On Fri, 21 Jul 2017 22:15:37 +0300
Siarhei Siamashka <siarhei.siamas...@gmail.com> wrote:
> 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)];
> ^
And if we do it this way, then the compile-time test can look a bit
cleaner:
test.c:17:5: error: size of array ‘compile_test_for_struct_a_alignment’ is
negative
int compile_test_for_struct_a_alignment[(__alignof__(struct a) == 2) ? 1 : -1];
--
Best regards,
Siarhei Siamashka
_______________________________________________
U-Boot mailing list
U-Boot@lists.denx.de
https://lists.denx.de/listinfo/u-boot
