From 732215a83f83eae3bdb56516fd617a258c813e44 Mon Sep 17 00:00:00 2001
From: Christian Helmuth <christian.helmuth@genode-labs.com>
Date: Fri, 4 Oct 2019 15:48:30 +0200
Subject: [PATCH] libc: limit fcntl(F_SETFL) to file-status flags

This fixes unintended (and unpermitted) changes of O_ACCMODE bits.
---
 repos/libports/src/lib/libc/vfs_plugin.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/repos/libports/src/lib/libc/vfs_plugin.cc b/repos/libports/src/lib/libc/vfs_plugin.cc
index a3b89254ad..00a9fb75b9 100644
--- a/repos/libports/src/lib/libc/vfs_plugin.cc
+++ b/repos/libports/src/lib/libc/vfs_plugin.cc
@@ -1108,7 +1108,11 @@ int Libc::Vfs_plugin::fcntl(File_descriptor *fd, int cmd, long arg)
 	case F_SETFD: fd->cloexec = arg == FD_CLOEXEC;  return 0;
 
 	case F_GETFL: return fd->flags;
-	case F_SETFL: fd->flags = arg; return 0;
+	case F_SETFL: {
+			/* only the specified flags may be changed */
+			long const mask = (O_NONBLOCK | O_APPEND | O_ASYNC | O_FSYNC);
+			fd->flags = (fd->flags & ~mask) | (arg & mask);
+		} return 0;
 
 	default:
 		break;