Page fault,why ?

[Kupak]

i've got a strange problem,if i dont set the u/s bit in the page table for a new process's kernel stack,the iretd (i think) in the context_start() code gives me page fault,if i set it no problem.

Another question: how does u/s bit  behave when set in page directories,overrides the settings of the page tables ?

THX !

[anton]

Is WP flag in CR0 set? What is the privelage of your code?
Now anout the second question :
From 3.6.4 Volume 3 :
User/supervisor (U/S) flag, bit 2
Specifies the user-supervisor privileges for a page or group of pages (in the
case of a page-directory entry that points to a page table). ...

Anton

[Kupak]

Hi Anton !

I dont touch that flag,but why would that matter !?
The code is ring 0,it happens when the kernel starts a new process !
Context_load() is the place where i build up the stack frame for the iretd,so that the ring 3 code can start.Iretd pops the ring 3 es:esp from the kernel stack and eflags then eip:cs.


Here is the code :
_context_start:

cli

mov eax,[esp+4]                         ;eip
mov ebx,[esp+8]                         ;user stack
mov ecx,32+3 ;ring 3 data segment selector

push ecx ;ss3
push ebx ;esp3
push dword 0x200 ;eflags
push dword 24+3 ;ring 3 code selector
push eax ;eip

mov ds,cx
mov es,cx
mov fs,cx
mov gs,cx

xor eax,eax
xor ebx,ebx
xor ecx,ecx
xor edx,edx

mov edi,0
mov esi,0
mov ebp,0

iretd

[anton]

Probobly iretd does not generate a page fault, code that is after that(the thread it self) generates it!
From 4.11.2 v3
...
When the processor is in supervisor mode, it can access all pages; when in
user mode, it can access only !!!user-level pages!!!.

So, if you don't set u/s the page is in supervisor mode, but your program runs in user mode(ring 3)=>page fault!!!

anton