es register what do you do?

[gerard4143]

CPU - Intel x86

Quick Question....

In Linux the only segment register used are %cs, %ss and %ds and these are represented with
the macros __KERNEL_CS, __KERNEL_DS, __USER_CS, __USER_DS (intel 32 bit). Now
I have no problems with this concept and I understand how the %cs, %ss and %ds registers
switch between user to kernel and back again...what I do have problems understanding is the %es register
and how it fits into this picture as you can see in this code snippet from a entry.S file the %es register
is not only saved it is loaded with the __KERNEL_DS value....Thank-you for any replies Gerard4143

#define SAVE_ALL \
cld; \
pushl %es; \
pushl %ds; \
pushl %eax; \
pushl %ebp; \
pushl %edi; \
pushl %esi; \
pushl %edx; \
pushl %ecx; \
pushl %ebx; \
movl $(__KERNEL_DS),%edx; \
movl %edx,%ds; \
movl %edx,%es;

[henrikau]

AFAIK, there are 6 segment regs, but only 3 have a dedicated purpose (cs, ds and ss). The remaining 3 (es, fs, gs) can be used for whatever you like, basically. So, no macro has been defined to save/restore or get the contents of these, but the kernel must handle it anyway.

As to why SAVE_ALL only handles es, I do not really know. According to the great book (UTLK 3rd ed), es is used 'for pointers to user data structures (the kernel explicitly uses the es register)'

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[gerard4143]

As to why SAVE_ALL only handles es, I do not really know. According to the great book (UTLK 3rd ed), es is used 'for pointers to user data structures (the kernel explicitly uses the es register)'

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Well yes and no...If __KERNEL_DS loads the %ds structure with its value(base address 0x00000000 and limit 0xfffff) then why would you need %es to point to user data structures?

Thank-you for the reply...Gerard4143

[gerard4143]

for pointers to user data structures

Just clarification....by user data structures do you mean the user data structures located in the user kernel stack or just user data? ...Gerard4143

[henrikau]

Just clarification....by user data structures do you mean the user data structures located in the user kernel stack or just user data? ...Gerard4143

by user data structures, I mean what UTLK said
Section 2.3.* Segmentation in Linux

Try asking this on kernelnewbies, lots of skilled aspiring (and seasoned) hackers there

[gerard4143]

Try asking this on kernelnewbies, lots of skilled aspiring (and seasoned) hackers there

Thank-you for the link and the advice...kernelnewbies I'll check it out...Gerard4143