How do I directly access reserved memory with a kernel module? - c

Question

I'm trying to limit the OS (Ubuntu Server 15.04) to a certain memory usage and reserve the rest but write a kernel module to read/write to the reserved memory. I figured out how to limit the usage/reserve memory using the kernel parameters "mem=4G memmap=4G#0 memmap=4G$4G" (4GB for OS and 4GB reserved, split at 4GB point) but I don't know how DMA to reserved memory works with kernel modules. I was thinking just create a proc file but I'm not sure if you can create one outside of the OS's allocated memory.
Any suggestions? Thanks!
Edit: This is for research so it doesn't need to be "nice"
Update:
Maybe I don't need to write a kernel module. I just found this and I'm going to give it a shot:
http://elinux.org/Memory_Management#Reserving_.28and_accessing.29_the_top_of_memory_on_startup
Update:
I tried the link above but I segfault whenever I try to write. Here's my code:
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mann.h>
#define RESERVED_MEMORY_SIZE 0x100000000
int main() {
int fd;
char *reserved_memory;
fd = open("/dev/mem", O_RDWR | O_SYNC);
reserved_memory = (char *) mmap(0, RESERVED_MEMORY_SIZE, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, 4096);
reserved_memory[0] = 'a';
return 0;
}
dmesg shows:
a.out[1167]: segfault at ffffffffffffffff ip 00000000004005d7 sp 00007ffeffccbd80 error 7 in a.out[400000+1000]
For kicks I tried reserved_memory[1]:
a.out[1180]: segfault at 0 ip 00000000004005db sp 00007ffc388d77b0 error 6 in a.out[400000+1000]
I'll look into the format of those messages so I can figure out what it's telling me.
Update:
I found this question by somebody with the same issue as me however the only solution appears to be a kernel rebuild. I'm going to try to avoid this so maybe my best option is a custom kernel module again.
accessing mmaped /dev/mem?

Answer 1

Ok, so I think I solved it. Turns out I just didn't understand how mmap works and I guess the kernel has no restriction on writing/reading /dev/mem if it's in reserved memory. Below are two programs that will write to my reserved spot in memory and read from it.
Write "Hello World!":
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#define RESERVED_MEMORY_OFFSET 0x100000000 /* Offset is 4GB */
int main() {
int fd;
char *reserved_memory;
char *buffer = "Hello World!";
fd = open("/dev/mem", O_RDWR | O_SYNC):
/* Returns a pointer to the 4GB point in /dev/mem - the start of my reserved memory. Only mapping 4096 bytes. */
reserved_memory = (char *) mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, RESERVED_MEMORY_OFFSET);
if (reserved_memory == MAP_FAILED) {
printf("Failed to creating mapping.\n");
printf("ERRNO: %s\n", strerror(errno));
return -1;
}
sprintf(reserved_memory, "%s", buffer);
return 0;
}
Read from beginning of reserved memory:
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#define RESERVED_MEMORY_OFFSET 0x100000000 /* Offset is 4GB */
int main() {
int fd;
char *reserved_memory;
char buffer[13];
fd = open("/dev/mem", O_RDWR | O_SYNC):
/* Returns a pointer to the 4GB point in /dev/mem - the start of my reserved memory. Only mapping 4096 bytes. */
reserved_memory = (char *) mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_FILE | MAP_SHARED, fd, RESERVED_MEMORY_OFFSET);
if (reserved_memory == MAP_FAILED) {
printf("Failed to creating mapping.\n");
printf("ERRNO: %s\n", strerror(errno));
return -1;
}
snprintf(buffer, 13, "%s", reserved_memory);
printf("%s\n", buffer);
return 0;
}
Special thanks to #knm241!