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Date: Fri, 5 Jan 2018 15:53:23 +0100
From: Sascha Hauer <s.hauer@pengutronix.de>
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References: <20171225183758.7773-1-antonynpavlov@gmail.com>
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Errors-To: barebox-bounces+u.kleine-koenig=pengutronix.de@lists.infradead.org
Subject: Re: [PATCH] net: make struct bootp __packed to prevent unaligned
 store on MIPS
To: Antony Pavlov <antonynpavlov@gmail.com>
Cc: barebox@lists.infradead.org

On Mon, Dec 25, 2017 at 09:37:58PM +0300, Antony Pavlov wrote:
> How to repropduce the unaligned store problem
> 
>     qemu-system-mips -nodefaults -M malta -m 256 \
>                      -nographic -serial stdio -monitor null \
>                      -bios barebox-flash-image \
>                      -net user -net nic,model=rtl8139
> 
>     ...
> 
>     barebox:/ dhcp
> 
>     Ooops, address error on store!
> 
>     $ 0   : 00000000 00000000 01010600 697f2a2e
>     $ 4   : a0850000 00000000 0000001c a040c1b8
>     $ 8   : 00000000 00000002 00000002 00000000
>     $12   : 00000000 00000040 00000100 00000001
>     $16   : a040bba0 a0850000 a0850000 a0850000
>     $20   : 00000000 00000075 00000076 a040ba20
>     $24   : 00000002 a080f210
>     $28   : 00000000 a03ffce0 fffffffd a0833b8c
>     Hi    : 000154f8
>     Lo    : 20000000
>     epc   : a0833b84
>     ra    : a0833b8c
>     Status: 00000002
>     Cause : 80000414
>     Config: 80008482
> 
>     ### ERROR ### Please RESET the board ###
> 
> The unaligned store instruction is located in the bootp_request() from net/dhcp.c:
> 
> a0833b50 <bootp_request>:
> ..
> a0833b7c:       3c020101        lui     v0,0x101
> a0833b80:       24420600        addiu   v0,v0,1536 /* 0x1010600 -> v0 */
> a0833b84:       0c20024a        jal     a0800928 <get_time_ns>
> a0833b88:       ae02002a        sw      v0,42(s0) /* store 0x1010600 to addr s0 + 42 */
> 
> This assembler code is generated by mips-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
> from this:
> 
>     bp = (struct bootp *)payload;
>     bp->bp_op = OP_BOOTREQUEST; /* 0x01 */
>     bp->bp_htype = HWT_ETHER; /* 0x01 */
>     bp->bp_hlen = HWL_ETHER; /* 0x06 */
>     bp->bp_hops = 0;
> 
> Compiler replaces four 'store byte' instruction by one 'store 32-bit word'
> instruction. Alas sometimes this leads to unaligned store situation.
> 
> Making struct bootp __packed prevents this optimization and fixes the problem.
> 
> Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>

Applied to master, thanks

Sascha

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