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

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Date:   Tue, 10 Sep 2019 18:26:32 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Joe Perches <joe@...ches.com>, 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>,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] printf: add support for printing symbolic error codes

On Mon, Sep 9, 2019 at 11:39 PM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> It has been suggested several times to extend vsnprintf() to be able
> to convert the numeric value of ENOSPC to print "ENOSPC". This is yet
> another attempt. Rather than adding another %p extension, simply teach
> plain %p to convert ERR_PTRs. While the primary use case is
>
>   if (IS_ERR(foo)) {
>     pr_err("Sorry, can't do that: %p\n", foo);
>     return PTR_ERR(foo);
>   }
>
> it is also more helpful to get a symbolic error code (or, worst case,
> a decimal number) in case an ERR_PTR is accidentally passed to some
> %p<something>, rather than the (efault) that check_pointer() would
> result in.
>
> With my embedded hat on, I've made it possible to remove this.
>
> I've tested that the #ifdeffery in errcode.c is sufficient to make
> this compile on arm, arm64, mips, powerpc, s390, x86 - I'm sure the
> 0day bot will tell me which ones I've missed.
>
> The symbols to include have been found by massaging the output of
>
>   find arch include -iname 'errno*.h' | xargs grep -E 'define\s*E'
>
> In the cases where some common aliasing exists
> (e.g. EAGAIN=EWOULDBLOCK on all platforms, EDEADLOCK=EDEADLK on most),
> I've moved the more popular one (in terms of 'git grep -w Efoo | wc)
> to the bottom so that one takes precedence.

> +#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.

> +const char *errcode(int err)
We got long, why not to use long type for it?

> +{
> +       /* 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?

> +       if (err >= 512 && err - 512 < ARRAY_SIZE(codes_512))
> +               return codes_512[err - 512];
> +       /* But why? */
> +       if (IS_ENABLED(CONFIG_MIPS) && err == EDQUOT) /* 1133 */
> +               return "EDQUOT";
> +       return NULL;
> +}

> +               long err = PTR_ERR(ptr);
> +               const char *sym = errcode(-err);

Do we need additional sign change if we already have such check inside
errcode()?

-- 
With Best Regards,
Andy Shevchenko