bsd-user: Implement rfork(2) system call.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230925182425.3163-28-kariem.taha2.7@gmail.com>

bsd-user/freebsd/os-proc.h

@@ -219,4 +219,43 @@ static inline abi_long do_freebsd_vfork(void *cpu_env)
     return do_freebsd_fork(cpu_env);
 }
 
+/* rfork(2) */
+static inline abi_long do_freebsd_rfork(void *cpu_env, abi_long flags)
+{
+    abi_long ret;
+    abi_ulong child_flag;
+
+    /*
+     * XXX We need to handle RFMEM here, as well.  Neither are safe to execute
+     * as-is on x86 hosts because they'll split memory but not the stack,
+     * wreaking havoc on host architectures that use the stack to store the
+     * return address as both threads try to pop it off.  Rejecting RFSPAWN
+     * entirely for now is ok, the only consumer at the moment is posix_spawn
+     * and it will fall back to classic vfork(2) if we return EINVAL.
+     */
+    if ((flags & TARGET_RFSPAWN) != 0) {
+        return -TARGET_EINVAL;
+    }
+    fork_start();
+    ret = rfork(flags);
+    if (ret == 0) {
+        /* child */
+        child_flag = 1;
+        target_cpu_clone_regs(cpu_env, 0);
+    } else {
+        /* parent */
+        child_flag = 0;
+    }
+
+    /*
+     * The fork system call sets a child flag in the second return
+     * value: 0 for parent process, 1 for child process.
+     */
+    set_second_rval(cpu_env, child_flag);
+    fork_end(child_flag);
+
+    return ret;
+
+}
+
 #endif /* BSD_USER_FREEBSD_OS_PROC_H */

bsd-user/freebsd/os-syscall.c

@@ -234,6 +234,10 @@ static abi_long freebsd_syscall(void *cpu_env, int num, abi_long arg1,
         ret = do_freebsd_vfork(cpu_env);
         break;
 
+    case TARGET_FREEBSD_NR_rfork: /* rfork(2) */
+        ret = do_freebsd_rfork(cpu_env, arg1);
+        break;
+
     case TARGET_FREEBSD_NR_execve: /* execve(2) */
         ret = do_freebsd_execve(arg1, arg2, arg3);
         break;