linux-kernel - Re: [PATCH] afs: fix no return statement in function returning non-void

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Message-ID: <c2002123-795c-20ae-677c-a35ba0e361af@infradead.org>
Date:   Tue, 15 Jun 2021 18:38:41 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Hulk Robot <hulkci@...wei.com>,
        Zheng Zengkai <zhengzengkai@...wei.com>,
        Tom Rix <trix@...hat.com>, linux-afs@...ts.infradead.org,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] afs: fix no return statement in function returning
 non-void

On 6/15/21 5:32 PM, Linus Torvalds wrote:
> On Tue, Jun 15, 2021 at 4:58 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>>
>> Some implementations of BUG() are macros, not functions,
> 
> Not "some", I think. Most.
> 
>> so "unreachable" is not applicable AFAIK.
> 
> Sure it is. One common pattern is the x86 one:
> 
>   #define BUG()                                                   \
>   do {                                                            \
>           instrumentation_begin();                                \
>           _BUG_FLAGS(ASM_UD2, 0);                                 \
>           unreachable();                                          \
>   } while (0)

duh.

> and that "unreachable()" is exactly what I'm talking about.
> 
> So I repeat: what completely broken compiler / config / architecture
> is it that needs that "return 0" after a BUG() statement?

I have seen it on ia64 -- most likely GCC 9.3.0, but I'll have to
double check that.

> Because that environment is broken, and the warning is bogus and wrong.
> 
> It might not be the compiler. It might be some architecture that does
> this wrong. It might be some very particular configuration that does
> something bad and makes the "unreachable()" not work (or not exist).
> 
> But *that* is the bug that should be fixed. Not adding a pointless and
> incorrect line that makes no sense, just to hide the real bug.