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Date: Sat, 29 Sep 2012 19:42:28 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
        Anton Arapov <anton@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>
Subject: Re: [PATCH 3/5] uprobes: Fix UPROBE_SKIP_SSTEP checks in
	handle_swbp()
Message-ID: <20120929174228.GA27035@redhat.com>
References: <20120914171513.GA29599@redhat.com> <20120914171557.GA29642@redhat.com> <20120915073957.GD7588@in.ibm.com> <20120915150120.GA20608@redhat.com> <20120917172052.GK28033@linux.vnet.ibm.com> <20120918160738.GA22995@redhat.com> <20120920144311.GF27880@linux.vnet.ibm.com> <20120924200825.GA25264@redhat.com>
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On 09/24, Oleg Nesterov wrote:
>
> Anyway,
>
> > for example
> > 0f 1f 40 00
>
> OK, thanks, objdump reports "nopl 0x0(%rax)", looks fine.
>
> But. I do not see how __skip_sstep() can handle this case correctly.
> Not only it should update regs->ip afaics, it should also account 2
> extra bytes _after_ 0f 1f.
>
> > 0f 1f 44 00 00
>
> OK, nopl   0x0(%rax,%rax,1), but in this case we need to skip 3
> extra bytes.
>
>
> I am starting to think this code is broken and we should simply remove
> all checks except 0x66 and 0x90. In this case we do not even need to
> update regs->ip.
>
> Otherwise this code needs to know the insn's length.
>
> Right?

OK, I verified that the application is really killed by SIGILL if
you probe such an instruction. I'll send the fix which simply removes
this code. I do not know how to figure out the actual insn length.

Oleg.