Question about local variables and paging...
Posted: Wed Jan 10, 2007 8:55 pm
I didn't think this would be an issue, but I have a local variable in some function called buffer:
The deadbeef is there for a memory dump.
This buffer variable is in a static member function in a class (C++ Kernel). Usually this function works fine. But after I enable my own paging that maps only what I need (the kernel intially starts with 4MB identity mapped) like video memory, kernel pages, and the free page stack, calling this function page faults. If I remove that local variable, it's fine. When I check the bochs log, I get this:
The code disassembly is here (mixed with source):
I have no idea why it'd try to load eax with gs:0x14.
Perhaps something with my linker script?...
I really have no idea what could be causing this or what. I've been hacking on this for a few days now and it's driving me insane, any help is appreciated.
Code: Select all
char buffer[8] = {0xd,0xe,0xa,0xd,0xb,0xe,0xe,0xf};
This buffer variable is in a static member function in a class (C++ Kernel). Usually this function works fine. But after I enable my own paging that maps only what I need (the kernel intially starts with 4MB identity mapped) like video memory, kernel pages, and the free page stack, calling this function page faults. If I remove that local variable, it's fine. When I check the bochs log, I get this:
Code: Select all
00016288115-@c0100b47-[CPU ]: page fault for address 00000014 @ c0100b47
00016288115-@c0100b47-[CPU ]: exception(0x0E)
00016288115-@c0100b41-[CPU ]: interrupt(): vector = 14, INT = 0, EXT = 1
00016288115-@c0100b41-[CPU ]: page fault for address 00000070 @ c0100b41
00016288115-@c0100b41-[CPU ]: exception(0x0E)
00016288115-@c0100b41-[CPU ]: interrupt(): vector = 8, INT = 0, EXT = 1
00016288115-@c0100b41-[CPU ]: page fault for address 00000040 @ c0100b41
00016288115-@c0100b41-[CPU ]: exception(0x0E)
00016288115-@c0100b41-[CPU ]: protected mode
00016288115-@c0100b41-[CPU ]: CS.d_b = 32 bit
00016288115-@c0100b41-[CPU ]: SS.d_b = 32 bit
00016288115-@c0100b41-[CPU ]: | EAX=00000030 EBX=00002000 ECX=00109000 EDX=00000001
00016288115-@c0100b41-[CPU ]: | ESP=c0107f98 EBP=00067ee4 ESI=000544c1 EDI=000544c2
00016288115-@c0100b41-[CPU ]: | IOPL=0 id vip vif ac vm RF nt of df if tf SF zf AF pf cf
00016288115-@c0100b41-[CPU ]: | SEG selector base limit G D
00016288115-@c0100b41-[CPU ]: | SEG sltr(index|ti|rpl) base limit G D
00016288115-@c0100b41-[CPU ]: | CS:0008( 0001| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | DS:0010( 0002| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | SS:0010( 0002| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | ES:0010( 0002| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | FS:0010( 0002| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | GS:0010( 0002| 0| 0) 00000000 000fffff 1 1
00016288115-@c0100b41-[CPU ]: | EIP=c0100b41 (c0100b41)
00016288115-@c0100b41-[CPU ]: | CR0=0x80000011 CR1=0 CR2=0x00000040
00016288115-@c0100b41-[CPU ]: | CR3=0x00109000 CR4=0x00000010
00016288115-@c0100b41-[CPU ]: >> mov eax, dword ptr gs:0x14 : 65A114000000
Code: Select all
char Convert::Blah(char x, char y)
c0100b2e: 83 ec 1c sub esp,0x1c
c0100b31: 8b 44 24 20 mov eax,DWORD PTR [esp+32]
c0100b35: 8b 54 24 24 mov edx,DWORD PTR [esp+36]
c0100b39: 88 44 24 08 mov BYTE PTR [esp+8],al
c0100b3d: 88 54 24 04 mov BYTE PTR [esp+4],dl
c0100b41: 65 a1 14 00 00 00 mov eax,gs:0x14
c0100b47: 89 44 24 18 mov DWORD PTR [esp+24],eax
c0100b4b: 31 c0 xor eax,eax
{
char buffer[8] = {0xd,0xe,0xa,0xd,0xb,0xe,0xe,0xf};
c0100b4d: c6 44 24 10 0d mov BYTE PTR [esp+16],0xd
c0100b52: c6 44 24 11 0e mov BYTE PTR [esp+17],0xe
c0100b57: c6 44 24 12 0a mov BYTE PTR [esp+18],0xa
c0100b5c: c6 44 24 13 0d mov BYTE PTR [esp+19],0xd
c0100b61: c6 44 24 14 0b mov BYTE PTR [esp+20],0xb
c0100b66: c6 44 24 15 0e mov BYTE PTR [esp+21],0xe
c0100b6b: c6 44 24 16 0e mov BYTE PTR [esp+22],0xe
c0100b70: c6 44 24 17 0f mov BYTE PTR [esp+23],0xf
return (x+y) & 0x7F;
c0100b75: 0f b6 54 24 08 movzx edx,BYTE PTR [esp+8]
c0100b7a: 0f b6 44 24 04 movzx eax,BYTE PTR [esp+4]
c0100b7f: 8d 04 02 lea eax,[edx+eax]
c0100b82: 0f be c0 movsx eax,al
c0100b85: 83 e0 7f and eax,0x7f
}
c0100b88: 8b 54 24 18 mov edx,DWORD PTR [esp+24]
c0100b8c: 65 33 15 14 00 00 00 xor edx,DWORD PTR gs:0x14
c0100b93: 74 05 je c0100b9a <ForgueOS::Convert::Blah(char, char)+0x6c>
c0100b95: e8 dc f5 ff ff call c0100176 <__stack_chk_fail>
c0100b9a: 83 c4 1c add esp,0x1c
c0100b9d: c3 ret
Perhaps something with my linker script?...
I really have no idea what could be causing this or what. I've been hacking on this for a few days now and it's driving me insane, any help is appreciated.