Hi,
These are the definitions of __KERNEL_DS and __USER_DS:
#define __KERNEL_DS (GDT_ENTRY_KERNEL_DS * 8)
#define __USER_DS (GDT_ENTRY_DEFAULT_USER_DS* 8 + 3)
The difference is firstly in the privilege level ("+ 3"). There also
is a comment in the same file (arch/x86/include/asm/segment.h):
/*
* we cannot use the same code segment descriptor for user and kernel
* -- not even in the long flat mode, because of different DPL /kkeil
* The segment offset needs to contain a RPL. Grr. -AK
* GDT layout to get 64bit syscall right (sysret hardcodes gdt offsets)
*/
2010/6/8 rahul dev <rahul_dev_agg@xxxxxxxxxxx>:
> Hi Joel,
>
> <snip>
>
>> since Linux doesn't use segmentation , all the segment descriptor values > are the same. So I think it doesn't matter what selector value you load > in DS, ES etc. Though I don't know the real reason for why __USER_DS is > used instead of __KERNEL_DS
>
> Although, linux doesn't use segmentation, but I believe the DPL values are different for user and kernel data segment descriptor. Please correct me if I am wrong.
>
> thanks,
> rahul
>
>
>
>
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Alexander Potashev
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