[PATCH] net: ipv6: fix alignment errors on ARM

[Vyacheslav V. Mitrofanov]

On Thu, 2023-01-19 at 14:05 +0300, Sergei Antonov wrote:
> 
> On Thu, 19 Jan 2023 at 11:18, Vyacheslav V. Mitrofanov
> <v.v.mitrofanov at yadro.com> wrote:
> > On Wed, 2023-01-18 at 20:52 +0300, Sergei Antonov wrote:
> > > Commands "ping6" and "tftpboot ... -ipv6" did not work on ARM
> > > because
> > > machine code expects 4-byte alignment and some structures from
> > > net6.h
> > > are not aligned in memory.
> > > 
> > > Fix by adding __packed, since it is already used in this file.
> > > 
> > > Signed-off-by: Sergei Antonov <saproj at gmail.com>
> > > ---
> > >  include/net6.h | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/include/net6.h b/include/net6.h
> > > index 9b3de028e6dc..2d7c5a096046 100644
> > > --- a/include/net6.h
> > > +++ b/include/net6.h
> > > @@ -24,7 +24,7 @@ struct in6_addr {
> > >  #define s6_addr                in6_u.u6_addr8
> > >  #define s6_addr16      in6_u.u6_addr16
> > >  #define s6_addr32      in6_u.u6_addr32
> > > -};
> > > +} __packed;
> > > 
> > >  #define IN6ADDRSZ      sizeof(struct in6_addr)
> > >  #define INETHADDRSZ    sizeof(net_ethaddr)
> > > @@ -62,7 +62,7 @@ struct ip6_hdr {
> > >         u8              hop_limit;
> > >         struct in6_addr saddr;
> > >         struct in6_addr daddr;
> > > -};
> > > +} __packed;
> > >  #define IP6_HDR_SIZE (sizeof(struct ip6_hdr))
> > > 
> > >  /* struct udp_hdr - User Datagram Protocol header */
> > > @@ -164,7 +164,7 @@ struct icmp6hdr {
> > >  #define icmp6_addrconf_managed icmp6_dataun.u_nd_ra.managed
> > >  #define icmp6_addrconf_other   icmp6_dataun.u_nd_ra.other
> > >  #define icmp6_rt_lifetime      icmp6_dataun.u_nd_ra.rt_lifetime
> > > -};
> > > +} __packed;
> > > 
> > >  extern struct in6_addr const net_null_addr_ip6;        /* NULL
> > > IPv6
> > > address */
> > >  extern struct in6_addr net_gateway6;   /* Our gateways IPv6
> > > address
> > > */
> > > --
> > > 2.34.1
> > > 
> > Hello, Sergei!
> > 
> > I didn't get you a little bit. You mean holes between fields of
> > structures or alignment of the beginning?
> 
> No, it is not about holes between fields. It is about the address at
> which a structure is placed in memory. In combination with store
> merging it leads to unaligned memory access. At least for struct
> ip6_hdr, which I studied using assembly listing files. The struct
> ip6_hdr is not placed at 4-byte aligned address because it is 14
> bytes
> off from the beginning of the Ethernet packet.
> 
> Also see how the corresponding IPv4 struct ip_hdr from net.h was
> fixed
> by commit 704f3acfcf553. I should have referred to it in my commit
> description.
> 
I got you. It makes sense.
Thanks!

Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov at yadro.com>