From: Felix Fietkau <nbd@nbd.name>
Date: Wed, 8 Oct 2025 22:15:42 +0200
Subject: [PATCH] fs: add dup2() function

Add dup2() function to duplicate file descriptors, useful for redirecting
standard streams in child processes.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---

--- a/lib/fs.c
+++ b/lib/fs.c
@@ -1278,6 +1278,54 @@ uc_fs_fdopen(uc_vm_t *vm, size_t nargs)
 	return ucv_resource_create(vm, "fs.file", fp);
 }
 
+/**
+ * Duplicates a file descriptor.
+ *
+ * This function duplicates the file descriptor `oldfd` to `newfd`. If `newfd`
+ * was previously open, it is silently closed before being reused.
+ *
+ * Returns `true` on success.
+ * Returns `null` on error.
+ *
+ * @function module:fs#dup2
+ *
+ * @param {number} oldfd
+ * The file descriptor to duplicate.
+ *
+ * @param {number} newfd
+ * The file descriptor number to duplicate to.
+ *
+ * @returns {?boolean}
+ *
+ * @example
+ * // Redirect stderr to a log file
+ * const logfile = open('/tmp/error.log', 'w');
+ * dup2(logfile.fileno(), 2);
+ * logfile.close();
+ */
+static uc_value_t *
+uc_fs_dup2(uc_vm_t *vm, size_t nargs)
+{
+	uc_value_t *oldfd_arg = uc_fn_arg(0);
+	uc_value_t *newfd_arg = uc_fn_arg(1);
+	int oldfd, newfd;
+
+	oldfd = get_fd(vm, oldfd_arg);
+
+	if (oldfd == -1)
+		err_return(errno ? errno : EBADF);
+
+	newfd = get_fd(vm, newfd_arg);
+
+	if (newfd == -1)
+		err_return(errno ? errno : EBADF);
+
+	if (dup2(oldfd, newfd) == -1)
+		err_return(errno);
+
+	return ucv_boolean_new(true);
+}
+
 
 /**
  * Represents a handle for interacting with a directory opened by `opendir()`.
@@ -2890,6 +2938,7 @@ static const uc_function_list_t global_f
 	{ "error",		uc_fs_error },
 	{ "open",		uc_fs_open },
 	{ "fdopen",		uc_fs_fdopen },
+	{ "dup2",		uc_fs_dup2 },
 	{ "opendir",	uc_fs_opendir },
 	{ "popen",		uc_fs_popen },
 	{ "readlink",	uc_fs_readlink },