Add a "fork" of musl as //fusl.

fusl/src/process/posix_spawn.c

+#define _GNU_SOURCE
+#include <spawn.h>
+#include <sched.h>
+#include <unistd.h>
+#include <signal.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+#include "syscall.h"
+#include "pthread_impl.h"
+#include "fdop.h"
+#include "libc.h"
+
+struct args {
+        int p[2];
+        sigset_t oldmask;
+        const char *path;
+        int (*exec)(const char *, char *const *, char *const *);
+        const posix_spawn_file_actions_t *fa;
+        const posix_spawnattr_t *restrict attr;
+        char *const *argv, *const *envp;
+};
+
+void __get_handler_set(sigset_t *);
+
+static int __sys_dup2(int old, int new)
+{
+#ifdef SYS_dup2
+        return __syscall(SYS_dup2, old, new);
+#else
+        if (old==new) {
+                int r = __syscall(SYS_fcntl, old, F_GETFD);
+                return r<0 ? r : old;
+        } else {
+                return __syscall(SYS_dup3, old, new, 0);
+        }
+#endif
+}
+
+static int child(void *args_vp)
+{
+        int i, ret;
+        struct sigaction sa = {0};
+        struct args *args = args_vp;
+        int p = args->p[1];
+        const posix_spawn_file_actions_t *fa = args->fa;
+        const posix_spawnattr_t *restrict attr = args->attr;
+        sigset_t hset;
+
+        close(args->p[0]);
+
+        /* All signal dispositions must be either SIG_DFL or SIG_IGN
+         * before signals are unblocked. Otherwise a signal handler
+         * from the parent might get run in the child while sharing
+         * memory, with unpredictable and dangerous results. To
+         * reduce overhead, sigaction has tracked for us which signals
+         * potentially have a signal handler. */
+        __get_handler_set(&hset);
+        for (i=1; i<_NSIG; i++) {
+                if ((attr->__flags & POSIX_SPAWN_SETSIGDEF)
+                     && sigismember(&attr->__def, i)) {
+                        sa.sa_handler = SIG_DFL;
+                } else if (sigismember(&hset, i)) {
+                        if (i-32<3U) {
+                                sa.sa_handler = SIG_IGN;
+                        } else {
+                                __libc_sigaction(i, 0, &sa);
+                                if (sa.sa_handler==SIG_IGN) continue;
+                                sa.sa_handler = SIG_DFL;
+                        }
+                } else {
+                        continue;
+                }
+                __libc_sigaction(i, &sa, 0);
+        }
+
+        if (attr->__flags & POSIX_SPAWN_SETPGROUP)
+                if ((ret=__syscall(SYS_setpgid, 0, attr->__pgrp)))
+                        goto fail;
+
+        /* Use syscalls directly because the library functions attempt
+         * to do a multi-threaded synchronized id-change, which would
+         * trash the parent's state. */
+        if (attr->__flags & POSIX_SPAWN_RESETIDS)
+                if ((ret=__syscall(SYS_setgid, __syscall(SYS_getgid))) ||
+                    (ret=__syscall(SYS_setuid, __syscall(SYS_getuid))) )
+                        goto fail;
+
+        if (fa && fa->__actions) {
+                struct fdop *op;
+                int fd;
+                for (op = fa->__actions; op->next; op = op->next);
+                for (; op; op = op->prev) {
+                        /* It's possible that a file operation would clobber
+                         * the pipe fd used for synchronizing with the
+                         * parent. To avoid that, we dup the pipe onto
+                         * an unoccupied fd. */
+                        if (op->fd == p) {
+                                ret = __syscall(SYS_dup, p);
+                                if (ret < 0) goto fail;
+                                __syscall(SYS_close, p);
+                                p = ret;
+                        }
+                        switch(op->cmd) {
+                        case FDOP_CLOSE:
+                                __syscall(SYS_close, op->fd);
+                                break;
+                        case FDOP_DUP2:
+                                if ((ret=__sys_dup2(op->srcfd, op->fd))<0)
+                                        goto fail;
+                                break;
+                        case FDOP_OPEN:
+                                fd = __sys_open(op->path, op->oflag, op->mode);
+                                if ((ret=fd) < 0) goto fail;
+                                if (fd != op->fd) {
+                                        if ((ret=__sys_dup2(fd, op->fd))<0)
+                                                goto fail;
+                                        __syscall(SYS_close, fd);
+                                }
+                                break;
+                        }
+                }
+        }
+
+        /* Close-on-exec flag may have been lost if we moved the pipe
+         * to a different fd. We don't use F_DUPFD_CLOEXEC above because
+         * it would fail on older kernels and atomicity is not needed --
+         * in this process there are no threads or signal handlers. */
+        __syscall(SYS_fcntl, p, F_SETFD, FD_CLOEXEC);
+
+        pthread_sigmask(SIG_SETMASK, (attr->__flags & POSIX_SPAWN_SETSIGMASK)
+                ? &attr->__mask : &args->oldmask, 0);
+
+        args->exec(args->path, args->argv, args->envp);
+        ret = -errno;
+
+fail:
+        /* Since sizeof errno < PIPE_BUF, the write is atomic. */
+        ret = -ret;
+        if (ret) while (__syscall(SYS_write, p, &ret, sizeof ret) < 0);
+        _exit(127);
+}
+
+
+int __posix_spawnx(pid_t *restrict res, const char *restrict path,
+        int (*exec)(const char *, char *const *, char *const *),
+        const posix_spawn_file_actions_t *fa,
+        const posix_spawnattr_t *restrict attr,
+        char *const argv[restrict], char *const envp[restrict])
+{
+        pid_t pid;
+        char stack[1024];
+        int ec=0, cs;
+        struct args args;
+
+        if (pipe2(args.p, O_CLOEXEC))
+                return errno;
+
+        pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+
+        args.path = path;
+        args.exec = exec;
+        args.fa = fa;
+        args.attr = attr ? attr : &(const posix_spawnattr_t){0};
+        args.argv = argv;
+        args.envp = envp;
+        pthread_sigmask(SIG_BLOCK, SIGALL_SET, &args.oldmask);
+
+        pid = __clone(child, stack+sizeof stack,
+                CLONE_VM|CLONE_VFORK|SIGCHLD, &args);
+        close(args.p[1]);
+
+        if (pid > 0) {
+                if (read(args.p[0], &ec, sizeof ec) != sizeof ec) ec = 0;
+                else waitpid(pid, &(int){0}, 0);
+        } else {
+                ec = -pid;
+        }
+
+        close(args.p[0]);
+
+        if (!ec && res) *res = pid;
+
+        pthread_sigmask(SIG_SETMASK, &args.oldmask, 0);
+        pthread_setcancelstate(cs, 0);
+
+        return ec;
+}
+
+int posix_spawn(pid_t *restrict res, const char *restrict path,
+        const posix_spawn_file_actions_t *fa,
+        const posix_spawnattr_t *restrict attr,
+        char *const argv[restrict], char *const envp[restrict])
+{
+        return __posix_spawnx(res, path, execve, fa, attr, argv, envp);
+}