fops read returns some data, busybox config frag for stat, bibliography start

README.md

@@ -24,6 +24,7 @@ The Linux kernel version can be found with:
 1.  [Introduction](introduction.md)
 1.  [Build](build.md)
 1.  [kmod](kmod.md)
+1.  [Bibliography](bibliography.md)
 1.  Examples
     1.  [Host](host/)
     1.  Buildroot

bibliography.md

@@ -0,0 +1,4 @@
+# Bibliography
+
+- <https://lwn.net/Kernel/LDD3/> the best, but always outdated, book. Updated source: <https://github.com/martinezjavier/ldd3>
+- <https://github.com/agelastic/eudyptula>

busybox_config_fragment

@@ -0,0 +1 @@
+CONFIG_STAT=y

kernel_module/fops.c

@@ -5,6 +5,9 @@ Usage:
 
 	/fops.sh
 
+No, there ain't no official docs:
+http://stackoverflow.com/questions/15213932/what-are-the-struct-file-operations-arguments
+
 fops define what the kernel will do on filesystem system calls on all of
 /dev, /proc, /sys, and consistute the main method of userland communication
 in drivers (syscalls being the other one).
@@ -13,9 +16,12 @@ Here we use debugfs.
 */
 
 #include <linux/debugfs.h>
+#include <linux/errno.h> /* EFAULT */
 #include <linux/fs.h>
-#include <linux/kernel.h>
+#include <linux/kernel.h> /* min */
 #include <linux/module.h>
+#include <linux/printk.h> /* printk */
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
 
 MODULE_LICENSE("GPL");
 
@@ -27,13 +33,26 @@ int fop_open(struct inode *inode, struct file *file)
 	return 0;
 }
 
+/**/
 ssize_t fop_read(struct file *file, char __user *buf, size_t len, loff_t *off)
 {
+	ssize_t ret;
+	char s[] = "abcd";
 	printk(KERN_INFO "read\n");
 	printk(KERN_INFO "len = %zu\n", len);
-	/* TODO. */
-	/*copy_to_user(arg, &msg, size);*/
-	return 0;
+	printk(KERN_INFO "off = %lld\n", (long long)*off);
+	if (sizeof(s) <= *off) {
+		ret = 0;
+	} else {
+		ret = min(len, sizeof(s) - (size_t)*off);
+		if (copy_to_user(buf, s, ret)) {
+			ret = -EFAULT;
+		} else {
+			*off += ret;
+		}
+	}
+	printk(KERN_INFO "ret = %lld\n", (long long)ret);
+	return ret;
 }
 
 ssize_t fop_write(struct file *file, const char __user *buf, size_t len, loff_t *off)
@@ -41,6 +60,7 @@ ssize_t fop_write(struct file *file, const char __user *buf, size_t len, loff_t
 	printk(KERN_INFO "write\n");
 	printk(KERN_INFO "buf = %.*s\n", (int)len, buf);
 	printk(KERN_INFO "len = %zu\n", len);
+	printk(KERN_INFO "off = %lld\n", (long long)*off);
 	return len;
 }
 

rootfs_overlay/fops.sh

@@ -6,14 +6,20 @@ mount -t debugfs none /fops
 cd /fops/kernel_module_cheat
 
 cat fops
-# => open
-# => read
-# => len = [0-9]+
-# => close
+# => abcd
+# dmesg => open
+# dmesg => read
+# dmesg => len = [0-9]+
+# dmesg => close
 
 printf a >fops
-# => open
-# => write
-# => len = a
-# => len = 1
-# => close
+# dmesg => open
+# dmesg => write
+# dmesg => len = 1
+# dmesg => buf = a
+# dmesg => close
+
+cd /
+umount /fops
+rmdir /fops
+rmmod fops

run

@@ -4,8 +4,16 @@ cd buildroot
 make BR2_EXTERNAL="$(pwd)/../kernel_module" qemu_x86_64_defconfig
 echo '
 BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="../kernel_config_fragment"
+BR2_PACKAGE_BUSYBOX_CONFIG_FRAGMENT_FILES="../busybox_config_fragment"
 BR2_PACKAGE_KERNEL_MODULE=y
 BR2_ROOTFS_OVERLAY="../rootfs_overlay"
 ' >> .config
 env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(($(nproc) - 2))" kernel_module-rebuild all
-qemu-system-x86_64 -M pc -kernel output/images/bzImage -drive file=output/images/rootfs.ext2,if=virtio,format=raw -append root=/dev/vda -net nic,model=virtio -net user
+qemu-system-x86_64 \
+  -M pc \
+  -append 'root=/dev/vda' \
+  -drive file=output/images/rootfs.ext2,if=virtio,format=raw \
+  -kernel output/images/bzImage \
+  -net nic,model=virtio \
+  -net user \
+;