Do I need the GDTPTR after loading it with LGDT

[teodori]

Hello, say I put my GDT with 256 descriptor from 0x1000 to 0x2000, then load with LGDT my GDTPTR made of 10 bytes from any location.
Is it then possible to overwrite the memory of my GDTPTR?
I know that the GDT must not be overwritten, but what about my GDTPTR containing the virtual address and the size of the GDT!

[thomasloven]

Testing...
My kernel won't boot...
Without actually reading the documentation again, I believe this would make sense. The GDT register is probably 32 bits, while the gdt pointer is 40 bits iirc.

But if you're short on space, you can store the gdt pointer in gdt entry 0 (which is unused). Works on qemu and bochs at least.

[teodori]

Hm I think we missunderstand us. Here is a part of my code.
I copy the the GDT from my .rodata to memory, but my GDTPTR is loaded with LGDT and it remains in the .rodata, which part will be delete when the kernel loader exits.

Code: Select all

...
gdt64:
	# Null Descriptor
	.word 0x0000, 0x0000
	.byte 0x00, 0b00000000, 0b00000000, 0x00
	# Code Descriptor
	.word 0xffff, 0x0000
	.byte 0x00, 0b10011010, 0b10101111, 0x00
	# Data Descriptor
	.word 0xffff, 0x0000
	.byte 0x00, 0b10010010, 0b10101111, 0x00
gdt64_end:

gdt64ptr:
	.word (gdt64_end - gdt64 - 1)
	.quad 0x0000
...
	movl $(INITSEG * 0x10 + gdt64), %esi
	movl $0x0000, %edi
	movl $(gdt64_end - gdt64), %ecx
	rep movsb
...
	lgdt (INITSEG * 0x10 + gdt64ptr)
...

[thomasloven]

Oh. I assumed you were talking about a 32 bit kernel.
I have no experience with 64 bit programming. Sorry.

[doxrobot]

yes it's fine. you can over-write it.

[teodori]

thank you

[AbstractYouShudNow]

Why would you want to overwrite it ?
Basically, a C kernel defines a struct gdtr, takes a static variable of that type, and use lgdt with a pointer to the structure, so you don't need to overwrite it at least.

Anyways, I don't know whether ALL x86 compatible processors will cache the value in memory so that you can overwrite gdtr. This would require extensive testing, but would remain an ugly hack though

[Brendan]

Hi,

Why would you want to overwrite it ?

I'd assume the idea is to free RAM that was used during boot so it's not left allocated for no reason forever. The GDTPTR is small, but if you've got code to free a "no longer needed after boot" area for other reasons then it makes sense to put whatever you can (including small things) in that area.

Anyways, I don't know whether ALL x86 compatible processors will cache the value in memory so that you can overwrite gdtr. This would require extensive testing, but would remain an ugly hack though

Intel CPUs have done this since the beginning (80386), and I doubt any CPU manufacturer would want to be "less compatible" without a very good reason.

If a CPU didn't keep track of the address of the GDT it'd have to keep track of the address of the GDTPTR instead (it'd be just as much work for CPU designers), and every time the CPU needs something from the GDT it'd have an extra fetch to access the GDTPTR first (it wouldn't be faster). Basically, there isn't a good reason for a CPU manufacturer to want to be "less compatible", and I'd be happy to assume all 80x86 CPUs keep track of the address of the GDT.


Cheers,

Brendan