On Wed, Oct 18, 2017 at 6:30 AM, Tejun Heo [off-list ref] wrote:

On Mon, Oct 16, 2017 at 11:33:21PM -0700, Nick Desaulniers wrote:

quoted

When compiling arch/x86/boot/compressed/eboot.c with HOSTCC=clang, the
following warning is observed:

./include/linux/cgroup-defs.h:391:16: warning: field 'cgrp' with
variable sized type 'struct cgroup' not at the end of a struct or class
is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
        struct cgroup cgrp;
                      ^
Flexible array members are a C99 feature, but must be the last member of
a struct. Structs with flexible members composed in other structs must
also be the final members, unless using GNU C extensions.

struct cgroup_root's member cgrp is a struct cgroup, struct cgroup's
member ancestor_ids is a flexible member.

This is silly tho.  We know the the root group embedded there won't
have any ancestor_ids.

Sure, but struct cgroup_root is still declared as having a struct
cgroup not declared as the final member.

Also, in general, nothing prevents us from
doing something like the following.

        struct outer_struct {
                blah blah;
                struct inner_struct_with_flexible_array_member inner;
                unsigned long storage_for_flexible_array[NR_ENTRIES];
                blah blah;
        };

At that point, then why have the struct with flexible array member in
the first place?

quoted

or specific location of the member cgrp within struct cgroup_root AFAICT.

I think we should just silence the bogus warning.

Is the order of the members actually important?  Otherwise it seems
that we're taking advantage of a GNU C extension for no real reason,
which is what I'm trying to avoid.  Please reconsider.