Programmatically drop Linux cache as non-root user

Question

For testing purposes, I can drop cached memory by writing to the drop_caches file in Linux under the procfs. I can only do this as root. This is on embedded Linux so there is no sudo.

sync; echo 3 > /proc/sys/vm/drop_caches

I can write to the file programmatically in c++ by doing something from the post --> How to programmatically clear the filesystem memory cache in C++ on a Linux system?

sync();
std::ofstream ofs("/proc/sys/vm/drop_caches");
ofs << "3" << std::endl;

The challenge is wanting to do this while running the app as a non-root user. On reboot, the permissions look like:

# cd /proc/sys/vm/
# ls -lrt drop_caches 
-rw-r--r--    1 root     root             0 Feb 13 19:50 drop_caches

And you cannot seem to change those permissions - even as root:

# chmod 777 drop_caches 
chmod: drop_caches: Operation not permitted
# chown user:user drop_caches 
chown: drop_caches: Operation not permitted

How can I accomplish this on Linux? Is it possible to change permissions of a procfs file? I can fully customize my kernel if necessary. Thanks -

Answer 1

You can create an auxiliary executable (be very careful, it is dangerous) which any user can run it with root permissions.

This is called setuid .

For safety reasons, you cannot setuid a shell script.

Extracting from the wiki how to use it:

The setuid and setgid bits are normally set with the command chmod by setting the high-order octal digit to 4 (for setuid) or 2 (for setgid). "chmod 6711 file" will set both the setuid and setgid bits (2+4=6)

Update

As @rici noted, you still will need to have execution permission to execute this process, so you can remove execution permission from others and keep it only on group . So, only who is member of the group can execute it.