netdev - Re: [PATCH] arm64: net: bpf: don't BUG() on large shifts

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Message-ID: <20160112192302.GA34276@ast-mbp.thefacebook.com>
Date:	Tue, 12 Jan 2016 11:23:03 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Rabin Vincent <rabin@....in>, davem@...emloft.net,
	netdev@...r.kernel.org, zlim.lnx@...il.com, yang.shi@...aro.org,
	catalin.marinas@....com, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: net: bpf: don't BUG() on large shifts

On Tue, Jan 12, 2016 at 05:17:10PM +0000, Will Deacon wrote:
> On Fri, Jan 08, 2016 at 11:09:44AM -0800, Alexei Starovoitov wrote:
> > On Fri, Jan 08, 2016 at 03:44:23PM +0000, Will Deacon wrote:
> > > On Tue, Jan 05, 2016 at 06:39:03PM +0100, Rabin Vincent wrote:
> > > > Attempting to generate UBFM/SBFM instructions with shifts that can't be
> > > > encoded in the immediate fields of the opcodes leads to a trigger of a
> > > > BUG() in the instruction generation code.  As the ARMv8 ARM says: "The
> > > > shift amounts must be in the range 0 to one less than the register width
> > > > of the instruction, inclusive."  Make the JIT reject unencodable shifts
> > > > instead of crashing.
> > > 
> > > I moaned about those BUG_ONs when they were introduced:
> > > 
> > >   https://lkml.org/lkml/2014/7/17/438
> > > 
> > > The response then was that the verifier would catch these issues so
> > > there was nothing to worry about. Has something changed so that is no
> > > longer the case? Do we need to consider a different way of rejecting
> > > invalid instructions at the encoding stage rather than bringing down the
> > > kernel?
> > 
> > that discussion lead to replacement of all BUG_ONs in
> > arch/arm64/net/bpf_jit_comp.c with pr_err_once(), but looks like
> > arch/arm64/kernel/insn.c wasn't addressed.
> > The amount of BUG_ONs there is indeed overkill regardless of what
> > verifier and other JITs do. btw, x64 JIT doesn't have runtime BUG_ONs.
> 
> Maybe, but insn.c is also used by the alternatives patching code, so we
> really need a way to communicate failure back to the BPF JIT when passed
> an invalid instruction description.

agree. I think there are several options to achieve that after
all BUG_ONs are removed:
- change interface for all insn generating macros to check for
  AARCH64_BREAK_FAULT opcode as error.
  That will require all of emit*() functions in bpf_jit_comp.c to
  be changed to accept/return error.
  Overall that looks like massive change.
- ignore AARCH64_BREAK_FAULT during emit and add another pass after
  all code is generated. If such insn is found in a jited code,
  discard the jit.
  I think that's better option.

Zi, any comments?