Linux Kernel Module Cheat

/dev/mem exposes access to physical addresses, and we use it through the convenient devmem BusyBox utility.

First launch linux/virt_to_phys_user.out as described at Userland physical address experiments.

Next, read from the physical address:

devmem 0x7c7b800

Possible output:

Memory mapped at address 0x7ff7dbe01000.
Value at address 0X7C7B800 (0x7ff7dbe01800): 0x12345678

which shows that the physical memory contains the expected value 0x12345678.

0x7ff7dbe01000 is a new virtual address that devmem maps to the physical address to be able to read from it.

Modify the physical memory:

devmem 0x7c7b800 w 0x9abcdef0

After one second, we see on the screen:

i 9abcdef0
[1]+  Done                       ./posix/virt_to_phys_test.out

so the value changed, and the while loop exited!

This example requires:

CONFIG_STRICT_DEVMEM=n, otherwise devmem fails with:
```
devmem: mmap: Operation not permitted
```
nopat kernel parameter

which we set by default.