From f7dc760dd7a2dd8ba5a133e396e717b7886059f4 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Mon, 4 Nov 2024 01:41:14 -0500
Subject: kernel: descrip: Add support for fd duplication

Sometimes we may need to duplicate a file descriptor and create a new
fdno that refers to the same vnode. The fd_dup() function enables us
to do this when needed.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/include/sys/filedesc.h |  2 ++
 sys/kern/kern_descrip.c    | 23 +++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/sys/include/sys/filedesc.h b/sys/include/sys/filedesc.h
index f5f24db..94946a0 100644
--- a/sys/include/sys/filedesc.h
+++ b/sys/include/sys/filedesc.h
@@ -48,6 +48,8 @@ int fd_read(unsigned int fd, void *buf, size_t count);
 
 int fd_alloc(struct filedesc **fd_out);
 int fd_open(const char *pathname, int flags);
+
+int fd_dup(int fd);
 struct filedesc *fd_get(unsigned int fdno);
 
 #endif  /* !_SYS_FILEDESC_H_ */
diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index c3d9b5c..da10f89 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -226,3 +226,26 @@ fd_open(const char *pathname, int flags)
     filedes->vp = nd.vp;
     return filedes->fdno;
 }
+
+/*
+ * Duplicate a file descriptor. New file descriptor
+ * points to the same vnode.
+ */
+int
+fd_dup(int fd)
+{
+    int error;
+    struct filedesc *new_desc, *tmp;
+
+    tmp = fd_get(fd);
+    if (tmp == NULL)
+        return -EBADF;
+
+    if ((error = fd_alloc(&new_desc)) != 0)
+        return error;
+
+    /* Ref that vnode before we point to it */
+    vfs_vref(tmp->vp);
+    new_desc->vp = tmp->vp;
+    return new_desc->fdno;
+}
-- 
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