netdev - RE: [PATCH 4/9] ipv6: sr: add core files for SR HMAC support

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB0205C9F@AcuExch.aculab.com>
Date:   Wed, 19 Oct 2016 14:43:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'zhuyj' <zyjzyj2000@...il.com>,
        David Lebrun <david.lebrun@...ouvain.be>
CC:     netdev <netdev@...r.kernel.org>
Subject: RE: [PATCH 4/9] ipv6: sr: add core files for SR HMAC support

From: zhuyj
> Sent: 19 October 2016 14:21
>  __attribute__((packed)) is potentially unsafe on some systems. The
> symptom probably won't show up on an x86, which just makes the problem
> more insidious; testing on x86 systems won't reveal the problem. (On
> the x86, misaligned accesses are handled in hardware; if you
> dereference an int* pointer that points to an odd address, it will be
> a little slower than if it were properly aligned, but you'll get the
> correct result.)
> 
> On some other systems, such as SPARC, attempting to access a
> misaligned int object causes a bus error, crashing the program.

__attribute__((packed)) causes the compiler to generate byte memory
access and shifts to access the unaligned data.
You don't get a fault, just rather more instructions that you had in mind.

You shouldn't really specify 'packed' unless there are real reasons
why the data structure will appear on misaligned addresses.

	David