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Date:   Sat, 1 Apr 2023 10:33:45 +0000
From:   Thomas =?utf-8?Q?Wei=C3=9Fschuh?= <linux@weissschuh.net>
To:     Willy Tarreau <w@1wt.eu>
Cc:     Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Subject: Re: [PATCH] tools/nolibc: validate C99 compatibility
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Hi Willy,

sorry for the late response to your previous mail, I was traveling.

On 2023-04-01 12:03:30+0200, Willy Tarreau wrote:
> On Wed, Mar 29, 2023 at 08:11:58AM +0200, Willy Tarreau wrote:
> > On Wed, Mar 29, 2023 at 05:35:33AM +0000, Thomas Weißschuh wrote:
> > > Hi Willy,
> > > 
> > > On 2023-03-29 07:16:11+0200, Willy Tarreau wrote:
> > > > On Tue, Mar 28, 2023 at 09:07:35PM +0000, Thomas Weißschuh wrote:
> > > > > Most of the code was migrated to C99-conformant __asm__ statements
> > > > > before. It seems string.h was missed.
> > > > > 
> > > > > Fix string.h and also validate during build that nolibc stays within
> > > > > C99.
> > > > 
> > > > I'm all for improving portability, however I have a concern with building
> > > > the test case with -std=c99 which is that it might hide some c99-only
> > > > stuff that we'd introduce by accident in the nolibc's code, and I'd
> > > > rather not do that because it will mean changing build options for some
> > > > external programs using it if it happens. However I totally agree with
> > > > you that we need to make sure that there's no build issues with c99
> > > > compilers. Modern compilers are c99-compatible but generally come with
> > > > GNU extensions and I understand why you're interested in switching to
> > > > std=c99 in order to drop some of these like "asm". Should we have two
> > > > build targets, the default one and a c99 one ? Maybe. The build is so
> > > > small and quick that nobody will care, so we could definitely imagine
> > > > building the two versions. Maybe you have a better idea ?
> > > 
> > > I'm not sure I understand.
> > > Do you want to stay compatible with c89/gnu89?
> > 
> > At least with gnu89, yes, since it's been used by default by a wide
> > range of compilers.
> > 
> > > If so we could use that baseline standard instead of -std=c99.
> > 
> > The only thing is that c99 is both more permissive and more restrictive
> > than gnu89 since (as you noticed) gnu89 allows for example "asm" instead
> > of "__asm__".
> > 
> > > Without specifying a standard we get whatever the compiler uses as
> > > default which is probably much newer than c99.
> > 
> > Yes but do we really care ? I think we want at least some gnuXX
> > (which gcc does by default) and some c99 for those who don't want to
> > depend on gnuXX. Diversity in tests provides faster reports than
> > forcing everyone to the same set. By keeping the default build option,
> > a backwards-compatibility test is just a matter of setting CC= with the
> > relevant compiler to confirm it's still OK, without being fooled by the
> > fact that a standard other than the default was used.
> > 
> > > Having two targets seems to be easy to do but I'm not sure what the
> > > advantage would be over compiling once against the intended baseline
> > > standard.
> > 
> > We're providing a set of includes to be used by userland so there isn't
> > a single intended baseline standard. I'm not advocating for supporting
> > everything on earth at all, but at least it should work with native
> > compilers currently found in distros or on the kernel.org crosstools,
> > and with some older toolchains that are used once in a while to rebuild
> > a few compact tools. For example I've used this codebase to build a
> > recovery kernel+tools in the past, which fits everything in a 1MB
> > binary, and that's the type of thing where you know that it's not always
> > easy nor relevant to port the code to newer compilers, so if it used to
> > work on gcc 4.7 you'll just reuse that one if you still have it. My
> > position regarding older tools is: we don't make particular efforts to
> > test them, but we at least do not try hard to evince them either as
> > long as it's not necessary.
> 
> I just ran some tests and there's actually better to achieve what you're
> looking for. Let's just use -fno-asm, it removes the GNU-specific "asm",
> "inline", and "typeof" in favor of the "__" variants. With gcc 11.3 it
> gives me this, which is exactly what we were looking for:
> 
>   gcc -Os -fno-ident -fno-asynchronous-unwind-tables -fno-stack-protector -DNOLIBC_STACKPROTECTOR -mstack-protector-guard=global -fstack-protector-all -fno-asm -s -o nolibc-test \
>     -nostdlib -static -Isysroot/x86/include nolibc-test.c -lgcc
>   In file included from sysroot/x86/include/stdlib.h:14,
>                    from sysroot/x86/include/nolibc.h:103,
>                    from sysroot/x86/include/errno.h:26,
>                    from sysroot/x86/include/stdio.h:14,
>                    from nolibc-test.c:15:
>   sysroot/x86/include/string.h: In function 'memset':
>   sysroot/x86/include/string.h:93:17: error: 'asm' undeclared (first use in this function)
>      93 |                 asm volatile("");
>         |                 ^~~
>   sysroot/x86/include/string.h:93:17: note: each undeclared identifier is reported only once for each function it appears in
>   sysroot/x86/include/string.h:93:20: error: expected ';' before 'volatile'
>      93 |                 asm volatile("");
>         |                    ^~~~~~~~~
>         |                    ;
>   sysroot/x86/include/string.h: In function 'strlen':
>   sysroot/x86/include/string.h:142:17: warning: implicit declaration of function 'asm' [-Wimplicit-function-declaration]
>     142 |                 asm("");
>         |                 ^~~
>   nolibc-test.c: In function 'main':
>   nolibc-test.c:898:25: error: 'asm' undeclared (first use in this function)
>     898 |                         asm volatile ("outb %%al, %%dx" :: "d"(0x501), "a"(0));
>         |                         ^~~
>   nolibc-test.c:898:28: error: expected ';' before 'volatile'
>     898 |                         asm volatile ("outb %%al, %%dx" :: "d"(0x501), "a"(0));
>         |                            ^~~~~~~~~
>         |                            ;
>   make: *** [Makefile:128: nolibc-test] Error 1
> 
> With this, we don't need to force -std=c99 nor to build two variants,
> a single variant will catch GCCisms even with older compilers while not
> being overly annoying.

The original goal was to have a fixed and explicit baseline that is
supported. Using -std= ensures that no deviations are accidentally
introduced.

Using -fno-asm only prevents this specific gnu-ism. All other gnu-isms
and features from whatever the current compilers default language level
is are still allowed.

Therefore I'm not convinced of this aproach.

These are the aproaches I can see that reach this goal:

* Declare C99 as baseline, build with -std=c99
* Declare C99 and GNU89 as baseline, build with both.
* Declare C99 and GNU89 as baseline, build with -std=c99,
  wait for people to complain if something breaks on gnu89
  (same as current status)
* Declare C89 as baseline, remove all C99-isms and gnu-isms
  (C++ comments, "inline" and some smaller stuff),
  build with -std=c89.

Personally I think C99 is a baseline that is easy to fulfill by nolibc
and should not restrict users.

Thomas