On Tue, 30 Oct 2012, Fan Yang wrote:

2012/10/29 Mulyadi Santosa [off-list ref]

quoted

Hi Fan...

On Sun, Oct 28, 2012 at 9:02 PM, Fan Yang [off-list ref] wrote:

quoted

[root at shell--box kernel_mod]# dmesg -c
**********************************
cs 60 96
ds 7b 123
ss 68 104
es 7b 123
fs d8 216
gs e0 224
**********************************

The cs and ds in the kernel space is 60 and 7b. But the kernel define the
KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?

you print CS and DS twice, once during init and once during exit of
your kernel module. So, which one do you want to confirm?

All in all, I have a guess that you see such number (DS belongs to
user space in kernel module) because IIRC kernel module loading is
done using syscall and with the help of modprobe helper.

Thus, it is important to access user space during that stage, hence DS
still using user space data segment.


--
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

Hi  Mulyadi Santosa
   I get the same result during the kernel module init and exit. Then I try
to add a syscall to print these registers, and nothing changed. It is
strange.

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(Weird, this is the third time, I have to send this. If anybody gets this
message multiple times, I apologise but my mail is not in the archives.)

If Mulyadi is right and we need DS to be USER_DS to access user space (I
really don't know, sorry, but maybe there is something in your <uaccess.h>?)
then your attempt to try with a syscall couldn't yield other values because
one trait of syscalls is that they can access user space.

This means you would get DS = USER_DS precisely _because_ you are in a
syscall. Module init and exit are, too, just some stack frames above one and
thus fall into this category as well.

But shouldn't it be possible to register a timer and then print the
segment registers? Timers are fired in softirq context and, hence, have no
connection to user space.

Regards,
Tobi