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

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Message-ID: <CAKU6vyYQ84o=W7=cqNFTcpBXmDn+CPE5g-66kUJsR_Kc8a9iZg@mail.gmail.com>
Date:	Fri, 26 Apr 2013 14:25:50 -0400
From:	Xi Wang <xi.wang@...il.com>
To:	Eric Dumazet <eric.dumazet@...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

Not sure how many you are speaking for when you say "We are very dumb".  :)

Thanks for catching this.  I'l remove this arch thing in v2.

To address your other concern about registers, I'll add some comments
to the code, something like:

"%rdi,%r8,%r9 are not used by seccomp filters; it's safe to not save them."

- xi

On Fri, Apr 26, 2013 at 12:14 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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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