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| [Solved] an issuse about GP exception in infinte loop https://forum.osdev.org/viewtopic.php?f=1&t=36733 |
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| Author: | whereware [ Fri May 01, 2020 10:31 pm ] |
| Post subject: | [Solved] an issuse about GP exception in infinte loop |
I am learning things about os dev and I write a simple loader promgram. It loads gdt, enters protected mode and runs inito a infinte loop. The problem is when program runs into the infinte loop like "jmp $"... it causes GP exception, then again, finally I get a triple fault. but if I enter "si" continusly in GDB to run "jmp $" it runs as expected... are there any mechanisms in processor to prevent this kind of inf loop? or it is just fault of my code. thanks in advance! |
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| Author: | iansjack [ Sat May 02, 2020 12:52 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
It would help if you posted a link to your code (in an online repository - don't just paste all the code here) and detailed the exact steps that you followed in gdb. On the face of it, if a program causes a GPF when run it will do the same when running under gdb. |
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| Author: | whereware [ Sat May 02, 2020 4:24 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
iansjack wrote: It would help if you posted a link to your code (in an online repository - don't just paste all the code here) and detailed the exact steps that you followed in gdb. On the face of it, if a program causes a GPF when run it will do the same when running under gdb. Thank you for your reply! Here is the code: https://github.com/D0ot/dotkernel/blob/osloader-dev/osloader/osloader.asm It would cause some GPFs and finally a tripple fault with QEMU + gdb. Steps: (type in gdb) 1. "b _flush" , _flush is a label in the file above , at line 38 2. "si" "si" , type many si, it will reach the "jmp $" 3. "si", it runs "jmp $" well. no GPF 4. "c", QEMU closes(i used the option "--no-reboot" when lanuching QEMU) and leaves log saying : Code: check_exception old: 0xffffffff new 0xd 1: v=0d e=0042 i=0 cpl=0 IP=0008:00007e38 pc=00007e38 SP=0010:00007c00 env->regs[R_EAX]=d88e0010 ... check_exception old: 0xd new 0xd 2: v=08 e=0000 i=0 cpl=0 IP=0008:00007e38 pc=00007e38 SP=0010:00007c00 env->regs[R_EAX]=d88e0010 ... check_exception old: 0x8 new 0xd Triple fault disassembly outpu by objdump: "i686-elf-objdump -d -M intel -M i386 osloader.elf" Code: 00007e29 <_flush>: 7e29: 66 b8 10 00 mov ax,0x10 7e2d: 8e d8 mov ds,eax 7e2f: 8e c0 mov es,eax 7e31: 8e d0 mov ss,eax 7e33: bc 00 7c 00 00 mov esp,0x7c00 7e38: eb fe jmp 7e38 <_flush+0xf> I don't know why... |
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| Author: | Octocontrabass [ Sat May 02, 2020 4:42 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
whereware wrote: Code: e=0042 This means the IDT entry for INT 0x08 (IRQ0) is invalid. Perhaps you should disable interrupts before switching to protected mode. |
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| Author: | iansjack [ Sat May 02, 2020 4:56 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
As exception 0x08 is a double fault, I don't think that is the root cause. But, as there is no IDT to be valid or invalid, the root cause is obviously some interrupt or exception occurring. Could be the timer interrupt and you just didn't single-step enough times to hit it. As already said, either disable interrupts or create a valid IDT with valid handlers. |
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| Author: | whereware [ Sat May 02, 2020 5:19 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
Octocontrabass wrote: whereware wrote: Code: e=0042 This means the IDT entry for INT 0x08 (IRQ0) is invalid. Perhaps you should disable interrupts before switching to protected mode. Thanks! I am just a beginner and I don't know the meaning of that "e=xxx".  thanks again. hahaha
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| Author: | whereware [ Sat May 02, 2020 5:28 am ] |
| Post subject: | Re: an issuse about GP exception in infinte loop |
iansjack wrote: As exception 0x08 is a double fault, I don't think that is the root cause. But, as there is no IDT to be valid or invalid, the root cause is obviously some interrupt or exception occurring. Could be the timer interrupt and you just didn't single-step enough times to hit it. As already said, either disable interrupts or create a valid IDT with valid handlers. Thanks! It could be a best practice? Maybe I should read code of someone else. hahaha. Thanks again! |
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