Re: [PATCH net 1/2] net: dsa: mxl862xx: avoid unaligned 16-bit access in api_wrap

[Jakub Kicinski <kuba@kernel.org>]

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.