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

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Date:   Wed, 16 Jun 2021 05:56:39 -0700
From:   Tom Rix <trix@...hat.com>
To:     Zheng Zengkai <zhengzengkai@...wei.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Hulk Robot <hulkci@...wei.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 8:15 PM, Zheng Zengkai wrote:
> 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
>
powerpc64 gcc 10.3.1 is what I used to find this problem.

A fix is to use the __noreturn attribute on this function and not add a 
return like this

-static int afs_dir_set_page_dirty(struct page *page)
+static int __noreturn afs_dir_set_page_dirty(struct page *page)

and to the set of ~300 similar functions in these files.

$ grep -r -P "^\tBUG\(\)" .

If folks are ok with this, I'll get that set started.

Tom

>>> 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.
>> .
>>
>