diff --git a/apps/app.c b/apps/app.c
index ea977091..3794c0e6 100644
--- a/apps/app.c
+++ b/apps/app.c
@@ -14,7 +14,7 @@ int main()
     atexit(bye);
     printf("Welcome to %s from userspace (pid %d)!\n", "Luna", getpid());
 
-    int fd = open("/etc/motd", 0);
+    int fd = open("/etc/motd", O_RDONLY);
     if (fd < 0)
     {
         perror("open");
diff --git a/kernel/src/fs/VFS.h b/kernel/src/fs/VFS.h
index b2e26326..c01c2db1 100644
--- a/kernel/src/fs/VFS.h
+++ b/kernel/src/fs/VFS.h
@@ -26,6 +26,8 @@ namespace VFS
 
         virtual Result<usize> write(const u8* buf, usize offset, usize length) = 0;
 
+        virtual Result<void> truncate(usize size) = 0;
+
         // Generic methods
         virtual FileSystem& fs() const = 0;
 
diff --git a/kernel/src/fs/tmpfs/FileSystem.cpp b/kernel/src/fs/tmpfs/FileSystem.cpp
index a31fd707..ccd8b152 100644
--- a/kernel/src/fs/tmpfs/FileSystem.cpp
+++ b/kernel/src/fs/tmpfs/FileSystem.cpp
@@ -118,4 +118,15 @@ namespace TmpFS
 
         return length;
     }
+
+    Result<void> FileInode::truncate(usize size)
+    {
+        usize old_size = m_data_buffer.size();
+
+        TRY(m_data_buffer.try_resize(size));
+
+        if (size > old_size) memset(m_data_buffer.data() + old_size, 0, size - old_size);
+
+        return {};
+    }
 }
diff --git a/kernel/src/fs/tmpfs/FileSystem.h b/kernel/src/fs/tmpfs/FileSystem.h
index 8e029eb4..0a217486 100644
--- a/kernel/src/fs/tmpfs/FileSystem.h
+++ b/kernel/src/fs/tmpfs/FileSystem.h
@@ -63,6 +63,8 @@ namespace TmpFS
 
         Result<usize> write(const u8*, usize, usize) override;
 
+        Result<void> truncate(usize size) override;
+
         virtual ~FileInode() = default;
 
       private:
@@ -103,6 +105,11 @@ namespace TmpFS
             return err(EISDIR);
         }
 
+        Result<void> truncate(usize) override
+        {
+            return err(EISDIR);
+        }
+
         VFS::FileSystem& fs() const override
         {
             return *m_fs;
diff --git a/kernel/src/sys/file.cpp b/kernel/src/sys/file.cpp
index 9c7979bb..65c886e9 100644
--- a/kernel/src/sys/file.cpp
+++ b/kernel/src/sys/file.cpp
@@ -19,6 +19,8 @@ Result<u64> sys_read(Registers*, SyscallArgs args)
 
     auto& descriptor = *TRY(current->resolve_fd(fd));
 
+    if (!descriptor.is_readable()) return err(EBADF);
+
     usize nread = TRY(descriptor.inode->read(buf, descriptor.offset, size));
 
     descriptor.offset += nread;
@@ -38,6 +40,10 @@ Result<u64> sys_write(Registers*, SyscallArgs args)
 
     auto& descriptor = *TRY(current->resolve_fd(fd));
 
+    if (!descriptor.is_writable()) return err(EBADF);
+
+    if (descriptor.should_append()) todo();
+
     usize nwritten = TRY(descriptor.inode->write(buf, descriptor.offset, size));
 
     descriptor.offset += nwritten;
diff --git a/kernel/src/sys/open.cpp b/kernel/src/sys/open.cpp
index f5cb7a0c..0fb0d481 100644
--- a/kernel/src/sys/open.cpp
+++ b/kernel/src/sys/open.cpp
@@ -3,6 +3,10 @@
 #include "memory/MemoryManager.h"
 #include "sys/Syscall.h"
 #include "thread/Scheduler.h"
+#include <bits/open-flags.h>
+
+// These flags are needed after open(), the rest only affect open().
+constexpr int FLAGS_TO_KEEP = O_RDWR | O_APPEND;
 
 Result<u64> sys_open(Registers*, SyscallArgs args)
 {
@@ -16,11 +20,28 @@ Result<u64> sys_open(Registers*, SyscallArgs args)
 
     kinfoln("open: trying to open file %s, flags %d", path, flags);
 
-    auto inode = TRY(VFS::resolve_path(path));
+    SharedPtr<VFS::Inode> inode;
+
+    // Caller did not pass either O_RDONLY, O_WRONLY or O_RDWR
+    if ((flags & O_RDWR) == 0) { return err(EINVAL); }
+
+    auto maybe_inode = VFS::resolve_path(path);
+    if (maybe_inode.has_error())
+    {
+        if (maybe_inode.error() == ENOENT && (flags & O_CREAT)) inode = TRY(VFS::create_file(path));
+        else
+            return maybe_inode.release_error();
+    }
+    else if (flags & O_EXCL)
+        return err(EEXIST);
+    else
+        inode = maybe_inode.release_value();
+
+    if ((flags & O_WRONLY) && (flags & O_TRUNC)) inode->truncate(0);
 
     int fd = TRY(current->allocate_fd(0));
 
-    current->fd_table[fd] = FileDescriptor { inode, 0, flags };
+    current->fd_table[fd] = FileDescriptor { inode, 0, flags & FLAGS_TO_KEEP };
 
     kinfoln("open: allocated file descriptor %d for inode %zu", fd, inode->inode_number());
 
diff --git a/kernel/src/thread/Thread.cpp b/kernel/src/thread/Thread.cpp
index f01c70c0..f2992adc 100644
--- a/kernel/src/thread/Thread.cpp
+++ b/kernel/src/thread/Thread.cpp
@@ -1,4 +1,5 @@
 #include "thread/Thread.h"
+#include <bits/open-flags.h>
 #include <luna/Alloc.h>
 #include <luna/Atomic.h>
 
@@ -43,3 +44,18 @@ Result<FileDescriptor*> Thread::resolve_fd(int fd)
 
     return maybe_descriptor.value_ptr();
 }
+
+bool FileDescriptor::should_append()
+{
+    return flags & O_APPEND;
+}
+
+bool FileDescriptor::is_readable()
+{
+    return flags & O_RDONLY;
+}
+
+bool FileDescriptor::is_writable()
+{
+    return flags & O_WRONLY;
+}
diff --git a/kernel/src/thread/Thread.h b/kernel/src/thread/Thread.h
index 58787351..5f745012 100644
--- a/kernel/src/thread/Thread.h
+++ b/kernel/src/thread/Thread.h
@@ -27,6 +27,10 @@ struct FileDescriptor
     SharedPtr<VFS::Inode> inode;
     usize offset { 0 };
     int flags { 0 };
+
+    bool should_append();
+    bool is_writable();
+    bool is_readable();
 };
 
 static constexpr int FD_MAX = 64;
diff --git a/libc/include/bits/open-flags.h b/libc/include/bits/open-flags.h
new file mode 100644
index 00000000..bfdfda2a
--- /dev/null
+++ b/libc/include/bits/open-flags.h
@@ -0,0 +1,14 @@
+/* bits/open-flags.h: O_* constants for open(). */
+
+#ifndef _BITS_OPEN_FLAGS_H
+#define _BITS_OPEN_FLAGS_H
+
+#define O_RDONLY 1
+#define O_WRONLY 2
+#define O_RDWR 3
+#define O_APPEND 4
+#define O_CREAT 8
+#define O_EXCL 16
+#define O_TRUNC 32
+
+#endif
diff --git a/libc/include/fcntl.h b/libc/include/fcntl.h
index 401f936b..b6760cf2 100644
--- a/libc/include/fcntl.h
+++ b/libc/include/fcntl.h
@@ -3,6 +3,8 @@
 #ifndef _FCNTL_H
 #define _FCNTL_H
 
+#include <bits/open-flags.h>
+
 #ifdef __cplusplus
 extern "C"
 {
diff --git a/libc/src/stdio.cpp b/libc/src/stdio.cpp
index 6ccc932e..be4210f1 100644
--- a/libc/src/stdio.cpp
+++ b/libc/src/stdio.cpp
@@ -21,7 +21,7 @@ extern "C"
     FILE* fopen(const char* path, const char*)
     {
         // FIXME: Parse the mode string.
-        int fd = open(path, 0);
+        int fd = open(path, O_RDWR);
         if (fd < 0) return nullptr;
 
         FILE* f = (FILE*)malloc(sizeof(FILE));