Maxime,
> On 11 Jul 2017, at 18:59, Tom Rini <tr...@konsulko.com> wrote:
>
> On Tue, Jul 11, 2017 at 06:54:55PM +0200, Maxime Ripard wrote:
>> Hi,
>>
>> I recently got a gcc 7.1 based toolchain, and it seems like it
>> generates unaligned code, specifically in the net_set_ip_header
>> function in my case.
>>
>> Whenever some packet is sent, this data abort is triggered:
>>
>> => setenv ipaddr 10.42.0.1; ping 10.42.0.254
>> using musb-hdrc, OUT ep1out IN ep1in STATUS ep2in
>> MAC de:ad:be:ef:00:01
>> HOST MAC de:ad:be:af:00:00
>> RNDIS ready
>> musb-hdrc: peripheral reset irq lost!
>> high speed config #2: 2 mA, Ethernet Gadget, using RNDIS
>> USB RNDIS network up!
>> Using usb_ether device
>> data abort
>> pc : [<7ff9db10>] lr : [<7ff9f00c>]
>> reloc pc : [<4a043b10>] lr : [<4a04500c>]
>> sp : 7bf37cc8 ip : 00000000 fp : 7ff6236c
>> r10: 7ffed2b8 r9 : 7bf39ee8 r8 : 7ffed2b8
>> r7 : 00000001 r6 : 00000000 r5 : 0000002a r4 : 7ffed30e
>> r3 : 14000045 r2 : 01002a0a r1 : fe002a0a r0 : 7ffed30e
>> Flags: nZCv IRQs off FIQs off Mode SVC_32
>> Resetting CPU ...
>>
>>
>> Running objdump on it gives us this:
>>
>> 4a043b04 <net_set_ip_header>:
>>
>> /*
>> * Construct an IP header.
>> */
>> /* IP_HDR_SIZE / 4 (not including UDP) */
>> ip->ip_hl_v = 0x45;
>> 4a043b04: e59f3074 ldr r3, [pc, #116] ; 4a043b80
>> <net_set_ip_header+0x7c>
>> {
>> 4a043b08: e92d4013 push {r0, r1, r4, lr}
>> 4a043b0c: e1a04000 mov r4, r0
>> ip->ip_hl_v = 0x45;
>> 4a043b10: e5803000 str r3, [r0] <---- Abort
>> ip->ip_tos = 0;
>> ip->ip_len = htons(IP_HDR_SIZE);
>> ip->ip_id = htons(net_ip_id++);
>> 4a043b14: e59f3068 ldr r3, [pc, #104] ; 4a043b84
>> <net_set_ip_header+0x80>
>>
>> It seems like r0 is indeed set to an unaligned address (0x7ffed30e)
>> for some reason.
>>
>> Using a Linaro 6.3 toolchain works on the same commit with the same
>> config, so it really seems to be a compiler-related issue.
>>
>> It generates this code:
>>
>> 4a043ec4 <net_set_ip_header>:
>>
>> /*
>> * Construct an IP header.
>> */
>> /* IP_HDR_SIZE / 4 (not including UDP) */
>> ip->ip_hl_v = 0x45;
>> 4a043ec4: e3a03045 mov r3, #69 ; 0x45
>> {
>> 4a043ec8: e92d4013 push {r0, r1, r4, lr}
>> 4a043ecc: e1a04000 mov r4, r0
>> ip->ip_hl_v = 0x45;
>> 4a043ed0: e5c03000 strb r3, [r0]
>> ip->ip_tos = 0;
>> ip->ip_len = htons(IP_HDR_SIZE);
>> 4a043ed4: e3a03b05 mov r3, #5120 ; 0x1400
>> ip->ip_tos = 0;
>> 4a043ed8: e3a00000 mov r0, #0
>> ip->ip_len = htons(IP_HDR_SIZE);
>> 4a043edc: e1c430b2 strh r3, [r4, #2]
>> ip->ip_id = htons(net_ip_id++);
>> 4a043ee0: e59f3064 ldr r3, [pc, #100] ; 4a043f4c
>> <net_set_ip_header+0x88>
>>
>> And it seems like it's using an strb instruction to avoid the
>> unaligned access.
>>
>> As far as I know, we are passing --wno-unaligned-access, so the broken
>> situation should not arise, and yet it does, so I'm a bit confused,
>> and not really sure what to do from there.
>
> Can you reduce the code into a testcase? I think the first step is
> filing a bug with gcc and seeing where it goes from there as yes, we
> should be passing -mno-unaligned-access.
I don’t think that this is a GCC bug, as “-mno-unaligned-access” will
change the behaviour for packed data-structures only. Here’s from
the GCC docs:
> -munaligned-access
> -mno-unaligned-access
> Enables (or disables) reading and writing of 16- and 32- bit values from
> addresses that are not 16- or 32- bit aligned. By default unaligned access is
> disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline
> architectures, and enabled for all other architectures. If unaligned access
> is not enabled then words in packed data structures are accessed a byte at a
> time.
The key word seems to be “in packed data structures”.
However, I don’t see an attribute “packed” for the 'struct ip_udp_hdr’
(in include/net.h).
Could you try to verify that the error reproduces with a packed variant
of the ‘struct ip_udp_hdr’?
Regards,
Phil.
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