iret to pl3

[Adek336]

hi guys... I've been trying to add support for PL#3 code. In the end I have this code

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int system_main()
{
(..)
   asm volatile ("cli\n");
   asm volatile ("push $0x23\n"
         "push $0xc0008800\n"
         "push $0x200\n"
         "push $0x1b\n"
         "push $DIE\n"
         "iret");
(...)}

void DIE() asm ("DIE");
.globl DIE
DIE:
   nop
   nop
   nop
   mov $0xa1b2c3d4, %edi

   jmp DIE

It fails so badly even my interrupt handlers do not catch the problem. As I checked it, (uint32) &DIE == 0xc0008000, 0x1b is for PL#3 code, 0x23 is PL#3 data. The data pushed on stack seem in compliance with the iret specs.

user code: limit 0xfffff, base 0, attr 0xfa, flags 0xc0
used data: limit 0xfffff, base 0, attr 0xf2, flags 0xc0

If I change the code to
asm volatile ("pushf 0x8"
"pushf &DIE"
"iret");
it seems to work.

Hm, as I write this I begin to think it's the rtc timer interrupt which fails, perhaps because the kernel tss isn't yet loaded at that moment.. to be investigated.

Cheers,
Adrian.

[Pype.Clicker]

do you have a TSS and a SS0 /ESP0 for that task ? if not, the CPU will not be able to switch back to ring 0 to report exceptions ...

[Adek336]

I did map the appropriate pte entries with PTE_USER set, but forgot to do so with the whole page table. Now I am able to iret to a piece of code running in PL3. It actually spins a character in the topmost left of the screen

But, bloody intel, I can't get it to call an exception properly :/

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func_user:
  pushl $0x23
  popl %ds       ;setup user data segment
spin:
  incl (0x80001000)    ;access video memory, mapped to 0xb8000
  int $0xb
  jmp spin

intr0xb:
  hlt

That piece of code reboots. Hm, I once had some scheduling code which would switch between many PL3 threads :/ That's weird, I'll try to get at it.

(offtopic) So then the french did lost Paris within one month yes? Perhaps the war would end earlier if France did not fail so quickly.

Cheers
[shadow=red,left,300]Adrian, who has 3 stars ;D [/shadow]

[BI lazy]

oooch come on, what's that bragging about 3 stars good for? *rofl*

are you talking 'bout WWII?

Then, listen: If someone comes and holds a gun under your nose and tells you what to do, you are under normal circumstances quick at satisfying the request for the sake of your own beloved life. This has nothing to do with failing or so... it's just wanting to be alive and not wanting to experience harm. People like their skin intact, know ya?

That's what might have happened in France, hm, and in Austria tooo, when at 12.3.1938(not sure about this one) Hitler marched in and annexed Austria. Folks just wanted to live - and so they cooperated.

[Adek336]

Yea, possible.. didn't think about it that way.

Cheers,
Adrian.

[Pype.Clicker]

trying to *call* an exception, huh ? and you really though the CPU designer would let you fool them that way ?? Remember the 'PL' field in the IDT that you left to 0 "as said in the tutorials" ? well that was precisely what it was meant to do: prevent your user program to issue fake exceptions and other fake hardware interrupts ...

If you want to call the OS, you should provide a new interrupt with DPL=3 and call it.
If you just want to get out of your user mode program, you can either call the former interrupt with a code saying "i'm done" or just issue a HLT, which will raise a GPF and have something in your handler that knows HLT == end-of-process, or jump at a conventionnal 0xcafebabe address or something ... i'd personnally recommend the syscall approach for it is cleaner...

If you want to check how exceptions are processed, just do something illegal

ps: don't take offense if you feel like i'm teasing you... i found myself so stupid when i made the same mistake that i can't prevent myself to find it funny, still now ...

[Adek336]

Hm, so that's why I got the text "EXCEPTION 0x0d" each time I

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int $0

I'll try to use the DIV instruction to get int 0 running.. cheers,
Adrian

[Pype.Clicker]

Yup. If you want to see 'division by zero" message, just do

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   xor ecx, ecx
   div ecx

oh, btw, when i say

If you want to check how exceptions are processed, just do something illegal

i don't mean "download Kylie's last album using KaZaa" ... of course, this would only raise the MyDoomException ;D