Re: [PATCH][RFC] Avoid indexing std::vector past the end

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2017-12-28 11:01, Ruslan Kabatsayev wrote:
> Hello all,
> 
> On my system I have added some asserts into GCC's stl_vector.h, which 
> check for
> various mistakes like out of bounds access, call to std::vector::front 
> on empty
> vector etc. to debug my own projects. After I built GDB with such
> modifications, I've noticed that in some cases it accesses some vectors 
> out of
> bound, namely element one past the end. Effectively the code is 
> something like
> `auto*p=&someVector[someVector.size()];`, which, although may seem 
> legitimate
> on the first glance since it simply takes address, is still Undefined 
> Behavior
> according to the C++ Standard (see e.g. [1] and links in that page).
> 
> So I wonder whether GDB deliberately exploits undefined behavior here 
> knowing
> that GCC might give(?) some guarantee that this will always work as 
> intended,
> or it's simply a mistake, and my patch would be OK.
> 
> [1]: https://stackoverflow.com/a/27069592/673852
> 
> Regards,
> Ruslan

Hi Ruslan,

Thanks for finding and reporting this.  We certainly don't want to rely 
on any compiler-specific undefined behavior, this is a mistake.

The patch looks good to me, it's just missing a ChangeLog entry.

Simon