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 Post subject: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 2:53 am 
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I'm in user mode, using gdb the debugger just steps over the int instruction.
Interrupts are enabled according to EFLAGS.

any thoughts ?

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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 3:03 am 
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Hello.

If this is Linux Assembly, please can I have the code where it doesn't work.

Thanks
Steve.


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 3:11 am 
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What gdb instruction are you using to single-step your code?


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 4:53 am 
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iansjack wrote:
What gdb instruction are you using to single-step your code?

si

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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 4:58 am 
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stevewoods1986 wrote:
Hello.

If this is Linux Assembly, please can I have the code where it doesn't work.

Thanks
Steve.

This is not a Linux assembly.
It's a flat binary file I jump into once I switch to user mode and execute it.

The code for it:

Code:
   0:   e8 02 00                call   0x5
   3:   00 00                   add    %al,(%bx,%si)
   5:   eb fe                   jmp    0x5
   7:   55                      push   %bp
   8:   89 e5                   mov    %sp,%bp
   a:   cd 80                   int    $0x80
   c:   b8 00 00                mov    $0x0,%ax
   f:   00 00                   add    %al,(%bx,%si)
  11:   5d                      pop    %bp
  12:   c3                      ret   
  13:   00 14                   add    %dl,(%si)
  15:   00 00                   add    %al,(%bx,%si)
  17:   00 00                   add    %al,(%bx,%si)
  19:   00 00                   add    %al,(%bx,%si)
  1b:   00 01                   add    %al,(%bx,%di)
  1d:   7a 52                   jp     0x71
  1f:   00 01                   add    %al,(%bx,%di)
  21:   7c 08                   jl     0x2b
  23:   01 1b                   add    %bx,(%bp,%di)
  25:   0c 04                   or     $0x4,%al
  27:   04 88                   add    $0x88,%al
  29:   01 00                   add    %ax,(%bx,%si)
  2b:   00 1c                   add    %bl,(%si)
  2d:   00 00                   add    %al,(%bx,%si)
  2f:   00 1c                   add    %bl,(%si)
  31:   00 00                   add    %al,(%bx,%si)
  33:   00 d3                   add    %dl,%bl
  35:   ff                      (bad) 
  36:   ff                      (bad) 
  37:   ff 0c                   decw   (%si)
  39:   00 00                   add    %al,(%bx,%si)
  3b:   00 00                   add    %al,(%bx,%si)
  3d:   41                      inc    %cx
  3e:   0e                      push   %cs
  3f:   08 85 02 42             or     %al,0x4202(%di)
  43:   0d 05 48                or     $0x4805,%ax
  46:   c5 0c                   lds    (%si),%cx
  48:   04 04                   add    $0x4,%al


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Last edited by stdcall on Sun Aug 13, 2017 8:19 am, edited 1 time in total.

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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 5:11 am 
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If the code isn't running under Linux, how is it running? (It looks like real mode.) Is there actually an int 0x80 handler?


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 5:30 am 
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stdcall wrote:
The code for it:
Code:
00000000 <.data>:

I see a contradiction here.

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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 6:04 am 
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BTW, note that the interrupt flag has no connection with a software interrupt.


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 6:17 am 
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iansjack wrote:
BTW, note that the interrupt flag has no connection with a software interrupt.


True, it could be an exception (fault). What platform is he/she using? Int 0x80 is Linux.


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 6:38 am 
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A software interrupt cannot be an exception.


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 8:04 am 
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stevewoods1986 wrote:
iansjack wrote:
BTW, note that the interrupt flag has no connection with a software interrupt.


True, it could be an exception (fault). What platform is he/she using? Int 0x80 is Linux.


This is no Linux, this is my OS. there's a dedicated isr 0x80 handler I've written and placed in the IDT. and event if there wasn't I would expect to see a general exception fault.

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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 8:06 am 
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XenOS wrote:
stdcall wrote:
The code for it:
Code:
00000000 <.data>:

I see a contradiction here.


That's not a contradiction, the format I use for executable is just plain binrary, there are no sections like in ELF.
so basically to dump the assembly out of the binary I typed:
Code:
objdump -D -b binary -mi386 -Maddr16,data16 ./binary_code

program:     file format binary

Disassembly of section .data:

00000000 <.data>:
   0:   e8 02 00                call   0x5
....


So basically, the .data section you think you saw is not really a section at all.

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“Utterly meaningless!
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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 8:30 am 
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I found a workaround around this but I don't seem to understand why it didn't work.
I changed the int number to 0x40 instead of 0x80 and now it works perfectly.
I don't get it.

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“Utterly meaningless!
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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 8:59 am 
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stdcall wrote:
I found a workaround around this but I don't seem to understand why it didn't work.
I changed the int number to 0x40 instead of 0x80 and now it works perfectly.
I don't get it.

You haven't really said anything about the context of your issue.

Are you running Linux, on which you run Qemu, on which you run your own OS, on which you run some test app you compiler that tries to use int 0x80? And you are using gdb from the host Linux to connect to Qemu and single stepping over said int 0x80?

Are you using a recent version of Qemu or something ancient? Did you compile it or use a packaged one? Are you using KVM with Qemu or..?


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 Post subject: Re: assemby int x80 doesn't do anything
PostPosted: Sun Aug 13, 2017 11:29 am 
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LtG wrote:
stdcall wrote:
I found a workaround around this but I don't seem to understand why it didn't work.
I changed the int number to 0x40 instead of 0x80 and now it works perfectly.
I don't get it.

You haven't really said anything about the context of your issue.

Are you running Linux, on which you run Qemu, on which you run your own OS, on which you run some test app you compiler that tries to use int 0x80? And you are using gdb from the host Linux to connect to Qemu and single stepping over said int 0x80?

Are you using a recent version of Qemu or something ancient? Did you compile it or use a packaged one? Are you using KVM with Qemu or..?


I'm Running Linux, on top of that I run Qemu and I'm connecting to the os using remote GDB protocol and Qemu offers.
The OS bootstraps and then runs a process in user mode. the process is a flat binary that is loaded via grub modules.
It appears that even before switching to user mode, int 0x80 doesn't do anything.
changing the number from 0x80 to something else (0x40) solved the issue, now the interrupt occurs.

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says the Teacher.
“Utterly meaningless!
Everything is meaningless.” - Ecclesiastes 1, 2

Educational Purpose Operating System - EPOS


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