Hi,Just try to understand the kernel filesystem.
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(¤t->mm->mmap_sem);
If I execute
up_write(¤t->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