From: Guenter Roeck > Sent: 14 September 2021 23:47 ... > I am not sure if there was agreement to accept this patch or not, but > I was asked to resend it with the above change, so here it is. An open > question was if it is acceptable to have a structure named xxx_hdr > include an element pointing to the data following that header. It may be a pragmatic solution to the problem. But it isn't 'correct'. OTOH I think gcc is broken. It ought to at least give a sane method of getting the warning ignored in specific cases. > If this patch is not acceptable, the patch in buildbot may be a possible > alternative to consider. > https://git.busybox.net/buildroot/commit/?id=6e1106b4a9aee25d1556310d5cd1cb6dde2e6e3f > > arch/sparc/kernel/mdesc.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/sparc/kernel/mdesc.c b/arch/sparc/kernel/mdesc.c > index 8e645ddac58e..83e1f699bc32 100644 > --- a/arch/sparc/kernel/mdesc.c > +++ b/arch/sparc/kernel/mdesc.c > @@ -39,6 +39,7 @@ struct mdesc_hdr { > u32 node_sz; /* node block size */ > u32 name_sz; /* name block size */ > u32 data_sz; /* data block size */ > + char data[]; > } __attribute__((aligned(16))); > > struct mdesc_elem { > @@ -612,7 +613,7 @@ EXPORT_SYMBOL(mdesc_get_node_info); > > static struct mdesc_elem *node_block(struct mdesc_hdr *mdesc) > { > - return (struct mdesc_elem *) (mdesc + 1); > + return (struct mdesc_elem *) (mdesc->data); > } In order for gcc to consider (mdesc + 1) to have size 0 I think it must have tracked the pointer from a structure that has another field (or structure end) following 'mdesc'. If that is the case then it should also know that the data[] must also be size 0. So the warning may reappear with the next gcc version. The busybox patch has: +@@ -75,6 +75,7 @@ struct mdesc_handle { + refcount_t refcnt; + unsigned int handle_size; + struct mdesc_hdr mdesc; ++ char data[]; + }; Which really ought to be more than enough. Although the extra space could be considered to even be outside that structure. But the gcc folks suggested a completely brain-dead change that requires taking the offset from the outer structure. -- return (struct mdesc_elem *) (mdesc + 1); ++ return (struct mdesc_elem *) hp + offsetof(struct mdesc_handle, data); which is probably missing a (char *) cast. I wonder if it might be better to 'launder' the pointer so that gcc can't track its size. It may be that: return (struct mdesc_elem *)(ulong)(mdesc + 1); is enough. Otherwise it will need to be passed into an asm block. But gcc is getting stupid for system programming. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
