linux-kernel - Re: [PATCH] Enable '-Werror' by default for all kernel builds

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Date:   Tue, 7 Sep 2021 22:46:10 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-sparc <sparclinux@...r.kernel.org>
Subject: Re: [PATCH] Enable '-Werror' by default for all kernel builds

On 9/7/21 9:55 PM, Linus Torvalds wrote:
> On Tue, Sep 7, 2021 at 9:28 PM Guenter Roeck <linux@...ck-us.net> wrote:
>>
>> It is seen with gcc 11.x whenever a memXXX or strXXX function parameter
>> is a pointer to a fixed address.
> 
> I wonder why I don't see it with gcc 11.2 here on x86-64.
> 

I see the problem only on some architectures. No idea what triggers it,
but it is definitely architecture dependent.

>> gcc is happy if "(void *) 0xfffc1f2c"
>> is passed to a global function which does nothing but return the address,
>> such as:
>>
>> void *sanitize_address(void *address)
>> {
>>          return address;
>> }
> 
> We have had reasons to do things like that before for somewhat similar
> (well, opposite) reasons - trying to disassociate some pointer from
> its originating symbol type.
> 
> Look at RELOC_HIDE().
> 
> It might be worth it having something similar for "absolute_pointer()".
> 
> Entirely untested "written-in-the-MUA" garbage:
> 
>     #define absolute_pointer(val) \
>          ({ void *__res; __asm__("":"=r" (__res):"0" ((unsigned
> long)(val))); __res; })
> 

or:

#define absolute_pointer(val) RELOC_HIDE(val, 0)

or maybe:

#define absolute_pointer(val) RELOC_HIDE((void *)val, 0)

would do the same (though the first variant needs a pointer as argument).
All of those compile.

I tested the first and the last option on qemu:parisc and confirmed that
both work as expected.

I'd be happy to send a formal patch. Which one do you prefer, and where
should I put it ?

Thanks,
Guenter