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SubjectRe: Accessing file mapped data inside the kernel
Anton Altaparmakov wrote:

>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
>>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.

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];
up_write(&current->mm->mmap_sem); /* Without this the user program is dead locked */

return addr;

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