Re: Accessing file mapped data inside the kernel

[Paulo da Silva <psdasilva@esoterica.pt>]

Anton Altaparmakov wrote:

> Hi,
>
> On Wed, 9 Nov 2005, Paulo da Silva wrote:
>  
>
> > I posted about this a few days ago but got no responses
> > so far! I think this should be a trivial question for those
> > involved in the kernel internals. May be I didn't develop
> > the problem enough to be understood.
> >
> > So, here is the question reformulated.
> >
> > A given file system must supply a procedure for mmap.
> >
> > int <fsname>_file_mmap(struct file * file, struct vm_area_struct * vma)
> > {
> >  int addr;
> >  addr=generic_file_mmap(file,vma);
> >  <Code to access addr pointed bytes or vma->vm_start>
> >  return addr;
> > }
> >
> > I could verify that "addr" is what is returned to the user as
> > a pointer to a string of bytes that maps a file when a user
> > program calls mmap or mmap2.
> >
> > In the user program, I can access those bytes (read/write)
> > as, for ex., a char pointer.
> >
> > I don't know how to access those bytes inside the kernel
> > at the point <Code to access addr pointed bytes or vma->vm_start>
> >
> > First trys led the program that invoked mmap to block.
> > I thought that there's something to do with a previous
> >   down_write(&current->mm->mmap_sem);
> > If I execute
> >   up_write(&current->mm->mmap_sem);
> > before accessing the data the block situation does not
> > occur anymore. I would like to hear something about
> > this.
> >
> > Anyway, I tryed to use "copy_from_user" but I got
> > garbage, not the file contents! Using "strncpy" crashes
> > the kernel (UML)!
> >
> > Can someone please write a fragment of code to safely
> > access those bytes, copying them to and from a
> > kernel char pointed area so that they are read/written
> > to the file?
> >    
>
> Why do you want to do that?  If you explain what you are trying to do it 
> may be possible to help you better.  It is almost 100% certain that your 
> are going about it in completely the wrong way, so please describe what 
> you are trying to do...
>
> Best regards,
>
> 	Anton
>  
Just try to understand the kernel filesystem.
So far I could understand the 1st layer of
reading and writing. mmap seems to be a
difficult task however. So, I made a 1st try
looking at mmap supplied by the filesystem,
but I couldn't even succeed with a printk
of the mapped area! I would like to understand
what is the meaning of the address (int) returned
by generic_file_mmap that is also into vma->vm_start
and is returned to the user as a char pointer.
I thought that this address, being accessible
by a user program as a char pointer, should also
be accessible by a copy-from-user inside the
kernel. Unfortunately, this didn't happen!
Why? That's my question. Did I make any mistake?
A basic fragment of code showing how to access
that area could enlight me so that I could go
deeply into the code.

Ex.
Suppose a file has a string of text ("foo")
and the user calls mmap.

Why does this code not work?

The supplied filesystem mmap is "generic_file_mmap".
So, I changed it to foo_file_mmap as follows:

int foo_file_mmap(struct file * file, struct vm_area_struct * vma)
{

 int addr;
 char tstr[100];
 addr=generic_file_mmap(file,vma);
 up_write(&current->mm->mmap_sem); /* Without this the user program is dead locked */
 copy_from_user(tstr,(char*)addr,4);
 printk("%s",tstr);

 return addr;
}


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