On Tue, Aug 14, 2018 at 02:36:55PM -0700, Andrew Morton wrote: > On Tue, 14 Aug 2018 10:09:04 -0700 Guenter Roeck <li...@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