[U-Boot] Data Abort with gcc 7.1
Dr. Philipp Tomsich
philipp.tomsich at theobroma-systems.com
Wed Jul 12 17:49:38 UTC 2017
> On 12 Jul 2017, at 19:49, Tom Rini <trini at konsulko.com> wrote:
>
> On Wed, Jul 12, 2017 at 05:48:16PM +0200, Dr. Philipp Tomsich wrote:
>> Tom & Maxime,
>>
>>> On 12 Jul 2017, at 16:34, Tom Rini <trini at 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 <trini at 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
>
> OK. And are there other structures that we should mark as packed as
> well in include/net.h? I suspect so.
>
>> — 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.
>
> So, this is partly why I think we need to perhaps add / update / change
> some wording in the current file. The overall desire is to avoid
> "doesn't work, packed fixes it, packed must be the right fix!". We want
> to emphasize that packed should be used when it's appropriate, ie the
> cases you mention above. Thanks!
I’ll have a shot at rewording this. Give me a few days to get around to it.
—Phil.
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