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

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

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@gmail.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/