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

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Message-ID: <051421e0-afe8-c6ca-95cd-4dc8cd20a43e@huawei.com>
Date:   Wed, 16 Jun 2021 11:15:25 +0800
From:   Zheng Zengkai <zhengzengkai@...wei.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     David Howells <dhowells@...hat.com>,
        Hulk Robot <hulkci@...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

Oops, Sorry for the late reply and missing the compilation details.

> 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.

Actually we build the kernel with the following compiler, config and 
architecture :

powerpc64-linux-gnu-gcc --version
powerpc64-linux-gnu-gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

CONFIG_AFS_FS=y
# CONFIG_AFS_DEBUG is not set
CONFIG_AFS_DEBUG_CURSOR=y

make ARCH=powerpc CROSS_COMPILE=powerpc64-linux-gnu- -j64

...

fs/afs/dir.c: In function ‘afs_dir_set_page_dirty’:
fs/afs/dir.c:51:1: error: no return statement in function returning 
non-void [-Werror=return-type]
    51 | }
       | ^
cc1: some warnings being treated as errors

>> 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.
> .
>