linux-kernel - Re: Build failures with gcc 4.5 and older

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Message-Id: <20180814143655.3acd4bb211d44747f77e74f2@linux-foundation.org>
Date:   Tue, 14 Aug 2018 14:36:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-kernel@...r.kernel.org, Rik van Riel <riel@...riel.com>,
        Mike Galbraith <efault@....de>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Build failures with gcc 4.5 and older

On Tue, 14 Aug 2018 10:09:04 -0700 Guenter Roeck <linux@...ck-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"?

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.


So we could add something like that, along with the appropriate #if
GCC_VERSION and a comment.  A simple enough change, to keep those old
gcc versions limping along for a bit longer?