llseek

rootfs_overlay/etc/inittab

@@ -8,6 +8,7 @@
 ::sysinit:/etc/init.d/rcS
 # https://unix.stackexchange.com/questions/299408/how-to-login-automatically-without-typing-root-in-buildroot-x86-64-qemu
 console::respawn:/bin/sh
+#console::respawn:/sbin/getty -n -L console 0 vt100
 ::ctrlaltdel:/sbin/reboot
 ::shutdown:/etc/init.d/rcK
 ::shutdown:/sbin/swapoff -a

rootfs_overlay/fops.sh

@@ -1,10 +1,12 @@
 #!/bin/sh
-set -ex
+
+set -x
 insmod /fops.ko
 mkdir -p /fops
 mount -t debugfs none /fops
 cd /fops/kernel_module_cheat
 
+## Basic read.
 cat fops
 # => abcd
 # dmesg => open
@@ -12,14 +14,32 @@ cat fops
 # dmesg => len = [0-9]+
 # dmesg => close
 
-printf a >fops
+## Basic write
+
+printf '01' >fops
 # dmesg => open
 # dmesg => write
 # dmesg => len = 1
 # dmesg => buf = a
 # dmesg => close
 
-cd /
-umount /fops
-rmdir /fops
-rmmod fops
+cat fops
+# => 01cd
+# dmesg => open
+# dmesg => read
+# dmesg => len = [0-9]+
+# dmesg => close
+
+## ENOSPC
+printf '1234' >fops
+printf '12345' >fops
+echo "$?"
+# => 8
+cat fops
+# => 1234
+
+## seek
+printf '1234' >fops
+printf 'z' | dd bs=1 of=fops seek=2
+cat fops
+# => 12z4