linux-kernel - Re: net/smc/smc_llc.c:26:2: warning: field within 'struct smc_llc_hdr' is less aligned than 'union smc_llc_hdr::(anonymous at net/smc/smc_llc.c:26:2)' and is usually due to 'struct smc_llc_hdr' being packed, which can lead to unaligned accesses

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Message-Id: <536e1ed3-5d6b-401e-b8b4-888b49aba6df@www.fastmail.com>
Date:   Sat, 17 Sep 2022 10:16:03 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "kernel test robot" <lkp@...el.com>,
        "Karsten Graul" <kgraul@...ux.ibm.com>
Cc:     llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: net/smc/smc_llc.c:26:2: warning: field  within 'struct smc_llc_hdr' is
 less aligned than 'union smc_llc_hdr::(anonymous at net/smc/smc_llc.c:26:2)'
 and is usually due to 'struct smc_llc_hdr' being packed, which can lead to
 unaligned accesses

On Sat, Sep 17, 2022, at 10:09 AM, kernel test robot wrote:
> Hi Karsten,
>
> FYI, the error/warning still remains.
>

>
>>> net/smc/smc_llc.c:26:2: warning: field  within 'struct smc_llc_hdr' is less aligned than 'union smc_llc_hdr::(anonymous at net/smc/smc_llc.c:26:2)' and is usually due to 'struct smc_llc_hdr' being packed, which can lead to unaligned accesses [-Wunaligned-access]
>            union {
>            ^
>    1 warning generated.
>
>
> vim +26 net/smc/smc_llc.c
>
>     23	
>     24	struct smc_llc_hdr {
>     25		struct smc_wr_rx_hdr common;
>   > 26		union {
>     27			struct {
>     28				u8 length;	/* 44 */
>     29		#if defined(__BIG_ENDIAN_BITFIELD)
>     30				u8 reserved:4,
>     31				   add_link_rej_rsn:4;
>     32	#elif defined(__LITTLE_ENDIAN_BITFIELD)
>     33				u8 add_link_rej_rsn:4,
>     34				   reserved:4;
>     35	#endif
>     36			};
>     37			u16 length_v2;	/* 44 - 8192*/
>     38		};
>     39		u8 flags;
>     40	} __packed;		/* format defined in
>     41				 * IBM Shared Memory Communications Version 2

I think the __packed annotation should be moved to the
length_v2 field instead of the outer struct to resolve that.

    Arnd