netdev - Re: [B.A.T.M.A.N.] [PATCH] Fix ARM BUILD_BUG

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Sun, 01 Dec 2013 15:28:58 +0100
From:	Antonio Quartulli <antonio@...hcoding.com>
To:	linux@....linux.org.uk
CC:	The list for a Better Approach To Mobile Ad-hoc
	 Networking <b.a.t.m.a.n@...ts.open-mesh.org>,
	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [B.A.T.M.A.N.] [PATCH] Fix ARM BUILD_BUG_ON() errors with	batman-adv

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 01/12/13 01:27, Antonio Quartulli wrote:
> On 01/12/13 00:05, Russell King - ARM Linux wrote:
>> On Sat, Nov 30, 2013 at 09:12:52PM +0100, Antonio Quartulli 
>> wrote:
>>> 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.
> 
>> The reason is this struct becomes a size of 4 bytes, even though 
>> it only contains three uint8_t members.  This offsets the
>> members of all structs that this struct is embedded in.
> 
>> If you don't wish to fix this, then please make your subsystem 
>> depend on !ARM because it's otherwise impossible to fix and can 
>> never work on ARM.
> 
> 
> I'd like to fix this.
> 
> Actually I can't really understand your explanation: struct 
> batadv_header is always used inside another parent structure and 
> the latter always has a uint8_t following the batadv_header, thus 
> filling that gap and aligning it to 4bytes. I think this is also 
> why we don't get this compiler error on any other architecture. Or 
> am I missing something?
> 
> I'll install a toolchain for ARM and I'll try to inspect it
> better. If we have to make a change we should do it before 3.13 is
> release since this change could possibly alter the packet layout.
> 
> 

It looks like that the ARM compiler cannot pack the structures properly.

So, given these two structures:

struct batadv_header {
        uint8_t  packet_type;
        uint8_t  version;
        uint8_t  ttl;
};

struct batadv_unicast_packet {
        struct batadv_header header;
        uint8_t  ttvn;
        uint8_t  dest[ETH_ALEN];
};

we have the compiler saying that offset_of dest in struct
batadv_unicast_packet is 5.

This means that struct batadv_header is padded to 4 bytes even if it
is enclosed in struct batadv_unicast_packet and a proper 1byte member
is put right after it.

Am I wrong or this is a problem in the ARM compiler not doing the
right assumption? On x86 and x86_64 offset_of dest is 4, as expected.


Regards,

- -- 
Antonio Quartulli
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.22 (GNU/Linux)

iEYEARECAAYFAlKbR6oACgkQpGgxIkP9cwdpYACgjcWLZ7EGFDatHTsByPz7idsY
YOgAnRq6JTmOv8zI3OrRiBRtOIprgOew
=Jxbq
-----END PGP SIGNATURE-----
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html