Hi Baoquan:
Thank you for your answer.
That is to say the designer use __USER_DS here deliberately to improve the efficiency jump from kernel space to user space?
BTW,how can you find this email? I write this email On 10/24/2012.
2013/6/20 Baoquan He <baoquan.he@xxxxxxxxx>
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.
error_code:
/* 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
cld
movl $(__KERNEL_PERCPU), %ecx
movl %ecx, %fs
UNWIND_ESPFIX_STACK
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
TRACE_IRQS_OFF
movl %esp,%eax # pt_regs pointer
call *%edi
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