No more than 255 device files with Debian Sarge

[Stefan Sperling <sperl@nateurope.com>]

Hello!

0. I discovered a strange phenomenon with a Debian
   Sarge installation. I am able to replicate it
   on several machines. It occurs on freshly
   installed machines as well as on "older" ones.

   The problem is this:

   I want to create more than 4000 device files
   for a project. I am aware that this is an
   uncommon scenario. But it is a customer request.

   Generating the device files works well for the
   first 255 ones. Then the major number is
   incremented by 1 and the minor number starts over
   with 0. mknod NEVER returns an error.

1. I tried this with kernel 2.6.8 as well as with
   kernel 2.6.16 - no change. devfs is removed.

   I can show this phenomenon on a Debian machine
   with some example values:

   nattest:/home/sperl/tmp# mknod testnode1 c 200 16
   nattest:/home/sperl/tmp# mknod testnode2 c 200 255
   nattest:/home/sperl/tmp# mknod testnode3 c 200 256
   nattest:/home/sperl/tmp# mknod testnode4 c 200 511
   nattest:/home/sperl/tmp# mknod testnode5 c 200 512
   nattest:/home/sperl/tmp# ls -l
   total 0
   crw-r--r--  1 root root 200,  16 2006-11-23 10:07 testnode1
   crw-r--r--  1 root root 200, 255 2006-11-23 10:07 testnode2
   crw-r--r--  1 root root 201,   0 2006-11-23 10:07 testnode3
   crw-r--r--  1 root root 201, 255 2006-11-23 10:07 testnode4
   crw-r--r--  1 root root 202,   0 2006-11-23 10:07 testnode5

2. Doing the same on my Ubuntu installation running a 2.6.15
   kernel I get this:

   root@natss:/home/sperl/tmp2# mknod testnode1 c 200 16
   root@natss:/home/sperl/tmp2# mknod testnode2 c 200 255
   root@natss:/home/sperl/tmp2# mknod testnode3 c 200 256
   root@natss:/home/sperl/tmp2# mknod testnode4 c 200 511
   root@natss:/home/sperl/tmp2# mknod testnode5 c 200 512
   root@natss:/home/sperl/tmp2# ls -l
   insgesamt 0
   crw-r--r-- 1 root root 200,  16 2006-11-23 10:10 testnode1
   crw-r--r-- 1 root root 200, 255 2006-11-23 10:11 testnode2
   crw-r--r-- 1 root root 200, 256 2006-11-23 10:11 testnode3
   crw-r--r-- 1 root root 200, 511 2006-11-23 10:11 testnode4
   crw-r--r-- 1 root root 200, 512 2006-11-23 10:11 testnode5

   As you can see, on Ubuntu everything is as expected.

3. I even wrote an own version of mknod to check if there is
   a bug in the tool. Result: All files are created correctly
   with Ubuntu but not with Debian.

4. I wrote a tiny device driver just to verify that the minor
   numbers are not only displayed wrong by "ls". Result: I was
   able to access all devices with major number 200 from my test
   application but not the ones with 201, 202,... For major
   number 200 everything looked sane; the minor number I have
   seen in my open function looked healthy.

5. When I do create a devicefile like testnode3 (with minor number
   256) from the Ubuntu machine via NFS this is what happens:
   On the Ubuntu machine the file looks ok (major 200, minor 256).
   On the Debian console I see the same file with major 201 and
   minor 0.

6. When I start my test application and try to open testnode3 on
   the Debian machine I cannot open the device.

7. I already experimented with the kernel configuration. Even
   excluding devfs did not help.

I found nothing similair in the Debian bug report database nor
on the rest of the Internet either.

Does anyone of you have an idea?

Thanks for reading!

Greetings,
Stefan

--
Stefan Sperling
Network Administration / Software Development

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