linux-kernel - Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and related sysctl

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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/