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

[Sergei Antonov]

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.

> Frankly speaking I always thought that in that kind of structure like
> below it is not necessary. To be honest I disassembled the same code on
> arm and there were no holes.

The following code from net6.c

int ip6_add_hdr(uchar *xip, struct in6_addr *src, struct in6_addr *dest,
                int nextheader, int hoplimit, int payload_len)
{
        struct ip6_hdr *ip6 = (struct ip6_hdr *)xip;

        ip6->version = 6;
        ip6->priority = 0;
        ip6->flow_lbl[0] = 0;
        ip6->flow_lbl[1] = 0;
        ip6->flow_lbl[2] = 0;

is optimized into:
801ab138:       e3a02060        mov     r2, #96 @ 0x60
801ab13c:       e1a04000        mov     r4, r0
801ab140:       e5802000        str     r2, [r0]

The last instruction does all 5 different assignments from C code at once.