Handle negative P(G)IDs via cast_(un)signed

kernel/src/syscall/setpgid.rs

@@ -12,6 +12,10 @@ pub fn sys_setpgid(pid: Pid, pgid: Pgid, ctx: &Context) -> Result<SyscallReturn>
     // The documentation quoted below is from
     // <https://www.man7.org/linux/man-pages/man2/setpgid.2.html>.
 
+    if pid.cast_signed() < 0 || pgid.cast_signed() < 0 {
+        return_errno_with_message!(Errno::EINVAL, "negative PIDs or PGIDs are not valid");
+    }
+
     // "If `pid` is zero, then the process ID of the calling process is used."
     let pid = if pid == 0 { current.pid() } else { pid };
     // "If `pgid` is zero, then the PGID of the process specified by `pid` is made the same as its

kernel/src/syscall/waitid.rs

@@ -15,14 +15,16 @@ pub fn sys_waitid(
     ctx: &Context,
 ) -> Result<SyscallReturn> {
     // FIXME: what does infoq and rusage use for?
-    let process_filter = ProcessFilter::from_which_and_id(which, upid)?;
+    let process_filter = ProcessFilter::from_which_and_id(which, upid as _)?;
     let wait_options = WaitOptions::from_bits(options as u32)
         .ok_or(Error::with_message(Errno::EINVAL, "invalid options"))?;
+
     let waited_process =
         wait_child_exit(process_filter, wait_options, ctx).map_err(|err| match err.error() {
             Errno::EINTR => Error::new(Errno::ERESTARTSYS),
             _ => err,
         })?;
+
     let pid = waited_process.map_or(0, |process| process.pid());
     Ok(SyscallReturn::Return(pid as _))
 }