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

[Geert Uytterhoeven]

Hi Rasmus,

Nice idea!

On Fri, Oct 11, 2019 at 3:38 PM Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx

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