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 <[email protected]> > 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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