[U-Boot] [PATCH] net: Mark the ip_udp_hdr struct as packed

[Siarhei Siamashka <siarhei.siamashka@gmail.com>]

On Wed, 19 Jul 2017 20:26:54 +0200
Jeroen Hofstee <jeroen@myspectrum.nl> wrote:

> Hi,
> 
> 
> On 07/18/2017 08:10 PM, Joe Hershberger wrote:
> > Hi Maxime,
> >
> > On Wed, Jul 12, 2017 at 9:34 AM, Maxime Ripard
> > <maxime.ripard@free-electrons.com> wrote:  
> >> The -mno-unaligned-access flag used on ARM to prevent GCC from generating
> >> unaligned accesses (obviously) will only do so on packed structures.
> >>
> >> It seems like gcc 7.1 is a bit stricter than previous gcc versions on this,
> >> and using it lead to data abort for unaligned accesses when generating
> >> network traffic.
> >>
> >> Fix this by adding the packed attribute to the ip_udp_hdr structure in
> >> order to let GCC do its job.
> >>
> >> Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> >> ---
> >>   include/net.h | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/include/net.h b/include/net.h
> >> index 997db9210a8f..7b815afffafa 100644
> >> --- a/include/net.h
> >> +++ b/include/net.h
> >> @@ -390,7 +390,7 @@ struct ip_udp_hdr {
> >>          u16             udp_dst;        /* UDP destination port         */
> >>          u16             udp_len;        /* Length of UDP packet         */
> >>          u16             udp_xsum;       /* Checksum                     */
> >> -};
> >> +} __attribute__ ((packed));  
> > Do you have an example of why this is unaligned? It seems that the
> > structure itself is naturally packed (each element is aligned to its
> > access size). It seems the only time this would hit a dabort is if the
> > head of the buffer is not 32-bit aligned. Maybe we should address the
> > place where that is the case instead of forcing byte-wise accesses in
> > general for this structure?  
> 
> |Perhaps __attribute__((aligned(2))) can prevent byte wise accesses? 
> Regards, Jeroen |

https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html

It says that "The aligned attribute can only increase alignment".

-- 
Best regards,
Siarhei Siamashka