On Mon, Mar 19, 2007 at 06:17:10PM +0530, pradeep singh wrote:
> >------------------------------
> >
> >Date: Sat, 17 Mar 2007 16:29:15 +0200
> >Subject: Re: i386: Why putting __USER_DS in kernel threads stack (%esp) ?
> >From: "Ahmed S. Darwish" <darwish.07@xxxxxxxxx>
> >
> >On Fri, Mar 16, 2007 at 03:38:27AM +0700, Mulyadi Santosa wrote:
> >> Hi ...
> >> >Hi list,
> >> >
> >> >Reading the kernel threads initialization code I see:
> >> >
> >> >int kernel_thread(...) {
> >> >
> >> > struct pt_regs regs;
> >> > memset(®s, 0, sizeof(regs));
> >> > [...]
> >> >** regs.xds = __USER_DS;
> >> >** regs.xes = __USER_DS;
> >> >
> >>
> >> I was checking those codes too and I think that is just for satisfying
> >> the requirement of filling struct regs. I guess it is filled with
> >> USER_DS just to pretend the way do_fork is usually called via clone/fork
> >> syscall, that is to create normal user thread.
> >>
> >
> >Commenting those two lines out don't even start init. While using
> >__KERNEL_DS
> >instead of __USER_DS starts init then causes a
> >`Panic: Segment violation at 0x8049798 - Sleeping for 30 seconds'.
> >It may then have an important role than just mimicing user processes,
> >right ?
> >
> >What makes me wonder is that as said in Understanding Linux Kernel (v3):
> >
> > When CPL (Current Privelege level) is equal to 3, the ds register must
> > contain the segment selector of the user data segment. When it's equal to
> > zero, the ds register must contain the segment selector of the kernel
> > data
> > segment.
>
> IMHO selector is different from segment descriptor right?
> descriptor's privilige level is checked ,as i make out of it. Selector
> comes after checking the CPL with the DPL in the GDT or LDT.
I think the logical sequence is
Selector [in segment registers ] -> GDT -> Descriptor
> Somebody can explain this better and correct me if i am wrong somewhere.
>
> >
> >So how come running a kernel thread with DS = __USER_DS ?
> AFAIK __USER_DS and __KERNEL_DS have exactly the same values.
Yes they have the same _start_ values to make segmentation transparent,
but they differ in other fields, espicifically the DPL (Descriptor privilege
level).
--
Ahmed S. Darwish
http://darwish.07.googlepages.com
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