linux-kernel - Re: [PATCH] arch_check_bp_in_kernelspace: fix the range check

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Message-ID: <20121121141627.GB21030@redhat.com>
Date:	Wed, 21 Nov 2012 15:16:27 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	u3557@...o.sublimeip.com
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch_check_bp_in_kernelspace: fix the range check

Hi Amnon,

Please read my previous email ;)
http://marc.info/?l=linux-kernel&m=135342649119153

On 11/21, u3557@...o.sublimeip.com wrote:
>
> Hi Oleg,
>
> > Or. Perhaps we can define TRAP_VSYSCALL and change emulate_vsyscall() to
> > do
> >
> >
> > 	if (current->ptrace && test_thread_flag(TIF_SYSCALL_TRACE))
> > 		send_sigtrap(TRAP_VSYSCALL, ...);
> >
> > if it returns true?
> >
>
> I wish it were possible, but the vsyscall page is entered in user-mode,

Only in NATIVE mode. emulate_vsyscall() runs in kernel mode.

And in the NATIVE mode PTRACE_SYSCALL should work just fine, because:

> The vsyscall page was designed in order to avoid user/kernel context
> switches,

True, it was. But not today. Please look at __vsyscall_page:

	__vsyscall_page:

		mov $__NR_gettimeofday, %rax
		syscall
		ret

If you want the "fast" sys_time() without entering the kernel, you can
use __vdso_time(). And since vdso has the user-space mapping you can
insert "int3" or use hw breakpoints.

At least this is my understanding after I glanced at the new implementation.

However. It is not that I think that TRAP_VSYSCALL is really good idea.
At least it needs another option...

Oleg.

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