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. -- Tom
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