linux-kernel - Re: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support BPF_S_ANC_SECCOMP_LD

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Date:	Fri, 26 Apr 2013 09:14:00 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Xi Wang <xi.wang@...il.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Russell King <linux@....linux.org.uk>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Will Drewry <wad@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 2/6] x86: bpf_jit_comp: support
 BPF_S_ANC_SECCOMP_LD_W instruction

On Fri, 2013-04-26 at 12:02 -0400, Xi Wang wrote:
> On Fri, Apr 26, 2013 at 11:11 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 1) 'current' at the time the code is jitted (compiled) is not the
> > 'current' at the time the filter will be evaluated.
> >
> > On x86_64, if CONFIG_IA32_EMULATION=y, syscall_get_arch() evaluates to :
> >
> > if (task_thread_info(task)->status & TS_COMPAT)
> >         return AUDIT_ARCH_I386;
> > return AUDIT_ARCH_X86_64;
> >
> > So your code is completely wrong.
> 
> Just to be clear, are you worrying about a process changing its
> personality after installing seccomp filters?

You didn't explained how things worked.

Are you assuming we network guys know everything ?

Just to make it very clear :

We are very dumb and you must explain us everything.

If process would not change personality, why do we have get_arch() at
all ? Why isn't it optimized outside of the JIT itself, in the generic
seccomp checker, its a single "A = K" instruction after all.

Why this part is even in the x86 BPF JIT ?

To me it looks like _if_ get_arch() is provided in BPF, its for a
reason, and your implementation looks very suspicious, if not buggy.



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