linux segment

Baoquan He baoquan.he at
Thu Jun 20 05:02:36 EDT 2013

On 10/24/2012 08:04 PM, Fan Yang wrote:

> Hi all: I print the cs ds and ss register in the user space, and it 
> is same as the __USER_CS and __USER_DS which defined in kernel as 73 
> and 7b. In the kernel __KERNEL_CS and __KERNEL_DS defined as 60 and 
> 68, but when I print this two value in my kernel module, I get 60
> and 7b. Why ? It should be 60 and 68, shouldn't it?

Hi Fan,

I just talked to you and very impressed. By checking source code, I
found what you mentioned is normal.

You can check arch/x86/kernel/entry_32.S, and go to the page_fault
definition, in error_code, you can
see the CS and DS assignment.

        /* the function address is in %gs's slot on the stack */
        pushl_cfi %fs
        /*CFI_REL_OFFSET fs, 0*/
        pushl_cfi %es
        /*CFI_REL_OFFSET es, 0*/
        pushl_cfi %ds
        /*CFI_REL_OFFSET ds, 0*/
        pushl_cfi %eax
        CFI_REL_OFFSET eax, 0
        pushl_cfi %ebp
        CFI_REL_OFFSET ebp, 0
        pushl_cfi %edi
        CFI_REL_OFFSET edi, 0
        pushl_cfi %esi
        CFI_REL_OFFSET esi, 0
        pushl_cfi %edx
        CFI_REL_OFFSET edx, 0
        pushl_cfi %ecx
        CFI_REL_OFFSET ecx, 0
        pushl_cfi %ebx
        CFI_REL_OFFSET ebx, 0
        movl $(__KERNEL_PERCPU), %ecx
        movl %ecx, %fs
        GS_TO_REG %ecx
        movl PT_GS(%esp), %edi          # get the function address
        movl PT_ORIG_EAX(%esp), %edx    # get the error code
        movl $-1, PT_ORIG_EAX(%esp)     # no syscall to restart
        REG_TO_PTGS %ecx
        SET_KERNEL_GS %ecx

Below is the assignment. This is changed in 2.6, the reason is that in
kernel the CPL is 0 and it's
safe to operate __USER_DS with DPL is 3. Here if use __KERNEL_DS, even
though they have the
same content, it need change back when return to user space.

So in your original post, you mentioned the cs:ds is 60, 7b, it's normal
and correct value. Hope
this can help.

        movl $(__USER_DS), %ecx
        movl %ecx, %ds
        movl %ecx, %es
        movl %esp,%eax                  # pt_regs pointer
        call *%edi

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