linux-kernel - Re: [PATCH v2] printf: add support for printing symbolic error codes

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Date:   Wed, 11 Sep 2019 11:37:28 +0200
From:   Uwe Kleine-König <uwe@...ine-koenig.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Joe Perches <joe@...ches.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] printf: add support for printing symbolic error codes

On 9/11/19 8:43 AM, Rasmus Villemoes wrote:
> On 11/09/2019 02.15, Joe Perches wrote:
>> On Tue, 2019-09-10 at 18:26 +0300, Andy Shevchenko wrote:
>>> On Mon, Sep 9, 2019 at 11:39 PM Rasmus Villemoes
>>> <linux@...musvillemoes.dk> wrote:
> 
>>>> +#define E(err) [err + BUILD_BUG_ON_ZERO(err <= 0 || err > 300)] = #err
>>>> +#define E(err) [err - 512 + BUILD_BUG_ON_ZERO(err < 512 || err > 550)] = #err
>>>
>>> From long term prospective 300 and 550 hard coded here may be forgotten.
> 
> No? The point of the BUILD_BUG_ON_ZEROs is that if you add a new
> Esomething, you'll get an instant build error if Esomething doesn't fit
> nicely in the array you put it in. Then one can go back and figure out
> whether the limit should be raised, a new codes_foo should be created,
> or if it's early enough so it's not ABI yet, simply change Esomething to
> a saner value.
> 
> A much bigger problem is that it's possible to add something to some
> errno.h without updating this table, but there's no good solution for
> that, I'm afraid. However, new Esomething are very rarely added, and
> printf() will still handle it gracefully until somebody notices.
> 
>>>> +const char *errcode(int err)
>>> We got long, why not to use long type for it?
> 
> Because errno values by definition have type int - and the linux syscall
> ABI very clearly limits values to [1,4095]. I can change the type used
> in vsnprintf.c if you prefer.
> 
>>>> +{
>>>> +       /* Might as well accept both -EIO and EIO. */
>>>> +       if (err < 0)
>>>> +               err = -err;
>>>> +       if (err <= 0) /* INT_MIN or 0 */
>>>> +               return NULL;
>>>> +       if (err < ARRAY_SIZE(codes_0))
>>>> +               return codes_0[err];
>>>
>>> It won't work if one of the #ifdef:s in the array fails.
>>> Would it?
> 
> I don't understand what you mean. How can an ifdef fail(?), and what
> exactly won't work?

I think Joe means: What happens if codes_0[57] is "" because there is no
ESOMETHING with value 57.

Best regards
Uwe



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