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Message-ID: <03ddd6e6-55ff-fff4-95f3-8c0b008443f8@gmail.com>
Date: Sun, 14 Nov 2021 16:45:17 +0100
From: "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Mickaël Salaün <mic@...ikod.net>,
Al Viro <viro@...iv.linux.org.uk>, Andrew Morton
<akpm@...ux-foundation.org>, Aleksa Sarai <cyphar@...har.com>,
Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
Casey Schaufler <casey@...aufler-ca.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Christian Heimes <christian@...hon.org>,
Deven Bowers <deven.desai@...ux.microsoft.com>,
Dmitry Vyukov <dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>,
Eric Chiang <ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>,
James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
"Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>,
Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>,
Miklos Szeredi <mszeredi@...hat.com>, Mimi Zohar <zohar@...ux.ibm.com>,
Paul Moore <paul@...l-moore.com>,
Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
Scott Shell <scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>,
Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>,
Yin Fengwei <fengwei.yin@...el.com>, kernel-hardening@...ts.openwall.com,
Linux API <linux-api@...r.kernel.org>,
Linux FS Devel <linux-fsdevel@...r.kernel.org>,
linux-integrity <linux-integrity@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and
related sysctl
Hi Geert,
On 11/14/21 16:32, Geert Uytterhoeven wrote:
> Hi Alejandro,
>
> On Sat, Nov 13, 2021 at 8:56 PM Alejandro Colomar (man-pages)
> <alx.manpages@...il.com> wrote:
>> On 11/13/21 14:02, Mickaël Salaün wrote:
>>>> TL;DR:
>>>>
>>>> ISO C specifies that for the following code:
>>>>
>>>> enum foo {BAR};
>>>>
>>>> enum foo foobar;
>>>>
>>>> typeof(foo) shall be int
>>>> typeof(foobar) is implementation-defined
>>>
>>> I tested with some version of GCC (from 4.9 to 11) and clang (10 and 11)
>>> with different optimizations and the related sizes are at least the same
>>> as for the int type.
>>
>> GCC has -fshort-enums to make enum types be as short as possible. I
>> expected -Os to turn this on, since it saves space, but it doesn't.
>
> Changing optimization level must not change the ABI, else debugging
> would become even more of a nightmare.
I agree, but if you invoke implementation-defined,
then it's not (only) the compiler's fault.
Instead of not allowing GCC to enable -fshort-enums ever,
one can write ISO C-complying code in the parts that
will be exposed as an interface,
by just using int.
That allows using -fshort-enums
for whatever reasons it might be good.
Not saying that the kernel wants to enable it,
but it costs nothing to write non-implementation-defined code
that doesn't forbid it.
It's comparable to passing a struct (not a pointer to it)
to a function.
If you change the size of the struct,
you screw the interface.
Better pass pointers, or standard types.
Cheers,
Alex
--
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
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