Hi Tejun,
El Sat, Oct 21, 2017 at 08:32:53AM -0700 Tejun Heo ha dit:
> Hello, Nick.
>
> On Fri, Oct 20, 2017 at 12:15:55AM -0700, Nick Desaulniers wrote:
> > > 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.
>
> Why is that a problem tho? We know that it doesn't have any flexible
> array member so there's no storage allocated to it.
>
> > > 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?
>
> Because there are different ways to use the struct?
>
> > >> 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.
>
> Here, not necessarily but I don't want to move it for a bogus reason.
> Why would we disallow embedding structs with flexible members in the
> middle when it can be done and is useful? If we want to discuss
> whether we want to avoid such usages in the kernel (but why?), sure,
> let's have that discussion but we can't decide that on "clang warns on
> it by default".
>From your earlier comment I understand that there is no problem in
this case because we know that cgroup_root->cgrp will always be
empty.
However in other instances the warning could point out actual errors
in the code, so I think it is good to have this warning generally
enabled. If cgroup_root was defined in a .c file we could consider to
disable the warning locally, but since the definition is in a header
that is widely included (indirectly through linux/cgroup.h and
net/sock.h) this doesn't seem to be an option.
Is there a good reason for the current position of cgrp within
cgroup_root? If there are no drawbacks in moving it to the end of
the struct I think Nick's patch is a reasonable solution.
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