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 Post subject: Scrolling VGA terminal causing semi-null pointer dereference
PostPosted: Wed Dec 01, 2021 6:48 pm 
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Joined: Thu Nov 25, 2021 7:11 pm
Posts: 17
Somehow, calling memmove to scroll the terminal ends up dereferencing some value in the first page of memory, which I deliberately leave unpaged. A page fault occurs.
Code:
memmove((void*)VGA_MEMORY,(void*)(&VGA_MEMORY[VGA_WIDTH]),(VGA_WIDTH*2)*(VGA_HEIGHT-1));
memset((void*)(&VGA_MEMORY[VGA_WIDTH*VGA_HEIGHT-1]),0,VGA_WIDTH*2);


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 Post subject: Re: Scrolling VGA terminal causing semi-null pointer derefer
PostPosted: Wed Dec 01, 2021 7:57 pm 
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Joined: Mon Mar 25, 2013 7:01 pm
Posts: 3574
sed4906h wrote:
Code:
VGA_WIDTH*VGA_HEIGHT-1

The multiplication operator has higher precedence than the subtraction operator. You need parentheses to perform the subtraction before the multiplication.


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 Post subject: Re: Scrolling VGA terminal causing semi-null pointer derefer
PostPosted: Wed Dec 01, 2021 8:27 pm 
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Joined: Thu Nov 25, 2021 7:11 pm
Posts: 17
I didn't see that, thanks. Still, the problem occurs in the memmove. Looking at the stack trace in GDB, the parameters are clearly trashed. Perhaps improperly returning from the page fault handler messed up the stack.


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 Post subject: Re: Scrolling VGA terminal causing semi-null pointer derefer
PostPosted: Wed Dec 01, 2021 9:27 pm 
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Joined: Mon Mar 25, 2013 7:01 pm
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Isn't the page fault happening after the parameters are trashed?

But you're right that your interrupt handlers don't preserve registers. That'll cause problems. They also need to clear the direction flag and set the data segment registers appropriately for your kernel's use.


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