Read /dev/kmem failed

sailershen · 03-20-2006, 08:37 AM

Code:

struct descriptor_idt {
        unsigned short offset_low, seg_selector;
        unsigned char reserved, flag;
        unsigned short offset_high;
};

       .......

        struct descriptor_idt *descriptor;
       .......

        fd_kmem = open("/dev/kmem", O_RDWR);
        ptr_idt = get_addr_idt();
        descriptor = (struct descriptor_idt *) malloc(sizeof(struct descriptor_idt));
        ......
        readkmem(descriptor, ptr_idt + 8 * x, sizeof(struct descriptor_idt));

        ......

void readkmem(void *m, unsigned off, int size)
{
        int i;
        if (lseek(fd_kmem, off, SEEK_SET) != off) {
                fprintf(stderr, "Error lseek. Are you root? \n");
                exit(-1);
        }
        if (read(fd_kmem, m, size) != size) {
                fprintf(stderr, "Error read kmem\n");
                exit(-1);
        }
}

unsigned long get_addr_idt(void)
{
        unsigned char idtr[6];
        unsigned long idt;
        __asm__ volatile ("sidt %0":"=m" (idtr));
        idt = *((unsigned long *) &idtr[2]);
        return (idt);
}

When it's run, it also said "Error read kmem", and I found that the "read(fd_kmem, m, size)" always return 0, I don't know why?

Thanks for help.

nx5000 · 03-20-2006, 08:46 AM

add
#include <errno.h>
and replace your printf by perror("read");

and see what it gives. perror is very uselfull.

"permission denied" ?

sailershen · 03-20-2006, 07:48 PM

Quote:

Originally Posted by nx5000

add
#include <errno.h>
and replace your printf by perror("read");

and see what it gives. perror is very uselfull.

"permission denied" ?

Thanks for help.

I change it:

Code:

#include <errno.h>

        ......

        if ((i = read(fd_kmem, m, size)) != size) {
                fprintf(stderr, "Error read kmem\n");
                perror("read");
                exit(-1);
        }

When it run in SLES9 and RHEL3, it said:

Code:

Error read kmem
read: Success

nx5000 · 03-21-2006, 02:49 AM

Well, your test doesn't really show an error.

I would change

fprintf(stderr, "Error read kmem\n");

->

fprintf(stderr, "Error read kmem, only read %d bytes\n",i);

because read:Success means what it means

sailershen · 03-21-2006, 04:21 AM

I change the code:

Code:

void readkmem(void *m, unsigned off, int size)
{
        int i;
        if (lseek(fd_kmem, off, SEEK_SET) != off) {
                fprintf(stderr, "Error lseek. Are you root? \n");
                exit(-1);
        }
        if ((i = read(fd_kmem, m, size)) != size) {
                fprintf(stderr, "Error read kmem, only read %d bytes\n",i);
                perror("read");
                exit(-1);
        }
}

When run, it said:

Code:

Error read kmem, only read 0 bytes
read: Success

nx5000 · 03-21-2006, 05:33 AM

Look at the kernel source
Maybe you will have more answers on other forums...
like kerneltrap.org