Re: nested functions used by multiboot2 loader corrupt stack

[Bean]

On Jan 17, 2008 8:21 PM, Robert Millan <address@hidden> wrote:
> On Thu, Jan 17, 2008 at 04:15:23PM +0800, Bean wrote:
> >
> > You need to add NESTED_FUNC_ATTR to nested callback function that use
> > local variable. here is the patch:
>
> Glad to see you found the reason!
>
> But I don't get it.  I see that:
>
> #define NESTED_FUNC_ATTR __attribute__ ((__regparm__ (2)))
>
> so does this mean in one of the calls the caller and callee disagreed about
> how the third param is passed?
>
> Also, we have a lot of nested functions without this macro.  How does one
> distinguish the ones that need it from the ones that don't?

Embedded function used %ecx to store the pointer to it's parent's
stack. However, the program is compiled using option -mregparm=3,
which means it can use up to 3 registry to pass parameter.In
grub_elf32_load_segment, there are three parameter elf, phdr and hook,
which will take up %eax, %edx and %ecx. The value of %ecx, hook, will
be overwritten. Use NESTED_FUNC_ATTR ensure that only the first two
parameter will be passed using registry

This problem can occur when the following conditions are true:

1, Use embedded function as callback.
2, The embedded function use local variable in it's parent's stack.
3, The embedded function has at least three parameters.

-- 
Bean