/dev/mem

[root@localhost ~]# cat /dev/mem
cat: /dev/mem: No such file or directory
[root@localhost ~]# dmidecode -s system-product-name
TaiShan 200 (Model 2280)
[root@localhost ~]#

[root@localhost ~]# mknod -m 660 /dev/mem c 1 1
[root@localhost ~]# cat /dev/mem
cat: /dev/mem: No such device or address
[root@localhost ~]# chown root:kmem /dev/mem
[root@localhost ~]# cat /dev/mem
cat: /dev/mem: No such device or address

[root@localhost ~]# cat /boot/config-4.14.0-115.el7a.0.1.aarch64 | grep -Ee 'DEV(|K)MEM'
# CONFIG_DEVMEM is not set
CONFIG_ARCH_HAS_DEVMEM_IS_ALLOWED=y
[root@localhost ~]# 

user@debian-macbook:~$ cat /boot/config-4.13.9 | grep -Ee 'DEV(|K)MEM'
CONFIG_DEVMEM=y
CONFIG_DEVKMEM=y
CONFIG_ARCH_HAS_DEVMEM_IS_ALLOWED=y
# CONFIG_STRICT_DEVMEM is not set

[root@localhost ~]# cat /dev/mem
cat: /dev/mem: Bad address
[root@localhost ~]# 

[root@localhost ~]# cat /boot/config-4.19.90-9.10.aarch64 | grep -Ee 'DEV(|K)MEM'
CONFIG_DEVMEM=y
CONFIG_ARCH_HAS_DEVMEM_IS_ALLOWED=y
CONFIG_STRICT_DEVMEM=y
CONFIG_IO_STRICT_DEVMEM=y
[root@localhost ~]#

[root@localhost ~]# ./mem 
/dev/mem opened.
[root@localhost ~]# cat mem.c 
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
  
int main()
{
    int memfd;
  
    memfd = open("/dev/mem", O_RDWR | O_SYNC);
        if (memfd == -1) {
        printf("Can't open /dev/mem.
");
        exit(0);
    }
    printf("/dev/mem opened.
");
    close(memfd);
}
[root@localhost ~]#