Re: [PATCH] Fix broken recursion detection when printing static members

Simon Marchi
2017-10-25T02:24:24Z

On 2017-10-24 06:59, Patrick Frants wrote:
> I am so sorry, looks like git send-email removed the first N lines.
> Here is
> the complete changelog:
>
> Fix broken recursion detection when printing static members
>
> Recursion detection for static members was broken. The implementation
> uses
> a growing (and shrinking) obstack object to simulate a stack of
> addresses
> (CORE_ADDR). Pushing addresses is implemented by calling
> obstack_grow(),
> while popping is implemented by calling obstack_free(). The latter is
> problematic because obstack_free() expects a pointer to the base of an
> object. When popping elements of the stack however, obstack_free() was
> called with the new top, which potentially is not the same as the base
> of
> the stack. This is unintended use and the effect is that
> obstack->next_free
> and obstack->object_base members are assigned the value of the new top,
> which equals an empty stack. Summary: popping elements would always
> result
> in an empty stack, which breaks the recursion detection.
>
> The fix shrinks the stack using obstack_blank_fast() with a negative
> value
> as described at the bottom of this page:
> https://gcc.gnu.org/onlinedocs/libiberty/Extra-Fast-Growing.html
> "You can use obstack_blank_fast with a “negativeâ€ size argument to make
> the
> current object smaller. Just don’t try to shrink it beyond zero
> length—there’s no telling what will happen if you do that. Earlier
> versions
> of obstacks allowed you to use obstack_blank to shrink objects. This
> will
> no longer work."

Thanks for the nice description!

Simon