[U-Boot] Data Abort with gcc 7.1

[Tom Rini <trini@konsulko.com>]

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@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 :)

-- 
Tom
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