[PATCH] 69517 - [5/6 regression] SEGV on a VLA with excess initializer elements

Martin Sebor msebor@gmail.com
Mon Mar 14 21:26:00 GMT 2016


Ping:
   https://gcc.gnu.org/ml/gcc-patches/2016-03/msg00441.html

On 03/06/2016 06:38 PM, Martin Sebor wrote:
> GCC 4.9 had added support for C++ VLAs as specified in WG21
> document N3639 expected to be included in C++ 14.  However,
> WG21 ultimately decided not to include N3639 in C++ 14 and
> the G++ support was partially removed in 5.1.  Unfortunately,
> the removal rendered some safe albeit erroneous G++ 4.9 code
> undefined.  This patch restores the well-defined behavior of
> such code by having it throw an exception in response to
> the erroneous conditions.
>
> While testing the patch I found a number of other problems in
> the G++ support for VLAs, including PR c++/70019 - VLA size
> overflow not detected, which was never implemented (not even
> in 4.9).  Since this is closely related to the regression
> discussed in 69517 the patch also provides that support.
>
> There are a few additional points to note about the patch:
>
> 1) It restores the std::bad_array_length exception from N3639,
>     even though the class isn't specified by the C++ standard.
>     At first I thought that introducing a different (private)
>     type would be more appropriate, but in the end couldn't come
>     up with a good argument for not keeping the same type.  Using
>     the same type also allows programs that rely on the exception
>     and that were built with GCC 4.9 to be ported to GCC 6 without
>     change.
>
> 2) It hardwires a rather arbitrarily restrictive limit of 64 KB
>     on the size of the biggest C++ VLA.  (This could stand to be
>     improved and made more intelligent, and perhaps integrated
>     with stack  checking via -fstack-limit, after the GCC 6
>     release.)
>
> 3) By throwing an exception for erroneous VLAs the patch largely
>     defeats the VLA Sanitizer.  The sanitizer is still useful in
>     C++ 98 mode where the N3639 VLA runtime checking is disabled,
>     and when exceptions are disabled via -fno-exceptions.
>     Disabling  the VLA checking in C++ 98 mode doesn't seem like
>     a useful feature, but I didn't feel like reverting what was
>     a deliberate decision.
>
> Martin



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