[U-Boot] Data Abort with gcc 7.1

[Maxime Ripard]

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.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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