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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110848] Consider enabling -Wvla by default in non-GNU C++
 modes
Date: Mon, 23 Oct 2023 10:46:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110848

--- Comment #23 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Martin Uecker from comment #20)
> And what alternative do you think is fundamentally safer than VLAs?
>=20
> VLAs know their bound. Thus, they integrate with _FORTIFY_SOURCE, and UBS=
an
> bounds checking. Also UBSan address checking at run-time. At compile-time
> there is -Ws

They don't integrate with idiomatic C++ such as ranges algorithms, and
std::end.

More generally, they simply don't integrate with the C++ type system, so are
unusable with most generic code using templates. Not only does std::is_array
not recognise them as arrays, but even attempting to ask the question with
std::is_array is ill-formed. Variably modified types are not part of the C++
type system, so can't be template arguments.

int n =3D 2;
int a[n]{};
static_assert(not std::is_array_v<decltype(a)>); // error

Clang doesn't even allow the {} initializer in the code above, so they're n=
ot
portable either, even among compilers that support -std=3Dgnu++17 modes.


> std::vector<int> has some protection, e.g. ASAN finds the out of bounds
> access (at a high run-time cost) and one could activate the GLIBC asserti=
ons
> someho:
>=20
> https://godbolt.org/z/8zanMG5x4

This will abort with the recommended hardening flags, specifically
-D_GLIBCXX_ASSERTIONS (which is nothing to do with Glibc, and is suitable f=
or
production use, unlike ASan). Those assertions will be enabled by the propo=
sed
-fhardening flag.


>=20
> But I would not call it safer and overhead is much worse.
>=20
> Fundamentally, VLAs are the dynamic buffer which can be protected most
> easily and should be *preferred*.

Maybe for C, but not for C++.

I know you are a big fan of VLAs, but please don't try to push them into a
language where they do not fit, and are not needed.=