gdb + mmap + /dev/mem

gdb + mmap + /dev/mem

[rtainewbie]

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I am trying to debug a simple application running on GNU-Linux kernel 2.6.8.1
(see attachment).

This application works in a memory region not visible for linux (parameter
mem=XXX at boot time).

I map this region using mmap on /dev/mem device as root user.

When I debug it, accessing pointer to this region, the system freezes. So reboot
is necessary and no debug is possible.

The application work correcty but the only solution for debug it is running a
concurrent application that writes the content of this memory region using printf.

Does anybody know an easier and more confortable solution.

Thanks a lot.





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#include<stdio.h>
#include <sys/mman.h>
#include <syscall.h>
#include <errno.h>

int main()
{
	int fd,i;
	unsigned char * mem1, *mem2;
	const int size = 2 * 1024 * 1024;
	unsigned long addrphys = 254 * 1024 * 1024;
	
	mlockall(MCL_CURRENT| MCL_FUTURE);
	
	if ((fd = open("/dev/mem", 02) ) < 0) 
	{
		printf("can't open /dev/mem \n");
		exit (-1);
	}
	
	mem1 = (unsigned char *) mmap((void *)0,size ,PROT_READ | PROT_WRITE,MAP_SHARED , fd, addrphys);
	if (mem1)
		memset(mem1,0, size-1);
	printf("%d",(int) mem1);
	 if (mem1)
	{
		for (i=0; i < (size-1); i++)
		{
			printf("%x ", *(mem1 +i));
		}
		printf("\n\r");
	}
	
	munmap((void *)mem1,size );
	close(fd);
}

Re: gdb + mmap + /dev/mem

[Daniel Jacobowitz]

On Fri, Apr 01, 2005 at 06:41:12PM +0200, rtainewbie wrote:
> I am trying to debug a simple application running on GNU-Linux kernel 2.6.8.1
> (see attachment).
> 
> This application works in a memory region not visible for linux (parameter
> mem=XXX at boot time).
> 
> I map this region using mmap on /dev/mem device as root user.
> 
> When I debug it, accessing pointer to this region, the system freezes. So reboot
> is necessary and no debug is possible.
> 
> The application work correcty but the only solution for debug it is running a
> concurrent application that writes the content of this memory region using printf.
> 
> Does anybody know an easier and more confortable solution.

This is presumably a Linux kernel bug; you may want to talk to the
Linux kernel developers about it.

-- 
Daniel Jacobowitz
CodeSourcery, LLC