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Date:   Tue, 14 Aug 2018 15:15:59 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org, Rik van Riel <riel@surriel.com>,
        Mike Galbraith <efault@gmx.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: Build failures with gcc 4.5 and older
Message-ID: <20180814221559.GA15888@roeck-us.net>
References: <20180814170904.GA12768@roeck-us.net>
 <20180814143655.3acd4bb211d44747f77e74f2@linux-foundation.org>
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On Tue, Aug 14, 2018 at 02:36:55PM -0700, Andrew Morton wrote:
> On Tue, 14 Aug 2018 10:09:04 -0700 Guenter Roeck <linux@roeck-us.net> wrote:
> 
> > Since commit c1a2f7f0c0645 ("mm: Allocate the mm_cpumask
> > (mm->cpu_bitmap[]) dynamically based on nr_cpu_ids"), building
> > the Linux kernel with gcc version 4.5 and older fails as follows.
> > 
> > In file included from ./include/linux/mm.h:17:0,
> >                  from ./include/linux/pid_namespace.h:7,
> > 		 from ./include/linux/ptrace.h:10,
> > 		 from arch/openrisc/kernel/asm-offsets.c:32:
> > ./include/linux/mm_types.h:497:16: error: flexible array member in otherwise empty struct
 
> Confused.  Why does it think that the mm_struct is "otherwise empty"?
> 

The problem isn't really that the structure is otherwise empty.
Some digging reveals that the error message is wrong; gcc should
instead complain about having no _named_ structure element before
the flexible array member.

> This shuts it up:
> 
> --- a/include/linux/mm_types.h~a
> +++ a/include/linux/mm_types.h
> @@ -490,6 +490,7 @@ struct mm_struct {
>  #endif
>  	} __randomize_layout;
>  
> +	int wibble;
>  	/*
>  	 * The mm_cpumask needs to be at the end of mm_struct, because it
>  	 * is dynamically sized based on nr_cpu_ids.
> 

Unfortunately, that only triggers secondary errors.
Seen with both gcc 4.4 and 4.5.

mm/init-mm.c:29: error: unknown field ‘mm_rb’ specified in initializer
mm/init-mm.c:29: warning: missing braces around initializer
mm/init-mm.c:29: warning: (near initialization for ‘init_mm.<anonymous>’)
mm/init-mm.c:29: error: incompatible types when initializing type ‘struct
vm_area_struct *’ using type ‘struct rb_root’
mm/init-mm.c:30: error: unknown field ‘pgd’ specified in initializer

and many more similar errors.

Guenter