linux segment

linux segment

[Fan Yang]

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?
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linux segment

[Mulyadi Santosa]

On Wed, Oct 24, 2012 at 7:04 PM, Fan Yang <lljyangfan@gmail.com> 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?

you're not manually switch the data segment, aren't you?

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

linux segment

[Jun Hu]

Can you post out your codes ?

From: Fan Yang 
Sent: Wednesday, October 24, 2012 8:04 PM
To: kernelnewbies at kernelnewbies.org 
Subject: linux segment

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? 





--------------------------------------------------------------------------------
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Kernelnewbies at kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
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linux segment

[Fan Yang]

2012/10/27 Jun Hu <duanshuidao@hotmail.com>

>   Can you post out your codes ?
>
>  *From:* Fan Yang <lljyangfan@gmail.com>
> *Sent:* Wednesday, October 24, 2012 8:04 PM
> *To:* kernelnewbies at kernelnewbies.org
> *Subject:* linux segment
>
> 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?
>
>
>
> ------------------------------
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
> Hi Jun Hu
    There is my code which run at the user space:

  1 #include<stdio.h>
  2 main()
  3 {
  4     unsigned long cs, ds, ss, es, fs, gs;
  5     asm volatile("movl %%CS,%0\n\t":"=r"(cs));
  6     asm volatile("movl %%DS,%0\n\t":"=r"(ds));
  7     asm volatile("movl %%SS,%0\n\t":"=r"(ss));
  8     asm volatile("movl %%ES,%0\n\t":"=r"(es));

  9     asm volatile("movl %%FS,%0\n\t":"=r"(fs));
 10     asm volatile("movl %%GS,%0\n\t":"=r"(gs));
 11     printf ("**********************************\n");
 12     printf ("cs %lx\t%ld\n", cs, cs);
 13     printf ("ds %lx\t%ld\n", ds, ds);
 14     printf ("ss %lx\t%ld\n", ss, ss);
 15     printf ("es %lx\t%ld\n", es, es);
 16     printf ("fs %lx\t%ld\n", fs, fs);
 17     printf ("gs %lx\t%ld\n", gs, gs);
 18     printf ("**********************************\n");
 19 }



and the result of the progress in my machine is


**********************************
cs 73 115
ds 7b 123
ss 7b 123
es 7b 123
fs 0 0
gs 33 51
**********************************


so, you can see the cs and ds register is 73 and 7b which are same as the
kernel defined.  And the code of the kernel module is


 1 #include<linux/init.h>

  2 #include<linux/kernel.h>
  3 #include<linux/module.h>
  4
  5 static void __init print_init (void)
  6 {
  7     unsigned long cs, ds, ss, es, fs, gs,currenttime;
  8     asm volatile("movl %%CS,%0\n\t":"=r"(cs));
  9     asm volatile("movl %%DS,%0\n\t":"=r"(ds));
 10     asm volatile("movl %%SS,%0\n\t":"=r"(ss));
 11     asm volatile("movl %%ES,%0\n\t":"=r"(es));
 12     asm volatile("movl %%FS,%0\n\t":"=r"(fs));
 13     asm volatile("movl %%GS,%0\n\t":"=r"(gs));
 14     printk ("**********************************\n");
 15     printk ("cs %lx\t%ld\n", cs, cs);
 16     printk ("ds %lx\t%ld\n", ds, ds);
 17     printk ("ss %lx\t%ld\n", ss, ss);
 18     printk ("es %lx\t%ld\n", es, es);
 19     printk ("fs %lx\t%ld\n", fs, fs);
 20     printk ("gs %lx\t%ld\n", gs, gs);
 21     printk ("**********************************\n");
 22
 23 }
24
 25 static void __exit print_exit (void)
 26 {
 27     unsigned long cs, ds, ss;
 28     asm volatile("movl %%cs,%0\n\t":"=r"(cs));
 29     asm volatile("movl %%ds,%0\n\t":"=r"(ds));
 30     asm volatile("movl %%ss,%0\n\t":"=r"(ss));
 31
 32     printk ("**********************************\n");
 33     printk ("cs %lx\t%ld\n", cs, cs);
 34     printk ("ds %lx\t%ld\n", ds, ds);
 35     printk ("ss %lx\t%ld\n", ss, ss);
 36     printk ("**********************************\n");
 37     printk ("*****************bye***************\n");
 38 }
 39
 40 module_init (print_init);
 41 module_exit (print_exit);

 42 MODULE_LICENSE ("GPL");


the result of the running this module is

[root at shell--box kernel_mod]# dmesg -c
**********************************
cs 60 96
ds 7b 123
ss 68 104
es 7b 123
fs d8 216
gs e0 224
**********************************

The cs and ds in the kernel space is 60 and 7b. But the kernel define the
 KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?


Thanks
Fan
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linux segment

[Fan Yang]

2012/10/28 Fan Yang <lljyangfan@gmail.com>

>
>
> 2012/10/27 Jun Hu <duanshuidao@hotmail.com>
>
>>   Can you post out your codes ?
>>
>>  *From:* Fan Yang <lljyangfan@gmail.com>
>> *Sent:* Wednesday, October 24, 2012 8:04 PM
>> *To:* kernelnewbies at kernelnewbies.org
>> *Subject:* linux segment
>>
>> 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?
>>
>>
>>
>> ------------------------------
>> _______________________________________________
>> Kernelnewbies mailing list
>> Kernelnewbies at kernelnewbies.org
>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>> Hi Jun Hu
>     There is my code which run at the user space:
>
>   1 #include<stdio.h>
>   2 main()
>   3 {
>   4     unsigned long cs, ds, ss, es, fs, gs;
>   5     asm volatile("movl %%CS,%0\n\t":"=r"(cs));
>   6     asm volatile("movl %%DS,%0\n\t":"=r"(ds));
>   7     asm volatile("movl %%SS,%0\n\t":"=r"(ss));
>   8     asm volatile("movl %%ES,%0\n\t":"=r"(es));
>
>   9     asm volatile("movl %%FS,%0\n\t":"=r"(fs));
>  10     asm volatile("movl %%GS,%0\n\t":"=r"(gs));
>  11     printf ("**********************************\n");
>  12     printf ("cs %lx\t%ld\n", cs, cs);
>  13     printf ("ds %lx\t%ld\n", ds, ds);
>  14     printf ("ss %lx\t%ld\n", ss, ss);
>  15     printf ("es %lx\t%ld\n", es, es);
>  16     printf ("fs %lx\t%ld\n", fs, fs);
>  17     printf ("gs %lx\t%ld\n", gs, gs);
>  18     printf ("**********************************\n");
>  19 }
>
>
>
> and the result of the progress in my machine is
>
>
> **********************************
> cs 73 115
> ds 7b 123
> ss 7b 123
> es 7b 123
> fs 0 0
> gs 33 51
> **********************************
>
>
> so, you can see the cs and ds register is 73 and 7b which are same as the
> kernel defined.  And the code of the kernel module is
>
>
>  1 #include<linux/init.h>
>
>   2 #include<linux/kernel.h>
>   3 #include<linux/module.h>
>   4
>   5 static void __init print_init (void)
>   6 {
>   7     unsigned long cs, ds, ss, es, fs, gs,currenttime;
>   8     asm volatile("movl %%CS,%0\n\t":"=r"(cs));
>   9     asm volatile("movl %%DS,%0\n\t":"=r"(ds));
>  10     asm volatile("movl %%SS,%0\n\t":"=r"(ss));
>  11     asm volatile("movl %%ES,%0\n\t":"=r"(es));
>  12     asm volatile("movl %%FS,%0\n\t":"=r"(fs));
>  13     asm volatile("movl %%GS,%0\n\t":"=r"(gs));
>  14     printk ("**********************************\n");
>  15     printk ("cs %lx\t%ld\n", cs, cs);
>  16     printk ("ds %lx\t%ld\n", ds, ds);
>  17     printk ("ss %lx\t%ld\n", ss, ss);
>  18     printk ("es %lx\t%ld\n", es, es);
>  19     printk ("fs %lx\t%ld\n", fs, fs);
>  20     printk ("gs %lx\t%ld\n", gs, gs);
>  21     printk ("**********************************\n");
>  22
>  23 }
> 24
>  25 static void __exit print_exit (void)
>  26 {
>  27     unsigned long cs, ds, ss;
>  28     asm volatile("movl %%cs,%0\n\t":"=r"(cs));
>  29     asm volatile("movl %%ds,%0\n\t":"=r"(ds));
>  30     asm volatile("movl %%ss,%0\n\t":"=r"(ss));
>  31
>  32     printk ("**********************************\n");
>  33     printk ("cs %lx\t%ld\n", cs, cs);
>  34     printk ("ds %lx\t%ld\n", ds, ds);
>  35     printk ("ss %lx\t%ld\n", ss, ss);
>  36     printk ("**********************************\n");
>  37     printk ("*****************bye***************\n");
>  38 }
>  39
>  40 module_init (print_init);
>  41 module_exit (print_exit);
>
>  42 MODULE_LICENSE ("GPL");
>
>
> the result of the running this module is
>
> [root at shell--box kernel_mod]# dmesg -c
> **********************************
> cs 60 96
> ds 7b 123
> ss 68 104
> es 7b 123
> fs d8 216
> gs e0 224
> **********************************
>
> The cs and ds in the kernel space is 60 and 7b. But the kernel define the
>  KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?
>
>
> Thanks
> Fan
>

sorry, the kernel define the KERNEL_DS as 68, but I get 7b in my machine.
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linux segment

[Mulyadi Santosa]

Hi Fan...

On Sun, Oct 28, 2012 at 9:02 PM, Fan Yang <lljyangfan@gmail.com> wrote:
>
> [root at shell--box kernel_mod]# dmesg -c
> **********************************
> cs 60 96
> ds 7b 123
> ss 68 104
> es 7b 123
> fs d8 216
> gs e0 224
> **********************************
>
> The cs and ds in the kernel space is 60 and 7b. But the kernel define the
> KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?
>


you print CS and DS twice, once during init and once during exit of
your kernel module. So, which one do you want to confirm?

All in all, I have a guess that you see such number (DS belongs to
user space in kernel module) because IIRC kernel module loading is
done using syscall and with the help of modprobe helper.

Thus, it is important to access user space during that stage, hence DS
still using user space data segment.


-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

linux segment

[Fan Yang]

2012/10/29 Mulyadi Santosa <mulyadi.santosa@gmail.com>

> Hi Fan...
>
> On Sun, Oct 28, 2012 at 9:02 PM, Fan Yang <lljyangfan@gmail.com> wrote:
> >
> > [root at shell--box kernel_mod]# dmesg -c
> > **********************************
> > cs 60 96
> > ds 7b 123
> > ss 68 104
> > es 7b 123
> > fs d8 216
> > gs e0 224
> > **********************************
> >
> > The cs and ds in the kernel space is 60 and 7b. But the kernel define the
> > KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?
> >
>
>
> you print CS and DS twice, once during init and once during exit of
> your kernel module. So, which one do you want to confirm?
>
> All in all, I have a guess that you see such number (DS belongs to
> user space in kernel module) because IIRC kernel module loading is
> done using syscall and with the help of modprobe helper.
>
> Thus, it is important to access user space during that stage, hence DS
> still using user space data segment.
>
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com
>

Hi  Mulyadi Santosa
   I get the same result during the kernel module init and exit. Then I try
to add a syscall to print these registers, and nothing changed. It is
strange.
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linux segment

[Mulyadi Santosa]

On Tue, Oct 30, 2012 at 7:44 AM, Fan Yang <lljyangfan@gmail.com> wrote:
> Hi  Mulyadi Santosa
>    I get the same result during the kernel module init and exit. Then I try
> to add a syscall to print these registers, and nothing changed. It is
> strange.

I think you need to observe deeper, something change this.

BTW, are you running this inside a virtualization? and which kernel
version do you use?

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com

linux segment

[Tobias Boege]

On Tue, 30 Oct 2012, Fan Yang wrote:
> 2012/10/29 Mulyadi Santosa <mulyadi.santosa@gmail.com>
> 
> > Hi Fan...
> >
> > On Sun, Oct 28, 2012 at 9:02 PM, Fan Yang <lljyangfan@gmail.com> wrote:
> > >
> > > [root at shell--box kernel_mod]# dmesg -c
> > > **********************************
> > > cs 60 96
> > > ds 7b 123
> > > ss 68 104
> > > es 7b 123
> > > fs d8 216
> > > gs e0 224
> > > **********************************
> > >
> > > The cs and ds in the kernel space is 60 and 7b. But the kernel define the
> > > KERNEL_CS as 60 and the KERNEL_DS as 7b.  Where am I wrong?
> > >
> >
> >
> > you print CS and DS twice, once during init and once during exit of
> > your kernel module. So, which one do you want to confirm?
> >
> > All in all, I have a guess that you see such number (DS belongs to
> > user space in kernel module) because IIRC kernel module loading is
> > done using syscall and with the help of modprobe helper.
> >
> > Thus, it is important to access user space during that stage, hence DS
> > still using user space data segment.
> >
> >
> > --
> > regards,
> >
> > Mulyadi Santosa
> > Freelance Linux trainer and consultant
> >
> > blog: the-hydra.blogspot.com
> > training: mulyaditraining.blogspot.com
> >
> 
> Hi  Mulyadi Santosa
>    I get the same result during the kernel module init and exit. Then I try
> to add a syscall to print these registers, and nothing changed. It is
> strange.

> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

(Weird, this is the third time, I have to send this. If anybody gets this
message multiple times, I apologise but my mail is not in the archives.)

If Mulyadi is right and we need DS to be USER_DS to access user space (I
really don't know, sorry, but maybe there is something in your <uaccess.h>?)
then your attempt to try with a syscall couldn't yield other values because
one trait of syscalls is that they can access user space.

This means you would get DS = USER_DS precisely _because_ you are in a
syscall. Module init and exit are, too, just some stack frames above one and
thus fall into this category as well.

But shouldn't it be possible to register a timer and then print the
segment registers? Timers are fired in softirq context and, hence, have no
connection to user space.

Regards,
Tobi

linux segment

[Baoquan He]

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

linux segment

[Fan Yang]

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@gmail.com>

> 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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linux segment

[Baoquan He]

Since you asked this question by phone, we search your email address and
found your questions here.

On 06/20/2013 06:01 PM, Fan Yang wrote:
> 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.
>
>
>