From 96fe0378b7c183aebb4ba27743ba2e9843fcdd8a Mon Sep 17 00:00:00 2001
From: Ashwin Naren <arihant2math@gmail.com>
Date: Sun, 11 Jan 2026 22:05:47 -0800
Subject: [PATCH] implement F_DUPFD for sys_fcntl

---
 src/process/fd_table.rs       | 27 +++++++++++++++++++++++++--
 src/process/fd_table/dup.rs   | 15 ++++++++++++---
 src/process/fd_table/fcntl.rs |  6 +++---
 3 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/src/process/fd_table.rs b/src/process/fd_table.rs
index 2f11172..eda2471 100644
--- a/src/process/fd_table.rs
+++ b/src/process/fd_table.rs
@@ -88,8 +88,8 @@ impl FileDescriptorTable {
         Ok(fd)
     }
 
-    // Insert the given etnry at the specified index. If there was an entry at
-    // that index `Some(entry)` is returned. Otherwise, `None` is returned.
+    /// Insert the given entry at the specified index. If there was an entry at
+    /// that index `Some(entry)` is returned. Otherwise, `None` is returned.
     fn insert_at(&mut self, fd: Fd, entry: FileDescriptorEntry) -> Option<FileDescriptorEntry> {
         let fd_idx = fd.0 as usize;
 
@@ -101,6 +101,29 @@ impl FileDescriptorTable {
         self.entries[fd_idx].replace(entry)
     }
 
+    /// Insert the given entry at or above the specified index, returning the
+    /// file descriptor used.
+    fn insert_above(&mut self, min_fd: Fd, file: Arc<OpenFile>) -> Result<Fd> {
+        let start_idx = min_fd.0 as usize;
+        let entry = FileDescriptorEntry {
+            file,
+            flags: FdFlags::default(),
+        };
+
+        for i in start_idx..self.entries.len() {
+            if self.entries[i].is_none() {
+                let fd = Fd(i as i32);
+                self.insert_at(fd, entry);
+                return Ok(fd);
+            }
+        }
+
+        // No free slot found, so we need to expand the table.
+        let fd = Fd(self.entries.len() as i32);
+        self.entries.push(Some(entry));
+        Ok(fd)
+    }
+
     /// Removes a file descriptor from the table, returning the file if it
     /// existed.
     pub fn remove(&mut self, fd: Fd) -> Option<Arc<OpenFile>> {
diff --git a/src/process/fd_table/dup.rs b/src/process/fd_table/dup.rs
index 22b38fd..b879e60 100644
--- a/src/process/fd_table/dup.rs
+++ b/src/process/fd_table/dup.rs
@@ -6,13 +6,22 @@ use libkernel::{
 
 use super::{Fd, FdFlags, FileDescriptorEntry};
 
-pub fn sys_dup(fd: Fd) -> Result<usize> {
+pub fn dup_fd(fd: Fd, min_fd: Option<Fd>) -> Result<Fd> {
     let task = current_task();
     let mut files = task.fd_table.lock_save_irq();
 
-    let fd = files.get(fd).ok_or(KernelError::BadFd)?;
+    let file = files.get(fd).ok_or(KernelError::BadFd)?;
 
-    let new_fd = files.insert(fd.clone())?;
+    let new_fd = match min_fd {
+        Some(min_fd) => files.insert_above(min_fd, file.clone())?,
+        None => files.insert(file.clone())?,
+    };
+
+    Ok(new_fd)
+}
+
+pub fn sys_dup(fd: Fd) -> Result<usize> {
+    let new_fd = dup_fd(fd, None)?;
 
     Ok(new_fd.as_raw() as _)
 }
diff --git a/src/process/fd_table/fcntl.rs b/src/process/fd_table/fcntl.rs
index 0ffe3f4..f3d08b0 100644
--- a/src/process/fd_table/fcntl.rs
+++ b/src/process/fd_table/fcntl.rs
@@ -1,9 +1,9 @@
 use bitflags::Flags;
 use libkernel::error::{KernelError, Result};
 
-use crate::{process::fd_table::FdFlags, sched::current::current_task_shared};
-
 use super::Fd;
+use crate::process::fd_table::dup::dup_fd;
+use crate::{process::fd_table::FdFlags, sched::current::current_task_shared};
 
 const F_DUPFD: u32 = 0; // Duplicate file descriptor.
 const F_GETFD: u32 = 1; // Get file descriptor flags.
@@ -15,7 +15,7 @@ pub async fn sys_fcntl(fd: Fd, op: u32, arg: usize) -> Result<usize> {
     let task = current_task_shared();
 
     match op {
-        F_DUPFD => todo!(),
+        F_DUPFD => dup_fd(fd, Some(Fd(arg as i32))).map(|new_fd| new_fd.as_raw() as _),
         F_GETFD => {
             let fds = task.fd_table.lock_save_irq();
             let fd = fds