linux-kernel - Re: [PATCH v4 0/1] printf: add support for printing symbolic error names

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Date:   Tue, 26 Nov 2019 15:04:06 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Joe Perches <joe@...ches.com>, Petr Mladek <pmladek@...e.com>,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v4 0/1] printf: add support for printing symbolic error names

Hi Rasmus,

Nice idea!

On Fri, Oct 11, 2019 at 3:38 PM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
> This is a bit much for under the ---, so a separate cover letter for
> this single patch.
>
> v4: Dropped Uwe's ack since it's changed quite a bit. Change
> errcode->errname as suggested by Petr. Make it 'default y if PRINTK'
> so it's available in the common case, while those who have gone to
> great lengths to shave their kernel to the bare minimum are not
> affected.
>
> Also require the caller to use %pe instead of printing all ERR_PTRs
> symbolically. I can see some value in having the call site explicitly
> indicate that they're printing an ERR_PTR (i.e., having the %pe), but
> I also still believe it would make sense to print ordinary %p,
> ERR_PTR() symbolically instead of as a random hash value that's not
> stable across reboots. But in the interest of getting this in, I'll
> leave that for now. It's easy enough to do later by just changing the
> "case 'e'" to do a break (with an updated comment), then do an
> IS_ERR() check after the switch.
>
> Something I've glossed over in previous versions, and nobody has
> commented on, is that I produced "ENOSPC" while the 'fallback' would
> print "-28" (i.e., there's no minus in the symbolic case). I don't
> care much either way, but here I've tried to show how I'd do it if we
> want the minus also in the symbolic case. At first, I tried just using
> the standard idiom
>
>   if (buf < end)
>     *buf = '-';
>   buf++;
>
> followed by string(sym, ...). However, that doesn't work very well if
> one wants to honour field width - for that to work, the whole string
> including - must come from the errname() lookup and be handled by
> string(). The simplest seemed to be to just unconditionally prefix all
> strings with "-" when building the tables, and then change errname()
> back to supporting both positive and negative error numbers.

Still, it looks a bit wasteful to me to include the dash in each and every
string value.

Do you think you can code the +/- logic in string_nocheck() in less than
the gain achieved by dropping the dashes from the tables?
(e.g. by using the SIGN spec.flags? ;-)
Or, do we need it? IS_ERR() doesn't consider positive values errors.

Oh, what about the leading "E"? That one looks harder to get rid of,
though ;-)

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds