linux-kernel - Re: [PATCH] printf: fix Woverride-init warning for EDEADLK errno

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Date:   Tue, 27 Oct 2020 11:55:56 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Petr Mladek <pmladek@...e.com>, Arnd Bergmann <arnd@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] printf: fix Woverride-init warning for EDEADLK errno

On 27/10/2020 10.12, Petr Mladek wrote:
> On Tue 2020-10-27 09:46:28, Arnd Bergmann wrote:
>> On Tue, Oct 27, 2020 at 8:23 AM Rasmus Villemoes
>> <linux@...musvillemoes.dk> wrote:
>>> On 26/10/2020 22.49, Arnd Bergmann wrote:
>>>> From: Arnd Bergmann <arnd@...db.de>
>>>
>>> NAK. That would end up using the "EDEADLOCK" string for the value 35 on
>>> those architectures where they are the same, despite EDEADLK being the
>>> by far the most used symbol. See the comments and original commit log,
>>> the placement of these is deliberate.
> 
> Good point.
> 
>> Ok, I see.
>>
>>> How about we do this instead?
>>>
>>> when building with W=1. As the use of multiple initializers for the
>>> same entry here is quite deliberate, explicitly disable that warning
>>> for errname.o.
>>>
>>> diff --git a/lib/Makefile b/lib/Makefile
>>> index ce45af50983a2a5e3582..a98119519e100103818d 100644
>>> --- a/lib/Makefile
>>> +++ b/lib/Makefile
>>> @@ -224,6 +224,7 @@ obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o
>>>
>>>  obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o
>>>  obj-$(CONFIG_SYMBOLIC_ERRNAME) += errname.o
>>> +CFLAGS_errname.o += $(call cc-disable-warning, override-init)
>>>
>>
>> This works, but it disables a potentially useful warning in case we
>> get another conflict in this file, so I'd prefer to find a way to
>> avoid the warning rather than force-disabling it.
> 
> Yeah, I think that it is better to explicitely disable the less used
> variant in the code than hiding the double initialization. It will
> be clear what is going on.
> 
> 
>> How about adding the #ifdef around the EDEADLOCK line
>> instead of the EDEADLK one? Something like
>>
>> diff --git a/lib/errname.c b/lib/errname.c
>> index 0c4d3e66170e..93043fb960cc 100644
>> --- a/lib/errname.c
>> +++ b/lib/errname.c
>> @@ -38,7 +38,10 @@ static const char *names_0[] = {
>>         E(ECOMM),
>>         E(ECONNABORTED),
>>         E(ECONNRESET),
>> +       E(EDEADLK), /* EDEADLOCK */
>> +#if EDEADLK != EDEADLOCK /* mips, sparc, powerpc */
>>         E(EDEADLOCK),
>> +#endif
>>         E(EDESTADDRREQ),
>>         E(EDOM),
>>         E(EDOTDOT),
>> @@ -169,7 +172,6 @@ static const char *names_0[] = {
>>         E(ECANCELED), /* ECANCELLED */
>>         E(EAGAIN), /* EWOULDBLOCK */
>>         E(ECONNREFUSED), /* EREFUSED */
>> -       E(EDEADLK), /* EDEADLOCK */
> 
> This should stay :-)
> 

No, Arnd moved it next to EDEADLOCK, which is fine (it can lose the
comment /* EDEADLOCK */, though; the comment on the ifdef is
sufficient). Especially when:

> And we should remove the ECANCELLED definition. It is always the same
> as ECANCELED and replaced. We do not define EWOULDBLOCK and
> EREFUSED either.

Yes, I'm not sure why I elided EWOULDBLOCK and EREFUSED but not
ECANCELLED. So let's move EAGAIN, ECONNREFUSED and ECANCELED to their
proper alphabetic place. But I also want to add a check that the things
we've elided match some value that we do handle. So add something like

#ifdef EREFUSED /* parisc */
static_assert(EREFUSED == ECONNREFUSED);
#endif

#ifdef ECANCELLED /* parisc */
static_assert(ECANCELLED == ECANCELED);
#endif

static_assert(EAGAIN == EWOULDBLOCK); /* everywhere */

so that if we ever import some arch that defines EREFUSED to something
other than ECONNREFUSED, it would be caught. Essentially, errname.c
should mention every #define E* that appears in any errno*.h.

Rasmus