux Signature

long clone(unsigned long flags, void *child_stack, int *ptid, int *ctid, unsigned long tls);

Description

Creates a new process. ostoo supports the specific flag combination used by musl’s posix_spawn: CLONE_VM | CLONE_VFORK | SIGCHLD (0x4111). The child shares the parent’s address space and the parent blocks until the child calls execve or _exit.

Current Implementation

Only the flag combination CLONE_VM | CLONE_VFORK | SIGCHLD is accepted. Other flag combinations return -ENOSYS.

1. Validate arguments: child_stack must be non-zero. Unsupported flags return -ENOSYS.
2. Read parent state: Copies pml4_phys, cwd, fd_table (Arc clones), brk_*, mmap_* from the parent process.
3. Capture user registers: Reads user_rip, user_rflags, and user_r9 from PerCpuData (saved by the SYSCALL entry stub). These are needed so the child can “return from syscall” at the same instruction as the parent.
4. Create child process: New PID, same pml4_phys as parent (CLONE_VM), inherited fd table and cwd. Sets vfork_parent_thread to the parent’s scheduler thread index.
5. Spawn clone thread: Creates a scheduler thread via spawn_clone_thread that enters clone_trampoline. The trampoline sets up kernel state and drops to ring 3 at user_rip with RAX=0 (child return value) and R9=user_r9 (musl’s __clone fn pointer).
6. Block parent: Calls block_current_thread() (CLONE_VFORK semantics). The parent is unblocked when the child calls execve or _exit.
7. Return: After unblocking, returns the child’s PID to the parent.

Source: osl/src/clone.rs — sys_clone, libkernel/src/task/scheduler.rs — spawn_clone_thread, clone_trampoline

Usage from C (musl)

Not called directly — musl’s posix_spawn uses it internally:

#include <spawn.h>
#include <sys/wait.h>

pid_t child;
int err = posix_spawn(&child, "/hello", NULL, NULL, argv, envp);
if (err == 0) {
    int status;
    waitpid(child, &status, 0);
}

Errors

Design Notes

• musl’s __clone assembly stores the child function pointer in R9 before syscall. The entry stub saves R9 to PerCpuData.user_r9 (offset 32), and clone_trampoline restores it via jump_to_userspace(rax=0, r9=user_r9).
• The child shares the parent’s PML4 (CLONE_VM). After execve, the child gets a fresh PML4. The old shared PML4 continues to be used by the parent.