[PATCH] configure: add ABIGAIL_DEBUG options

[Mark Wielaard]

Hi Matthias,

On Mon, 2020-05-11 at 17:24 +0200, Matthias Maennich via Libabigail wrote:
> When exporting ABIGAIL_DEBUG=1, the binaries compiled are especially
> suitable for debugging. The CFLAGS and CXXFLAGS that are added disable
> optimization and increase debug information levels.
> 
> 	* configure.ac: add ABIGAIL_DEBUG environment variable for
> 	improved debugging capabilities
> 
> Signed-off-by: Matthias Maennich <maennich@google.com>
> ---
>  configure.ac | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/configure.ac b/configure.ac
> index 9f30ea38cf86..9aea79f49e9a 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -621,6 +621,11 @@ if test x$ABIGAIL_DEVEL != x; then
>     CXXFLAGS="-g -Wall -Wextra -Werror"
>  fi
>  
> +if test x$ABIGAIL_DEBUG != x; then
> +    CFLAGS="$CFLAGS -O0 -g3 -ggdb"
> +    CXXFLAGS="$CXXFLAGS -O0 -g3 -ggdb"
> +fi
> +

I think you either want -O0 plus -fno-inline to make debugging even
easier. Or (better IMHO) use -Og so you do get some optimization, just
none that makes debugging harder. The advantage of -Og is that you can
then also add -D_FORTIFY_SOURCE=2 to detect various memory and string
operation bugger overflows early (or even at compile time).

You also will want to add -D_GLIBCXX_DEBUG which catches various
illegal uses of std iterators and algorithms. In fact I just tried it
and while running make check it found:

   safe_iterator.h:360:error: attempt to advance 
       signed char dereferenceable (start-of-sequence) iterator -1
   steps, which falls     
       outside its valid range.

   Objects involved in the operation:
   iterator @ 0x0x7fffffffc740 {
   type =
   __gnu_debug::_Safe_iterator<__gnu_cxx::__normal_iterator<abigail::ir
   ::function_decl* const*,
   std::__cxx1998::vector<abigail::ir::function_decl*,
   std::allocator<abigail::ir::function_decl*> > >,
   std::__debug::vector<abigail::ir::function_decl*,
   std::allocator<abigail::ir::function_decl*> > > (constant iterator);
     state = dereferenceable (start-of-sequence);
     references sequence with type
   `std::__debug::vector<abigail::ir::function_decl*,
   std::allocator<abigail::ir::function_decl*> >' @ 0x0x7fffffffc740 }

Poking around in gdb found that in compute_diff () the a_base
RandomAccessOutputIterator could go back (-1) and then forward (+1)
again because there were missing brackets. That is undefined behavior
if we are at the beginning of the iterator.

Proposed fix attached.

Cheers,

Mark
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