Subject: Re: [PATCH] Fix ARM BUILD_BUG_ON() errors with batman-adv

From: Antonio Quartulli <[email-masked]>
Date: Sat, 30 Nov 2013 21:12:52 +0100

On 30/11/13 20:15, Russell King - ARM Linux wrote:
> From: Russell King <rmk+kernel-lFZ/[email-masked]>
> 
> The following errors were observed on ARM during a randconfig
> build. This patch addresses them by ensuring that the batadv_header
> structure is appropriately packed; this structure contains three
> 8-bit elements so there should be no undesired side effect from
> this packing.
> 
> net/batman-adv/main.c: In function 'batadv_init': 
> net/batman-adv/main.c:425:279: error: call to
> '__compiletime_assert_425' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_unicast_4addr_packet,
> src) != 10 net/batman-adv/main.c:426:267: error: call to
> '__compiletime_assert_426' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_unicast_packet, dest)
> != 4 net/batman-adv/main.c:427:275: error: call to
> '__compiletime_assert_427' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_unicast_tvlv_packet,
> dst) != 4 net/batman-adv/main.c:428:261: error: call to
> '__compiletime_assert_428' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_frag_packet, dest) !=
> 4 net/batman-adv/main.c:429:271: error: call to
> '__compiletime_assert_429' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_icmp_packet, icmph.dst)
> != 4 net/batman-adv/main.c:430:277: error: call to
> '__compiletime_assert_430' declared with attribute error:
> BUILD_BUG_ON failed: offsetof(struct batadv_icmp_packet_rr,
> icmph.dst) != 4
> 
> Signed-off-by: Russell King <rmk+kernel-lFZ/[email-masked]> --- 
> net/batman-adv/packet.h |    2 +- 1 files changed, 1 insertions(+),
> 1 deletions(-)
> 
> diff --git a/net/batman-adv/packet.h b/net/batman-adv/packet.h 
> index 207459b62966..4039f25794e0 100644 ---
> a/net/batman-adv/packet.h +++ b/net/batman-adv/packet.h @@ -171,7
> +171,7 @@ struct batadv_header { /* the parent struct has to add a
> byte after the header to make * everything 4 bytes aligned again 
> */ -}; +} __attribute__((packed));

Russell,

this attribute was there in the past and then was removed on purpose
because on some architectures it makes the compiler generating very
inefficient code (you can check
http://article.gmane.org/gmane.org.freifunk.batman/8317 for more details).

So we removed the __packed attribute and we took care of properly
aligning all the structures by adding "alignment/reserved members"
when needed.

I don't know the ARM architecture at all (I don't know if the other
batman-adv developers do), so could you please provide here some more
details about why that static check is failing? We would like to
address this issue differently rather than re-adding the __packed
attribute.

Thanks a lot.

Best Regards,

- -- 
Antonio Quartulli