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Date: Wed, 24 Jun 2026 17:52:39 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: David Laight <david.laight.linux@gmail.com>
Cc: Daniel Golle <daniel@makrotopia.org>, Andrew Lunn <andrew@lunn.ch>,
 Vladimir Oltean <olteanv@gmail.com>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, Paolo Abeni
 <pabeni@redhat.com>, netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH net 1/2] net: dsa: mxl862xx: avoid unaligned 16-bit
 access in api_wrap
Message-ID: <20260624175239.1b97aaa6@kernel.org>
In-Reply-To: <20260619100154.794168e5@pumpkin>
References: <599327521db465a534d277de53ab9b6cac01928b.1781702256.git.daniel@makrotopia.org>
	<20260619100154.794168e5@pumpkin>
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On Fri, 19 Jun 2026 10:01:54 +0100 David Laight wrote:
> > The MXL862XX_API_* macros pass the address of a stack-allocated, __packed
> > firmware-ABI struct to mxl862xx_api_wrap() as a void *. The struct has an
> > alignment of 1, so the compiler is free to place it at an odd address.
> > 
> > mxl862xx_api_wrap() reinterprets that buffer as a __le16 * and accesses it
> > with data[i], for which the compiler assumes the natural 2-byte alignment
> > of __le16 and emits aligned 16-bit loads/stores (e.g. lhu/sh on MIPS).
> > When the buffer lands on an odd address these fault on architectures that
> > do not support unaligned access, such as MIPS32.  
> 
> Isn't the correct fix to not pack the structure?
> (or probably any of the associated structures??)

Agreed, this is very silly:

struct mxl862xx_register_mod {
	__le16 addr;
	__le16 data;
	__le16 mask;
} __packed;

But some structs won't get aligned:

struct mxl862xx_mac_table_clear {
	u8 type;
	u8 port_id;
} __packed;

So I guess the "just don't pack" will have some corner cases, too.