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

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Date:   Tue, 10 Sep 2019 17:15:15 -0700
From:   Joe Perches <joe@...ches.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
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>,
        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 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:
> > 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()?

How is EBUSY differentiated from ZERO_SIZE_PTR ?