On Wed, 12 Jul 2017 16:34:50 +0200
Maxime Ripard <maxime.rip...@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.
This statement seems to be poorly worded.
> 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.
Why don't we just clearly say that this patch fixes undefined behaviour
in a buggy C code, caused by U-Boot failing to meet the 32-bit alignment
expectations of GCC for this particular structure?
> 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.rip...@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));
Alternatively we could try to only mark the 32-bit structure fields as
"packed" rather than marking the whole structure. Here is a test code:
/***********************************/
#include <stdio.h>
#include <stdint.h>
struct a
{
uint32_t x;
uint16_t y;
} a;
struct b
{
uint32_t x __attribute((packed));
uint16_t y;
};
int main(void)
{
printf("sizeof(struct a) = %d\n", (int)sizeof(struct a));
printf("sizeof(struct b) = %d\n", (int)sizeof(struct b));
return 0;
}
/***********************************/
Running it produces the following output:
sizeof(struct a) = 8
sizeof(struct b) = 6
__alignof__(struct a) = 4
__alignof__(struct b) = 2
Also as an additional safety measure, we can add something like this
to U-Boot:
assert(__alignof__(struct ip_udp_hdr) == 2);
Maybe it can be also done as a compile-time test rather than a
runtime test. In the example above, I can add the following code:
int dummy_b[3 - __alignof__(struct b)];
int dummy_a[3 - __alignof__(struct a)];
And then GCC complains at compile time, even though the error
message is not exactly intuitive:
test.c:17:5: error: size of array ‘dummy_a’ is too large
int dummy_a[3 - __alignof__(struct a)];
^
--
Best regards,
Siarhei Siamashka
_______________________________________________
U-Boot mailing list
U-Boot@lists.denx.de
https://lists.denx.de/listinfo/u-boot
