kernel module development: how to open block devices and use them (open, seek, read).

eantoranz · 12-06-2010, 08:55 PM

Hi!

I'm experimenting with linux module development. I want to open a block device and be able to basically open it, seek it, read from it and close it. Is there an easy example I could follow on how to do it?

Thanks in advance.

Aquarius_Girl · 12-07-2010, 09:12 PM

Check this out:
http://tldp.org/LDP/khg/HyperNews/ge...ces/block.html

and
block_read()
http://www.users.pjwstk.edu.pl/~jms/...lock_read.html

Don't you think it is a high time you should learn how to search Google ?

eantoranz · 12-08-2010, 03:07 AM

Thanks for your reply, anishakaul!

First, let me get it off my chest: I do know how to search in google (:-P).

Anyways, though the information I needed was not exactly _there_, your two links were able to point me in the right direction after a while of google and linux code digging (see? I can do it! :-D). If I do make it, I'll let you know of the final results.

Thanks!

PS Just in case someone is looking around here, the keyword is _filp_. Check filp_open and filp_close.

eantoranz · 12-08-2010, 03:18 AM

Oh, and people, make sure you read http://kerneltrap.org/node/4717 and http://kernelnewbies.org/FAQ/WhyWrit...romKernelIsBad before going ahead with it.

Aquarius_Girl · 12-08-2010, 03:20 AM

Why? Is block_read() not used for reading block device drivers? I have not got the privilege to use them myself yet, but the tldp site says they are the one!

and this link talks about filp_open being used in character devices??

eantoranz · 12-08-2010, 03:28 AM

Keep in mind I'm still a newbie on kernel development (and expect to remain like that for quite a while) but I _think_ the function is not available anymore:

Code:

$ grep -Hni block_read /proc/kallsyms 
/proc/kallsyms:6952:c022da60 T block_read_full_page
/proc/kallsyms:27439:c0734380 r __ksymtab_block_read_full_page
/proc/kallsyms:32536:c073d32c r __kcrctab_block_read_full_page
/proc/kallsyms:38048:c0747170 r __kstrtab_block_read_full_page
$

And also I downloaded the whole code of 2.6.32.26 and made a search for the function... perhaps it's been renamed?

prodev05 · 12-08-2010, 03:45 AM

Give me some time. I'll check-out my code repository and share you. kernel from 2.4 to 2.6 has major source changes. Also in this book the last page contain source pages. You can search for the function.

I believe what you are trying is very basic level. So you have to checkout the old source. Current kernel source doesn't have single identity. Its being share among the variables. Fully customized with more modules.

reg

Happy Programming

prodev05 · 12-08-2010, 03:51 AM

anishakaul

tldp document is not updated frequently. Instead we can try the doc section from the kernel source tree. That would be very helpful for the programming.

reg

prodev05 · 12-08-2010, 03:58 AM

eantoranz

here is your system call filp_open goto page 484 chapter 17 and there its starts.

Best regards

Happy programming

prodev05 · 12-08-2010, 03:58 AM

eantoranz

here is your system call filp_open goto page 484 chapter 17 and there its starts.

Best regards

Happy programming

Aquarius_Girl · 12-08-2010, 04:37 AM

How about looking into the function named: block_read_full_page(), You must be right, it seems that function has been renamed.

The /fs/buffer.c has its definition.

eantoranz · 12-08-2010, 09:29 AM

Anishakaul, prodev05, thanks for your kind feedback. I'll be playing a little bit with filp_open to see where I can get to. I'll be giving you my feedback as I move forward on the small project. Then again, thanks. :-)

eantoranz · 12-08-2010, 03:57 PM

Guys, is there a way to find out the size of a file after I open it with filp_open? I'm checking in the file operations and also in the properties of file but found nothing related. The only thing is the size of an inode, but I think file has a list of inodes and not just one to get the size from.

prodev05 · 12-09-2010, 03:06 AM

File Size (du -h) can be used in the apps layer on top of the kernel. Inode is the appropriate key to work with the file system functions.

Opening a file to perform some action and the only permitted actions is to read and write. Once the written data is saved then the file size will be committed to allocate the disk space (this result would be expected by Inode to capture and update. Also limitation of blocks will be done here) Until that the written data will be cache, that will be managed by the file system VFS layer.

Aquarius_Girl · 12-09-2010, 03:14 AM

Check this out:

stat()
http://linux.die.net/man/2/stat