netdev - RE: [PATCH] carl9170: fix struct alignment conflict

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Date:   Fri, 5 Feb 2021 15:31:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     Arnd Bergmann <arnd@...db.de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] carl9170: fix struct alignment conflict

From: Arnd Bergmann
> Sent: 04 February 2021 16:29
> 
> Multiple structures in the carl9170 driver have alignment
> impossible alignment constraints that gcc warns about when
> building with 'make W=1':
> 
> drivers/net/wireless/ath/carl9170/fwcmd.h:243:2: warning: alignment 1 of 'union <anonymous>' is less
> than 4 [-Wpacked-not-aligned]
> drivers/net/wireless/ath/carl9170/wlan.h:373:1: warning: alignment 1 of 'struct
> ar9170_rx_frame_single' is less than 2 [-Wpacked-not-aligned]
> 
> In the carl9170_cmd structure, multiple members that have an explicit
> alignment requirement of four bytes are added into a union with explicit
> byte alignment, but this in turn is part of a structure that also has
> four-byte alignment.
> 
> In the wlan.h header, multiple structures contain a ieee80211_hdr member
> that is required to be two-byte aligned to avoid alignmnet faults when
> processing network headers, but all members are forced to be byte-aligned
> using the __packed tag at the end of the struct definition.
> 
> In both cases, leaving out the packing does not change the internal
> layout of the structure but changes the alignment constraint of the
> structure itself.
> 
> Change all affected structures to only apply packing where it does
> not violate the alignment requirement of the contained structure.

I think I'd add compile-time assert that some of these structures
are exactly the expected size.
Then look at removing the outer packed/aligned attributes
and just putting the attribute on the 16/32 bit member(s)
that themselves might be misaligned.
Much the way that the 32bit aligned 64bit values are handled
in the x86 compat code in x86-64.

	David

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