linux-kernel - Re: [PATCH] x86: KPROBE_ENTRY should be paired wth KPROBE

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Date:	Sun, 23 Nov 2008 18:41:12 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Alexander van Heukelum <heukelum@...lshack.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: KPROBE_ENTRY should be paired wth KPROBE_END

[Ingo Molnar - Sun, Nov 23, 2008 at 04:31:40PM +0100]
| 
| * Cyrill Gorcunov <gorcunov@...il.com> wrote:
| 
| > [Alexander van Heukelum - Sun, Nov 23, 2008 at 04:04:18PM +0100]
| > | On Sun, Nov 23, 2008 at 05:12:37PM +0300, Cyrill Gorcunov wrote:
| > | > [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@...lshack.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@...tmail.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)
| > | 
| > | Looks like a good approach to me. But I assume the ENTRY cppmacro
| > | will include magic?
| > | 
| > | Greetings,
| > | 	Alexander
| > | 
| > 
| > yes, but now I'm in doubts since we have this definition in common 
| > linkage.h I dont know if such approach would be usable on other 
| > platforms.
| 
| i'd suggest to introduce another entry macro name for that, for the 
| time being. If other architectures want to pick up the method, they 
| can generalize and test it.
| 
| ( but this is assembly magic so i'm doubtful - while the features used 
|   are generic GAS features that should work everywhere, binutils 
|   variants tend to be rather fragile. So lets go with some other name 
|   like X86_ENTRY()/X86_END() or so - or maybe ENTRY_CFI()/END_CFI(). )
| 
| 	Ingo
| 

Indeed! I've just sent some quite raw version. Maybe we should introduce
asmmacro.h as ia64 did?

		- Cyrill -
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