RustPython · youknowone · May 22, 2026 · May 21, 2026
diff --git a/crates/stdlib/src/select.rs b/crates/stdlib/src/select.rs
@@ -5,7 +5,7 @@ pub(crate) use decl::module_def;
 use crate::vm::{
     PyObject, PyObjectRef, PyResult, TryFromObject, VirtualMachine, builtins::PyListRef,
 };
-use rustpython_host_env::select::{self as host_select, FdSet, RawFd};
+use rustpython_host_env::select::{self as host_select, FdSet, RawFd, platform::FD_SETSIZE};
 use std::io;
 
 #[derive(Traverse)]
@@ -81,8 +81,22 @@ mod decl {
 
         let seq2set = |list: &PyObject| -> PyResult<(Vec<Selectable>, FdSet)> {
             let v: Vec<Selectable> = list.try_to_value(vm)?;
+
+            let too_many_fds = cfg_select! {
+                windows => v.len() > FD_SETSIZE as usize,
+                _ => v.len() > FD_SETSIZE,
+            };
+            if too_many_fds {
+                return Err(vm.new_value_error("too many file descriptors in select()"));
+            }
+
             let mut fds = FdSet::new();
             for fd in &v {
+                #[cfg(unix)]
+                if fd.fno as usize >= FD_SETSIZE {
+                    return Err(vm.new_value_error("file descriptor out of range in select()"));
+                }
+
                 fds.insert(fd.fno);
             }
             Ok((v, fds))
@@ -97,11 +111,15 @@ mod decl {
             return Ok((empty.clone(), empty.clone(), empty));
         }
 
-        let nfds: i32 = [&mut r, &mut w, &mut x]
-            .iter_mut()
-            .filter_map(|set| set.highest())
-            .max()
-            .map_or(0, |n| n + 1) as _;
+        let nfds = cfg_select! {
+            windows => 0, // value is ignored on windows
+
+            _ => [&mut r, &mut w, &mut x]
+                .iter_mut()
+                .filter_map(|set| set.highest())
+                .max()
+                .map_or(0, |n| n + 1) as _,
+        };
 
         loop {
             let mut tv = timeout.map(host_select::sec_to_timeval);

diff --git a/extra_tests/snippets/stdlib_select.py b/extra_tests/snippets/stdlib_select.py
@@ -4,6 +4,8 @@
 
 from testutils import assert_raises
 
+TOO_MANY_SELECT_FDS = 4096
+
 
 class Nope:
     pass
@@ -42,3 +44,26 @@ def fileno(self):
     assert recvr in rres
 
 assert sendr in wres
+
+# Too many descriptors for select.select()
+if sys.platform != "win32":
+    import resource
+
+    soft_max_fds, hard_max_fds = resource.getrlimit(resource.RLIMIT_NOFILE)
+    if soft_max_fds != resource.RLIM_INFINITY:
+        # 100 additional fds should be enough for interpreter needs
+        need_fds = TOO_MANY_SELECT_FDS + 100
+
+        soft_max_fds = max(soft_max_fds, need_fds)
+        if hard_max_fds != resource.RLIM_INFINITY:
+            assert hard_max_fds >= soft_max_fds, (
+                "Not enough file descriptors for this test"
+            )
+        resource.setrlimit(resource.RLIMIT_NOFILE, (soft_max_fds, hard_max_fds))
+sockets = [s for _ in range(TOO_MANY_SELECT_FDS // 2) for s in socket.socketpair()]
+assert_raises(ValueError, select.select, sockets, [], [], 0)
+del sockets
+a, b = socket.socketpair()
+# CPython disallows this on *nix systems too.
+assert_raises(ValueError, select.select, [a] * TOO_MANY_SELECT_FDS, [], [], 0)
+del a, b