When I am trying to start one weird program under dosemu, I always get an Oops.
I have tracked this Oops down to a line of a source code in the kernel, see
below.
At first, the Oops itself:
---
ksymoops 2.4.3 on i686 2.2.20. Options used
-V (default)
-k /proc/ksyms (default)
-l /proc/modules (default)
-o /lib/modules/2.2.20/ (default)
-m /boot/System.map (specified)
Dec 2 21:04:05 localhost kernel: Unable to handle kernel paging request at virtual address 000078c3
Dec 2 21:04:05 localhost kernel: current->tss.cr3 = 0ad2f000, %cr3 = 0ad2f000
Dec 2 21:04:05 localhost kernel: *pde = 0b802067
Dec 2 21:04:05 localhost kernel: Oops: 0003
Dec 2 21:04:05 localhost kernel: CPU: 0
Dec 2 21:04:05 localhost kernel: EIP: 0010:[handle_vm86_fault+1149/2212]
Dec 2 21:04:05 localhost kernel: EFLAGS: 00010206
Dec 2 21:04:05 localhost kernel: eax: 00000009 ebx: 00003013 ecx: 00000020 edx: 00000021
Dec 2 21:04:05 localhost kernel: esi: 000000f3 edi: 000077d0 ebp: 000c39e0 esp: cad31ea8
Dec 2 21:04:05 localhost kernel: ds: 0018 es: 0018 ss: 0018
Dec 2 21:04:05 localhost kernel: Process xdos (pid: 872, process nr: 57, stackpage=cad31000)
Dec 2 21:04:05 localhost kernel: Stack: 000077d0 000000f4 00003013 000c39e0 00002827 cad31f00 00000000 c010a8cc
Dec 2 21:04:05 localhost kernel: 00009fff 00000000 00000000 cad30000 cad30000 00002827 c010a90f cad31f00
Dec 2 21:04:05 localhost kernel: 00000000 cad30000 00000000 c010a05d cad31f00 00000000 00000001 00000005
Dec 2 21:04:05 localhost kernel: Call Trace: [do_general_protection+0/116] [do_general_protection+67/116] [error_code+53/64] [system_call+52/56]
Dec 2 21:04:06 localhost kernel: Code: 88 7c 37 00 66 4e 88 5c 37 00 68 9d eb 1b c0 e8 3f 8b 00 00
Using defaults from ksymoops -t elf32-i386 -a i386
Code; 00000000 Before first symbol
00000000 <_EIP>:
Code; 00000000 Before first symbol
0: 88 7c 37 00 mov %bh,0x0(%edi,%esi,1)
Code; 00000004 Before first symbol
4: 66 4e dec %si
Code; 00000006 Before first symbol
6: 88 5c 37 00 mov %bl,0x0(%edi,%esi,1)
Code; 0000000a Before first symbol
a: 68 9d eb 1b c0 push $0xc01beb9d
Code; 0000000e Before first symbol
f: e8 3f 8b 00 00 call 8b53 <_EIP+0x8b53> 00008b52 Before first symbol
---
The faulting code is here:
arch/i386/kernel/vm86.c:341:
---
#define pushw(base, ptr, val) \
__asm__ __volatile__( \
"decw %w0\n\t" \
"movb %h2,0(%1,%0)\n\t" \ <-- fault is HERE!
"decw %w0\n\t" \
"movb %b2,0(%1,%0)" \
: "=r" (ptr) \
: "r" (base), "q" (val), "0" (ptr))
---
It is executed here, when it GPFs:
vm86.c:509:
---
case 0x9c:
SP(regs) -= 2;
IP(regs)++;
pushw(ssp, sp, get_vflags(regs)); <--handle_vm86_fault+1149/2212
VM86_FAULT_RETURN;
---
The registers:
ebx contains 0x3013. This is the value returned by get_vflags(regs).
edi=0x77d0 and esi=0xf3 contains dosemu's ss and sp.
So virtual flags are going to be moved to ss:sp to simulate pushf.
This all seems OK to me and I don't understand why the code
mov %bh,0x0(%edi,%esi,1)
can raise GPF with such values in regs.
So, I have collected all the info I ever could from this Oops, but I still
don't see a bug. I hope someone can help me.
Any ideas such as how this code (atleast theoretically) can raise GPF?
Any suggestions such as how can I collect more info on this subject?
I am pretty shure I can reproduce this Oops at any kernel, atleast at any 2.2.x,
but currently I am using 2.2.20.
Thanks for any advice.
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