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

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Message-ID: <CAHk-=whPPWYXKQv6YjaPQgQCf+78S+0HmAtyzO1cFMdcqQp5-A@mail.gmail.com>
Date:   Tue, 15 Jun 2021 17:32:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Randy Dunlap <rdunlap@...radead.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 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)

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?

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.

               Linus

                 Linus