Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] can: ucan: fix alignment constraints | Date | Tue, 9 Feb 2021 10:34:42 +0000 |
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From: Marc Kleine-Budde > Sent: 08 February 2021 13:16 > > On 04.02.2021 17:26:13, Arnd Bergmann wrote: > > From: Arnd Bergmann <arnd@arndb.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@arndb.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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