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Date: Sun, 23 Nov 2008 17:12:37 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Ingo Molnar <mingo@elte.hu>,
       Alexander van Heukelum <heukelum@mailshack.com>,
       LKML <linux-kernel@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
       Jan Beulich <jbeulich@novell.com>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86: KPROBE_ENTRY should be paired wth KPROBE_END
Message-ID: <20081123141237.GI24818@localhost>
References: <20081120152601.GA3953@mailshack.com> <20081120153954.GA22511@elte.hu> <20081121154155.GA12999@mailshack.com> <20081121154318.GA13014@mailshack.com> <20081121154428.GB13014@mailshack.com> <20081121160629.GA24839@elte.hu> <20081123090828.GA31490@mailshack.com> <20081123091532.GA31515@mailshack.com> <20081123132752.GF1178@elte.hu> <20081123135134.GH24818@localhost>
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[Cyrill Gorcunov - Sun, Nov 23, 2008 at 04:51:34PM +0300]
| [Ingo Molnar - Sun, Nov 23, 2008 at 02:27:52PM +0100]
| | 
| | * Alexander van Heukelum <heukelum@mailshack.com> wrote:
| | 
| | > Impact: moves some code out of .kprobes.text
| | > 
| | > KPROBE_ENTRY switches code generation to .kprobes.text, and KPROBE_END
| | > uses .popsection to get back to the previous section (.text, normally).
| | > Also replace ENDPROC by END, for consistency.
| | > 
| | > Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
| | 
| | applied to tip/x86/irq, thanks Alexander!
| | 
| | > One more small change for today. The xen-related functions 
| | > xen_do_hypervisor_callback and xen_failsafe_callback are put
| | > in the .kprobes.text even in the current kernel: ignore_sysret
| | > is enclosed in KPROBE_ENTRY / ENDPROC, instead of KPROBE_ENTRY /
| | > KPROBE_END, but I guess the situation is harmless.
| | 
| | yeah. It narrows no-kprobes protection for that code, but it should 
| | indeed be fine (and that's the intention as well).
| | 
| | Note that this is a reoccuring bug type, and rather long-lived. Can 
| | you think of any way to get automated nesting protection of both the 
| | .cfi_startproc/endproc macros and kprobes start/end? A poor man's 
| | solution would be to grep the number of start and end methods and 
| | enforce that they are equal.
| | 
| | 	Ingo
| | 
| 
| I think we could play with preprocessor and check if ENTRY/END matches.
| Looking now.
| 
| 		- Cyrill -

Here is what I've done

1) Add some macros like:

	.macro __set_entry
	.set _ENTRY_IN, 1
	.endm

	.macro __unset_entry
	.set _ENTRY_IN, 0
	.endm

	.macro __check_entry
	.ifeq _ENTRY_IN
	.error "END should be used"
	.abort
	.endif
	.endm

So the code

ENTRY(mcount)
	__unset_entry
	retq
	__check_entry
END(mcount)

will fail like

cyrill@lenovo linux-2.6.git $ make arch/x86/kernel/entry_64.o
  CHK     include/linux/version.h
  CHK     include/linux/utsrelease.h
  SYMLINK include/asm -> include/asm-x86
  CALL    scripts/checksyscalls.sh
  AS      arch/x86/kernel/entry_64.o
arch/x86/kernel/entry_64.S: Assembler messages:
arch/x86/kernel/entry_64.S:84: Error: END should be used
arch/x86/kernel/entry_64.S:84: Fatal error: .abort detected.  Abandoning ship.
make[1]: *** [arch/x86/kernel/entry_64.o] Error 1
make: *** [arch/x86/kernel/entry_64.o] Error 2
cyrill@lenovo linux-2.6.git $ 

So if such an approach is acceptable (in general) -- I could take a more
deeper look. So every ENTRY would check if other ENTRY/KPROBE is active
and report that.

		- Cyrill -
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