linux-kernel - Re: Questions about mmap

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Date:	Tue, 7 Oct 2008 16:43:17 -0400
From:	"linux-os (Dick Johnson)" <linux-os@...logic.com>
To:	"Thiago Lacerda" <thiagotbl@...il.com>
Cc:	"Linux kernel" <linux-kernel@...r.kernel.org>,
	"Stefan Richter" <stefanr@...6.in-berlin.de>
Subject: Re: Questions about mmap

On Tue, 7 Oct 2008, Thiago Lacerda wrote:

> Thank you Stefan.... by I'd something more concrete.
>
> I'm trying to do like this:
>
> //Code of the char device
> unsigned int **test;
> static int device_mmap(struct file *filp, struct vm_area_struct *vma) {
>      printk(KERN_INFO"Calling mmap\n");
>      vma->vm_flags |= VM_LOCKED;
>      if(remap_pfn_range(vma, vma->vm_start,
> virt_to_phys((void*)((unsigned long)test)) >> PAGE_SHIFT, vma->vm_end
> - vma->vm_start, PAGE_SHARED))
>                return -EAGAIN;
>      printk(KERN_INFO"mmap returned\n");
>      return 0;
> }
>
> static int __init testeInit(void) {
> /* after creating char device and registering it*/
> ...
> ....
> test = (unsigned int**) kmalloc(sizeof(unsigned int*)*1024, GFP_KERNEL);
> int i;
> for(i = 0; i < 1024; i++)
>     test[i] = NULL;
>
> unsigned int* temp1 = (unsigned int*) kmalloc(sizeof(unsigned int), GFP_KERNEL);
> (*temp1) = 9;
> test[0] = temp1;
> return 0;
> }
>
> and at user program:
>
> int main() {
>    int fd;
>
>    unsigned int **mptr;
>    size_t size = 1024*sizeof(unsigned int*);
>    fd = open("/dev/myDev", O_RDWR);
>    if( fd == -1) {
>        printf("open error...\n");
>        exit(0);
>    }
>
>    mptr = mmap(0, sizeof(unsigned int*)*1024, PROT_READ | PROT_WRITE,
> MAP_FILE | MAP_SHARED, fd, 0);
>    if(mptr == MAP_FAILED) {
>        printf("mmap() failed\n");
>        exit(1);
>    }
>    printf("teste[0]: %u\n", *mptr[0]);
>    munmap(mptr, size);
>    close(fd);
>    return 0;
> }
>
> It's not working, could you tell me what am I doing wrong? It is for
> my undergraduate thesis ans it is really driving me mad.
>
> I hope that you can help me.
>
> best regards.
>
> --
> Thiago de Barros Lacerda

It looks like you are trying to memory-map a pointer that
you expect to de-reference in user space. Perhaps you can
tell us what it is that you are trying to do. You need
to use copy/to/from_user to copy things from or to the kernel.

Attempts to dereference kernel data will fail because the
segments for kernel data or code are not the same as user
data or code.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.25.17 on an i686 machine (4786.81 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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