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Why does my interrupt handler fail?

I am trying to install an interrupt handler in Bochs , but for some reason my interrupt handler is not firing.

First I set a breakpoint:

00036440222i[CPU0 ] [36440222] Stopped on MAGIC BREAKPOINT (0) [0x0000000000703044] 0008:00000000c0003044 (unk. ctxt): int3

Then I try to execute int 3 .

However, when I do this, the error says int_trap_gate(): selector null , which seems to tell me that the selector in some table is null.

However, the GDT is shown below:

 <bochs:2> info gdt Global Descriptor Table (base=0x0000000000007c08, limit=23): GDT[0x00]=??? descriptor hi=0x00000000, lo=0x00000000 GDT[0x01]=Code segment, base=0x00000000, limit=0xffffffff, Execute/Read, Accessed, 32-bit GDT[0x02]=Data segment, base=0x00000000, limit=0xffffffff, Read/Write, Accessed

as well as IDT:

 <bochs:3> info idt Interrupt Descriptor Table (base=0x00000000c0101020, limit=2047): IDT[0x00]=32-Bit Interrupt Gate target=0x0001:0xc0004148, DPL=0 IDT[0x01]=32-Bit Interrupt Gate target=0x0001:0xc0004154, DPL=0 IDT[0x02]=32-Bit Interrupt Gate target=0x0001:0xc0004160, DPL=0 IDT[0x03]=32-Bit Interrupt Gate target=0x0001:0xc000416c, DPL=0 IDT[0x04]=32-Bit Interrupt Gate target=0x0001:0xc0004178, DPL=0 IDT[0x05]=32-Bit Interrupt Gate target=0x0001:0xc0004184, DPL=0 IDT[0x06]=32-Bit Interrupt Gate target=0x0001:0xc0004190, DPL=0 IDT[0x07]=32-Bit Interrupt Gate target=0x0001:0xc000419c, DPL=0 IDT[0x08]=32-Bit Interrupt Gate target=0x0001:0xc00041a8, DPL=0 IDT[0x09]=32-Bit Interrupt Gate target=0x0001:0xc00041b0, DPL=0 IDT[0x0a]=32-Bit Interrupt Gate target=0x0001:0xc00041bc, DPL=0 IDT[0x0b]=32-Bit Interrupt Gate target=0x0001:0xc00041c4, DPL=0 IDT[0x0c]=32-Bit Interrupt Gate target=0x0001:0xc00041cc, DPL=0 IDT[0x0d]=32-Bit Interrupt Gate target=0x0001:0xc00041d4, DPL=0 IDT[0x0e]=32-Bit Interrupt Gate target=0x0001:0xc00041dc, DPL=0 IDT[0x0f]=32-Bit Interrupt Gate target=0x0001:0xc00041e4, DPL=0 IDT[0x10]=32-Bit Interrupt Gate target=0x0001:0xc00041f0, DPL=0 IDT[0x11]=32-Bit Interrupt Gate target=0x0001:0xc00041fc, DPL=0 IDT[0x12]=32-Bit Interrupt Gate target=0x0001:0xc0004208, DPL=0 IDT[0x13]=32-Bit Interrupt Gate target=0x0001:0xc0004214, DPL=0 IDT[0x14]=32-Bit Interrupt Gate target=0x0001:0xc0004220, DPL=0 IDT[0x15]=32-Bit Interrupt Gate target=0x0001:0xc000422c, DPL=0 IDT[0x16]=32-Bit Interrupt Gate target=0x0001:0xc0004238, DPL=0 IDT[0x17]=32-Bit Interrupt Gate target=0x0001:0xc0004244, DPL=0 IDT[0x18]=32-Bit Interrupt Gate target=0x0001:0xc0004250, DPL=0 IDT[0x19]=32-Bit Interrupt Gate target=0x0001:0xc000425c, DPL=0 IDT[0x1a]=32-Bit Interrupt Gate target=0x0001:0xc0004268, DPL=0 IDT[0x1b]=32-Bit Interrupt Gate target=0x0001:0xc0004274, DPL=0 IDT[0x1c]=32-Bit Interrupt Gate target=0x0001:0xc0004280, DPL=0 IDT[0x1d]=32-Bit Interrupt Gate target=0x0001:0xc000428c, DPL=0 IDT[0x1e]=32-Bit Interrupt Gate target=0x0001:0xc0004298, DPL=0 IDT[0x1f]=??? descriptor hi=0x00000000, lo=0x00000000 <...> IDT[0xff]=??? descriptor hi=0x00000000, lo=0x00000000

It seems to me that the IDT records have valid GDT selectors, so I don’t understand what causes the problem when executing the interrupt:

 <bochs:5> s ; step (0).[36440222] [0x0000000000703044] 0008:00000000c0003044 (unk. ctxt): int3 CPU 0: Interrupt 0x03 occured (error_code=0x0000) 00036440222e[CPU0 ] int_trap_gate(): selector null CPU 0: Exception 0x0d - (#GP) general protection fault occured (error_code=0x0000) CPU 0: Interrupt 0x0d occured (error_code=0x0000) 00036440222e[CPU0 ] int_trap_gate(): selector null CPU 0: Exception 0x0d - (#GP) general protection fault occured (error_code=0x0000) CPU 0: Exception 0x08 - (#DF) double fault occured (error_code=0x0000) CPU 0: Interrupt 0x08 occured (error_code=0x0000) 00036440222e[CPU0 ] int_trap_gate(): selector null CPU 0: Exception 0x0d - (#GP) general protection fault occured (error_code=0x0000)

What is the problem?

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1 answer

Here:

 IDT[0x03]=32-Bit Interrupt Gate target=0x0001:0xc000416c, DPL=0

0x0001 is the value of the selector. Its 2 least significant bits are the RPL bit. The remaining bits are used as an index in the segment descriptor table (GDT or LDT). And these bits are 0. For all purposes, that selector is NULL.

Most likely you need 8, not 1.

For information on how selectors are precisely used, see the processor documentation.

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Source: https://habr.com/ru/post/1412544/

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