Re: Questions about mmap

[Chris Friesen]

linux-os (Dick Johnson) wrote:
> 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.

I do not think this is entirely true.  I haven't tried it recently, but 
I have reserved a page of kernel memory in a driver and mapped it into 
userspace such that it can be accessed by both.

This can be useful for cases where you want to make continuously-updated 
information available to userspace without the overhead of a syscall.

There can be some gotchas though...last time I tried this on ARM I had 
to explicitly flush the cache on the memory area after writing it in the 
kernel to ensure that userspace saw the updated data since it was 
accessing it via a different virtual address.

Chris
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