Is it possible to create a directory and file inside /proc/sys?

Question

I am running CentOS 6.2 and I need to create a subdirectory named "crypto" inside /proc/sys. Inside /proc/sys/crypto, I need to create a file named test which contains the value "1".

I have tried many things and nothing works. Can someone help me?

Answer 1

/proc is a virtual filesystem. You will need to write a kernel module that creates the appropriate structures within it.

Answer 2

Ignacio Vazquez-Abrams's answer is correct: files under /proc and /sys are provided by the kernel. If you want to add a file there, write a kernel module. You can find a slightly dated presentation of the /proc programming interface in Linux Device Drivers 3rd ed. ch. 4.

Technically, it is possible to jump through hoops to make a file appear in /proc/sys/crypto (or anywhere else, really). Make a bind mount from /proc/sys/crypto to a staging directory, and make a union mount of the staging directory and an overlay directory back onto /proc/sys/crypto. Here's an example using unionfs-fuse.

# mkdir /tmp/original /tmp/overlay
# mount --bind /proc/sys/crypto /tmp/original
# unionfs-fuse -o nonempty /tmp/overlay=RW:/tmp/original=RO /proc/sys/crypto
# echo hello >/proc/sys/crypto/test
# cat /proc/sys/crypto/test
hello
# umount /proc/sys/crypto
# umount /proc/sys/crypto
# cat /tmp/overlay/test
hello

Note: I disclaim any responsibility for system or brain damage caused by experimenting with this stuff. The commands above are perfectly safe, but messing with other areas of /sys and /proc can cause weird behavior.