Tom & Maxime,
> On 12 Jul 2017, at 16:34, Tom Rini <tr...@konsulko.com> wrote:
>
> On Wed, Jul 12, 2017 at 04:20:52PM +0200, Maxime Ripard wrote:
>> On Tue, Jul 11, 2017 at 07:59:21PM +0200, Dr. Philipp Tomsich wrote:
>>> 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’?
>>
>> It indeed fixed the issue. There might just have been a subtle change
>> of behaviour in GCC, and this is probably going to bite us in other
>> areas.
>>
>> I'll send a patch to add the packed attribute.
>
> Please bear in mind that packed should be used carefully. We've had
> some discussions about this before and have
> doc/README.unaligned-memory-access.txt which may need a little more
> updating now as well, depending on what the final resolution here is as
> I seem to recall some other problem reports with gcc-7.x, but I've not
> personally been able to hit these just yet. But I need to get on that
> soon. http://toolchains.free-electrons.com/ is awesome and I eagerly
> await gcc-7.x toolchains there if I can't get something else spun up in
> a chroot soon :)
So there’s the remaining question of how to fix this permanently:
— with my compiler engineering hat on, I’d recommend to mark this
as a packed struct, as that’s what it is: the compiler needs to keep it
packed, because that is how it is received/sent on the wire
— rereading the doc/README.unaligned-memory-access.txt, the
preferred solution in U-Boot would be to use put/get_unaligned to
access these fields (although I have concerns with this—see below).
I honestly wonder if the recommendation (to avoid ‘packed’) from the
README is appropriate for many of the data structure declarations in
U-Boot which deal with the external representation of data (i.e. DMA
descriptors, memory-mapped register files and data sent on a wire):
the C language does not offer any protection against a compiler adding
patting between structure members, as it sees fit. The original wording
from the standard is:
14 Each non-bit-field member of a structure or union object is aligned in
an implementation-defined manner appropriate to its type.
15 Within a structure object, the non-bit-field members and the units in
which bit-fields reside have addresses that increase in the order in which they
are declared. A pointer to a structure object, suitably converted, points to
its initial member (or if that member is a bit-field, then to the unit in which
it resides), and vice versa. There may be unnamed padding within a structure
object, but not at its beginning.
In other words: there’s nothing in the standard from stopping a compiler
to insert additional padding within structures, unless the ‘packed’ attribute
is added.
Regards,
Philipp.
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