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SubjectRe: [x86] Access off the bottom of stack causes a segfault?
On Tue, 14 Oct 2003, Chris Lattner wrote:
> Generated code:
> .intel_syntax
> ...
> main:
> mov DWORD PTR [%ESP - 16004], %EBP # Save EBP to stack
> mov %EBP, %ESP # Set up EBP
> sub %ESP, 16004 # Finally adjust ESP
> lea %EAX, DWORD PTR [%EBP - 16000] # Get the address of the array
> ...
> mov %EAX, 0 # Setup return value
> mov %ESP, %EBP # restore ESP
> mov %EBP, DWORD PTR [%ESP - 16004] # Restore EBP from stack
> ret

Ok, I found my own "answer": arch/i386/mm/fault.c:

/*
* accessing the stack below %esp is always a bug.
* The "+ 32" is there due to some instructions (like
* pusha) doing post-decrement on the stack and that
* doesn't show up until later..
*/
if (address + 32 < regs->esp)
goto bad_area;
Why exactly is accessing the stack below %esp always a bug?

-Chris

--
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