add character driver fops struct + implementation

5 files changed, 81 insertions, 3 deletions

diff --git a/1/Makefile b/1/Makefile
index a345845..d968e31 100644
--- a/1/Makefile
+++ b/1/Makefile
@@ -3,8 +3,11 @@ KERNEL := /lib/modules/$(RELEASE)/build
 # PWD doesn't have to be defined as it's inherited from the current environment
 # MAKE shouldn't be defined as it's special in GNU make
 
+# the names of these variables are required by the kernel's (imo weird) build
+# system, see [kbuild-obj-var] and [kbuild-module-makefile] in ../readme.md
 obj-m += lork.o
-lork-y := main.o
+lork-y += main.o
+lork-y += fopdrv.o
 
 all:
 	$(MAKE) -C $(KERNEL) M=$(PWD) modules
diff --git a/1/fopdrv.c b/1/fopdrv.c
new file mode 100644
index 0000000..31c76cd
--- /dev/null
+++ b/1/fopdrv.c
@@ -0,0 +1,28 @@
+#include "fopdrv.h"
+
+// driver/char/mem.c read_null (/dev/null)
+ssize_t fop_read(struct file *file, char __user *buf, size_t count, loff_t *ppos) {
+	return 0;
+}
+
+// driver/char/mem.c write_null
+ssize_t fop_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos) {
+	return count;
+}
+
+int fop_open(struct inode * inode, struct file * file) {
+	printk("%s\n", __PRETTY_FUNCTION__);
+	return 0;
+	// 0 seems to be a safe return value as it's used in driver/char/mem.c. The
+	// manual page for open(2) says that the system call returns a nonnegative
+	// integer representing the file descriptor on success, but it does not
+	// appears to be required.
+}
+
+int fop_release(struct inode * inode, struct file * file) {
+	printk("%s\n", __PRETTY_FUNCTION__);
+	return 0;
+	// same as above, but found in driver/char/lp.c
+}
+
+
diff --git a/1/fopdrv.h b/1/fopdrv.h
new file mode 100644
index 0000000..f2ca18c
--- /dev/null
+++ b/1/fopdrv.h
@@ -0,0 +1,11 @@
+#pragma once
+
+#include <linux/fs.h>
+#include <linux/types.h>
+
+// declarations copied from linux/fs.h
+ssize_t fop_read(struct file *, char __user *, size_t, loff_t *);
+ssize_t fop_write(struct file *, const char __user *, size_t, loff_t *);
+int fop_open(struct inode *, struct file *);
+int fop_release(struct inode *, struct file *);
+
diff --git a/1/main.c b/1/main.c
index 7299f36..88ae0d0 100644
--- a/1/main.c
+++ b/1/main.c
@@ -1,15 +1,24 @@
 #include <linux/init.h>
 #include <linux/module.h>
 
+#include "fopdrv.h"
+
 MODULE_LICENSE("MIT");
 
+struct file_operations fops = {
+  .read = fop_read,
+  .write = fop_write,
+  .open = fop_open,
+  .release = fop_release,
+};
+
 static int mod_init(void) {
-	printk(KERN_ALERT "Hello, world\n");
+	printk("%s\n", __PRETTY_FUNCTION__);
 	return 0;
 }
 
 static void mod_exit(void) {
-	printk(KERN_ALERT "Goodbye, world\n");
+	printk("%s\n", __PRETTY_FUNCTION__);
 }
 
 module_init(mod_init);
diff --git a/readme.md b/readme.md
new file mode 100644
index 0000000..f8f735d
--- /dev/null
+++ b/readme.md
@@ -0,0 +1,27 @@
+# sources
+
+- [Kbuild object variable name rationale][kbuild-obj-var]
+- [Kbuild kernel module setup guide][kbuild-module-makefile]
+- [`struct file_operations`][c-struct-file-operations] documentation, also
+    - write(2)
+    - read(2)
+- (Sadly) [the Linux kernel source][kernel] itself, especially
+    - drivers/char/random.c
+    - drivers/char/mem.c
+
+[kbuild-obj-var]: https://docs.kernel.org/kbuild/makefiles.html#loadable-module-goals-obj-m
+[kbuild-module-makefile]: https://www.kernel.org/doc/html/latest/kbuild/modules.html#creating-a-kbuild-file-for-an-external-module
+[c-struct-file-operations]: https://www.kernel.org/doc/html/latest/filesystems/vfs.html#struct-file-operations
+[kernel]: https://github.com/torvalds/linux
+
+# tips
+
+- When the kernel documentation references manpages, these actually contain
+  useful information. Manpages are not only for commands, but also include
+  detailed documentation for the syscall interface and other C APIs.
+
+  Since most of the manpages referenced by the kernel are from section 2 (on
+  system calls), you should append the number between brackets to the `man`
+  command, i.e. to read 'write(2)' use the command `man 2 write`.
+- Use `dmesg` with the `-w` or `-W` option (see man dmesg(1))
+