Re: Fix and Continue, part 1

[Mike Stump <mrs at apple dot com>]

On Thursday, March 6, 2003, at 07:33 PM, Daniel Jacobowitz wrote:
> Why is this necessary?  I'd think it would be generally easier and less
> intrusive to just make sure functions were a minimum of (however many
> instructions) long; if you're overwriting the code, why not overwrite
> the code instead of a padding buffer?

The gdb folks here who designed it wanted it that way!  :-)  They face 
the problem up dating and patching in the new .o files into a running 
system, its hard enough, and the nops remove some of the hair and make 
it a bit easier for them to do it.  I don't  have a better answer than 
that, though, if someone finds a way to make it work, even when the 
nops aren't there, I'd be happy to remove them.

> > There is another set of changes that indirect all global data accesses
> > through pointers at runtime, so that the newly loaded code will get 
> > the
> > old data, instead of any new data.  I'll be submitting that next.
>
> Hrm, I can't intuitively see why that's necessary either.  I'm curious,
> tell me more :)

Oh, this is far easier:

int i = 10;

foo() {
	++i;
}

and then reload:

int i = 10;

foo() {
	++i;
	printf("%d\n", i);\
}

on top of the previous code.  We want the printf to print the `old' i, 
not the newly loaded i.  If i is 100 in the running program, and we 
load up the new .o file, we want the next iteration through foo to 
print 101, not 11.