Re: [PATCH v2] vsprintf: introduce %dE for error constants

[Andy Shevchenko]

On Wed, Aug 28, 2019 at 12:14 AM Uwe Kleine-KÃnig <uwe@xxxxxxxxxxxxxxxxx> wrote:
>
> The new format specifier %dE introduced with this patch pretty-prints
> the typical negative error values. So
>
>         pr_info("probing failed (%dE)\n", ret);
>
> yields
>
>         probing failed (EIO)
>
> if ret holds -EIO. This is easier to understand than the for now common
>
>         probing failed (-5)
>
> (which used %d instead of %dE) for kernel developers who don't know the
> numbers by heart, The error names are also known and understood by
> userspace coders as they match the values the errno variable can have.
> Also to debug the sitation that resulted in the above message very
> probably involves EIO, so the message already gives the string to look
> out for.
>
> glibc (and other libc variants)'s printf have a similar feature: %m
> expands to strerror(errno). I preferred using %dE however because %m in
> userspace doesn't consume an argument (which is however necessary in the
> kernel as there is no global errno variable). Also this is handled
> correctly by the compiler's format string checker as there is a matching
> int variable for the %d the compiler notices.
>
> There are quite some code locations that could be adapted to benefit
> from this:
>
>         $ git grep -E '^\s*(printk|(kv?as|sn?|vs(c?n)?)printf|(kvm|dev|pr)_(emerg|alert|crit|err|warn(ing)?|notice|info|cont|devel|debug|dbg))\(.*(\(%d\)|: %d)\\n' v5.3-rc5 | wc -l
>         9141
>
> I expect there are some false negatives where the match is distributed
> over two or more lines and so isn't found.
>
> Signed-off-by: Uwe Kleine-KÃnig <uwe@xxxxxxxxxxxxxxxxx>
> ---
> Hello,
>
> in v1 I handled both positive and negative errors which was challenged.
> I decided to drop that and only handle the (common) negative values.
> Readding handling of positive values later is easier than dropping it.
>
> Sergey Senozhatsky and Enrico Weigelt suggested to use strerror as name
> for the function that I called errno2errstr. (In v1 it was not a
> separate function). I didn't pick this up because the semantic of
> strerror in userspace is different. It returns something like
> "Interrupted system call" while errno2errstr only yields "EINTR".
>
> I dropped EWOULDBLOCK from the list of codes as it is on all Linux archs
> identical to EAGAIN and the way the lookup works now breaks otherwise.
> (And having it wasn't useful already in v1.)
>
> Rasmus Villemoes pointed out that the way I wrote the resulting string
> to the buffer was broken. This is fixed according to his suggestion by
> using string_nocheck(). I also added a test to lib/test_printf.c as he
> requested.
>
> Petr Mladek had some concerns:
> > The array is long, created by cpu&paste, the index of each code
> > is not obvious.
>
> Yeah right, the array is long. This cannot really be changed because we
> have that many error codes. I don't understand your concern about the
> index not being obvious. The array was just a list of (number, string)
> pairs where the position in the array didn't have any semantic.
>
> I agree it would be nice to have only one place that has a collection of
> error codes. I thought about reorganizing how the error constants are
> defined, i.e. doing something like:
>
>         DEFINE_ERROR(EIO, 5, "I/O error")
>
> but I don't see a possibility to let this expand to
>
>         #define EIO 5
>
> without resorting to another macro language. If that were possible the
> list of DEFINE_ERRORs could be reused to help generating the code for
> errno2errstr. So if you have a good idea, please speak up.
>
> > There are ideas to make the code even more tricky to reduce
> > the size, keep it fast.
>
> I think Enrico Weigelt's suggestion to use a case is the best
> performance-wise so that's what I picked up. Also I hope that
> performance isn't that important because the need to print an error
> should not be so common that it really hurts in production.
>
> > Both, %dE modifier and the output format (ECODE) is non-standard.
>
> Yeah, obviously right. The problem is that the new modifier does
> something that wasn't implemented before, so it cannot match any
> standard. %pI is only known on Linux either, so I think being
> non-standard is a weak argument. For user consumption it would be nice
> if we'd get
>
>         probing failed (I/O Error)
>
> from pr_info("probing failed (%dE)\n", -EIO); but that would be still
> more expensive because the collection of string constants would become
> bigger that it is already now and "EIO" is the right one to search for
> when debugging the underlaying problem. So I think "EIO" is even more
> useful than "I/O Error".
>
> > Upper letters gain a lot of attention. But the error code is
> > only helper information. Also many error codes are misleading because
> > they are used either wrongly or there was no better available.
>
> This isn't really an argument against the patch I think. Sure, if a
> function returned (say) EIO while ETIMEOUT would be better, my patch
> doesn't improve that detail. Still
>
>         mydev: Failed to initialize blablub: EIO
>
> is more expressive than
>
>         mydev: Failed to initialize blablub: -5
>
> . With "EIO" in the message it is not worse than with "-5" independant of the
> question if EIO is the right error code used.
>
> > There is no proof that this approach would be widely acceptable for
> > subsystem maintainers. Some might not like mass and "blind" code
> > changes. Some might not like the output at all.
>
> I don't intend to mass convert existing code. I would restrict myself to
> updating the documentation and then maybe send a patch per subsystem as an
> example to let maintainers know and judge for themselves if they like it or
> not. And if it doesn't get picked up, we can just remove the feature again next
> year (or so).
>
> I dropped the example conversion, I think the idea should be clear now
> even without an explicit example.
>

Hold on, can you in less than 20 words put WHY this is needed?

-- 
With Best Regards,
Andy Shevchenko