How can Autoconf help with the transition to stricter compilation defaults?

[Aaron Ballman via cfe-commits]

On Thu, Nov 10, 2022 at 12:16 PM Zack Weinberg via cfe-commits
<cfe-commits at lists.llvm.org> wrote:
>
> I’m the closest thing Autoconf has to a lead maintainer at present.
>
> It’s come to my attention (via https://lwn.net/Articles/913505/ and
> https://fedoraproject.org/wiki/Changes/PortingToModernC) that GCC and
> Clang both plan to disable several “legacy” C language features by
> default in a near-future release (GCC 14, Clang 16) (see the Fedora
> wiki link for a list).  I understand that this change potentially
> breaks a lot of old dusty code, and in particular that
> Autoconf-generated configure scripts use constructs that may *silently
> give the wrong answer to a probe* when a stricter compiler is in use.
>
> Nobody has a whole lot of time to work on Autoconf at present, but I
> would like to ask, anyway, what Autoconf could potentially do to make
> this transition easier.  I’m already aware that the test code Autoconf
> 2.71 uses to probe for C89/C99/C11 support is broken; this has been
> fixed in development trunk to the extent it is possible for me to test
> it with GCC 12 (commit:
> <https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=bf5a75953b6d504f0405b1ca33b039b8dd39eef4>).
> Several other places using K&R function definitions and/or
> unprototyped function declarations (including the ubiquitously used
> AC_CHECK_FUNC) have also been fixed on trunk,
> <https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=8b5e2016c7ed2d67f31b03a3d2e361858ff5299b>.
> Changes to handle C23 built-in ‘bool’ better are under development but
> the design has not yet been finalized.

Thank you for all of your efforts in modernizing autoconf in response
to all these changes, it's greatly appreciated!

> The biggest remaining (potential) problem, that I’m aware of, is that
> AC_CHECK_FUNC unconditionally declares the function we’re probing for
> as ‘char NAME (void)’, and asks the compiler to call it with no
> arguments, regardless of what its prototype actually is.  It is not
> clear to me whether this will still work with the planned changes to
> the compilers.  Both GCC 12 and Clang 14 have on-by-default warnings
> triggered by ‘extern char memcpy(void);’ (or any other standard
> library function whose prototype is coded into the compiler) and this
> already causes problems for people who run configure scripts with
> CC='cc -Werror'.  Unfortunately this is very hard to fix — we would
> have to build a comprehensive list of library functions into Autoconf,
> mapping each to either its documented prototype or to a header where
> it ought to be declared; in the latter case we would also have to make
> e.g. AC_CHECK_FUNCS([getaddrinfo]) imply AC_CHECK_HEADERS([sys/types.h
> sys/socket.h netdb.h]) which might mess up configure scripts that
> aren’t expecting headers to be probed at that point.

In terms of the Clang side of things, I don't think we've formed any
sort of official stance on how to handle that yet. It's UB (you can
declare the C standard library interface without UB but calling any
function with a mismatched signature is UB) and that UB has some
amount of security implications associated with it, so I would say
there's a potential we might want to upgrade the diagnostic severity,
but it's not assured. FWIW, we're working on improving communication
about potentially disruptive changes to Clang, so you might want to
consider either subscribing to the clang-vendors code review group at
https://reviews.llvm.org/project/members/113/ (if you want to be
involved in code review before things land) or the Announcements
discourse channel at https://discourse.llvm.org/c/announce/ (if you
want to be notified after something lands but before Clang ships).

> How important do you think it is for this to be fixed?
>
> Are there any other changes you would like to see in a near-future
> Autoconf 2.72 in order to make this transition easier?

I don't have a specific list, but as a general request: moving away
from deprecated facilities of C or reliance on UB is a very pragmatic
idea given that the C committee is recapturing some of that design
space (like what happened with K&R C signatures) and implementers are
trying to improve the security posture for C.

> zw
>
> p.s. GCC and Clang folks: As long as you’re changing the defaults out
> from under people, can you please also remove the last few predefined
> user-namespace macros (-Dlinux, -Dunix, -Darm, etc) from all the
> -std=gnuXX modes?

If we can do so without breaking the world, I personally think it
would be nice to remove them.

~Aaron

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