[OCLUG-devel] arrays

[James Colannino]

Christopher Smith wrote:

> C doesn't have any bounds checking so it's not going to stop you from
> accessing data outside the array. Now, depending on how the allocator
> is setup, there may be some "padding" around your array that prevents
> really bad things from happening. In terms of what could go wrong?
> Well, this is a classic example of how stack overflows occur. You're
> writing past the end of the stack-allocated array, which means you're
> writing all over the stack. I'm guessing you aren't having a problem
> because you're exiting immediately after this. However, if this was in
> a subroutine, then when you return, the subroutine would pop off the 5
> bytes of your array and any arguments that were passed in, and then
> pop off the return address. If the return address has been overwritten
> by your array data, then it'll jump somewhere unanticipated. In most
> cases this will be a segmentation fault because it'll be a totally
> invalid address (which should be happening in this case). Sometimes
> though, you can be unfortunate enough to write in an address that is
> valid, at which your program will jump to that location and attempt to
> execute instructions. Very, very messy.


By the way, I'm just curious, does the kernel ever do anything about a
situation in which a process attempts to execute corrupted
instructions?  I guess there's probably not much it could do, but does
the kernel at least protect you from truly dangerous things that could
result from the random contents of a variable being executed as
instructions?  I'm guessing that since the kernel takes full control of
the hardware it would probably protect you at least to a certain
extent.  Perhaps such a thing could cause a kernel panic?

James

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