linux-kernel - Re: [PATCH] ARM: net: delegate filter to kernel interpreter when imm_offset() return value can't fit into 12bits.

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Message-Id: <20150510.192217.1306037281172187835.davem@davemloft.net>
Date:	Sun, 10 May 2015 19:22:17 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	nschichan@...ebox.fr
Cc:	linux@....linux.org.uk, ast@...mgrid.com, daniel@...earbox.net,
	mgherzan@...il.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] ARM: net: delegate filter to kernel interpreter when
 imm_offset() return value can't fit into 12bits.

From: Nicolas Schichan <nschichan@...ebox.fr>
Date: Thu,  7 May 2015 17:14:21 +0200

> The ARM JIT code emits "ldr rX, [pc, #offset]" to access the literal
> pool. #offset maximum value is 4095 and if the generated code is too
> large, the #offset value can overflow and not point to the expected
> slot in the literal pool. Additionally, when overflow occurs, bits of
> the overflow can end up changing the destination register of the ldr
> instruction.
> 
> Fix that by detecting the overflow in imm_offset() and setting a flag
> that is checked for each BPF instructions converted in
> build_body(). As of now it can only be detected in the second pass. As
> a result the second build_body() call can now fail, so add the
> corresponding cleanup code in that case.
> 
> Using multiple literal pools in the JITed code is going to require
> lots of intrusive changes to the JIT code (which would better be done
> as a feature instead of fix), just delegating to the kernel BPF
> interpreter in that case is a more straight forward, minimal fix and
> easy to backport.
> 
> Signed-off-by: Nicolas Schichan <nschichan@...ebox.fr>

Applied, thanks Nicolas.
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