linux-kernel - RE: [PATCH] can: ucan: fix alignment constraints

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Message-ID: <bd7e6497b3f64fb5bb839dc9a9d51d6a@AcuMS.aculab.com>
Date:   Tue, 9 Feb 2021 10:34:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Marc Kleine-Budde' <mkl@...gutronix.de>,
        Arnd Bergmann <arnd@...nel.org>
CC:     Wolfgang Grandegger <wg@...ndegger.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        "Oliver Hartkopp" <socketcan@...tkopp.net>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] can: ucan: fix alignment constraints

From: Marc Kleine-Budde
> Sent: 08 February 2021 13:16
> 
> On 04.02.2021 17:26:13, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > struct ucan_message_in contains member with 4-byte alignment
> > but is itself marked as unaligned, which triggers a warning:
> >
> > drivers/net/can/usb/ucan.c:249:1: warning: alignment 1 of 'struct ucan_message_in' is less than 4 [-
> Wpacked-not-aligned]
> >
> > Mark the outer structure to have the same alignment as the inner
> > one.
> >
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> Applied to linux-can-next/testing.

I've just had a look at that file.

Are any of the __packed or __aligned actually required at all.

AFAICT there is one structure that would have end-padding.
But I didn't actually spot anything validating it's length.
Which may well mean that it is possible to read off the end
of the USB receive buffer - plausibly into unmapped addresses.

I looked because all the patches to 'fix' the compiler warning
are dubious.
Once you've changed the outer alignment (eg of a union) then
the compiler will assume that any pointer to that union is aligned.
So any _packed() attributes that are required for mis-aligned
accesses get ignored - because the compiler knows the pointer
must be aligned.

So while the changes remove the warning, they may be removing
support for misaligned addresses.

	David

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