netdev - Re: [PATCH net-next v2] net: bpf: arm: make hole-faulting more robust

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Date:	Tue, 23 Sep 2014 12:40:48 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	dborkman@...hat.com
Cc:	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux@....linux.org.uk, catalin.marinas@....com,
	will.deacon@....com, mgherzan@...il.com, ast@...mgrid.com
Subject: Re: [PATCH net-next v2] net: bpf: arm: make hole-faulting more
 robust

From: Daniel Borkmann <dborkman@...hat.com>
Date: Fri, 19 Sep 2014 14:56:57 +0200

> Will Deacon pointed out, that the currently used opcode for filling holes,
> that is 0xe7ffffff, seems not robust enough ...
 ...
> ... which is a bit fragile. The ARM specification defines some *permanently*
> guaranteed undefined instruction (UDF) space, for example for ARM in ARMv7-AR,
> section A5.4 and for Thumb in ARMv7-M, section A5.2.6.
> 
> Similarly, ptrace, kprobes, kgdb, bug and uprobes make use of such instruction
> as well to trap. Given mentioned section from the specification, we can find
> such a universe as (where 'x' denotes 'don't care'):
> 
>   ARM:    xxxx 0111 1111 xxxx xxxx xxxx 1111 xxxx
>   Thumb:  1101 1110 xxxx xxxx
> 
> We therefore should use a more robust opcode that fits both. Russell King
> suggested that we can even reuse a single 32-bit word, that is, 0xe7fddef1
> which will fault if executed in ARM *or* Thumb mode as done in f928d4f2a86f
> ("ARM: poison the vectors page"). That will still hold our requirements:
 ...
> So on ARM 0xe7fddef1 conforms to the above UDF pattern, and the low 16 bit
> likewise correspond to UDF in Thumb case. The 0xe7fd part is an unconditional
> branch back to the UDF instruction.
> 
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>

Applied, thanks Daniel.
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