linux-kernel - Re: Regression [Was: Boot hang with stack protector on x86

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Message-ID: <20080223073746.GA28310@elte.hu>
Date:	Sat, 23 Feb 2008 08:37:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	James Morris <jmorris@...ei.org>
Cc:	Sam Ravnborg <sam@...nborg.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Regression [Was: Boot hang with stack protector on x86_64]

* Ingo Molnar <mingo@...e.hu> wrote:

> could you try the fix below ontop of x86.git#testing, does it solve 
> your boot hang?

find below another fix that is somewhat better as it does not affect the 
native kernel and !PARAVIRT.

btw., this also explains why this bug wasnt reported sooner against 
x86.git#testing: people done normally use PARAVIRT on 64-bit yet.
(there is no 64-bit host support)

	Ingo

---------------->
Subject: x86: stackprotector & PARAVIRT fix
From: Ingo Molnar <mingo@...e.hu>
Date: Sat Feb 23 07:06:55 CET 2008

on paravirt enabled 64-bit kernels the paravirt ops do function
calls themselves - which is bad with the stackprotector - for
example pda_init() loads 0 into %gs and then does MSR_GS_BASE
write (which modifies gs.base) - but that MSR write is a function
call on paravirt, which with stackprotector tries to read the
stack canary from the PDA ... crashing the bootup.

the solution was suggested by Arjan van de Ven: to exclude paravirt.c
from stackprotector, too many lowlevel functionality is in it. It's
not like we'll have paravirt functions with character arrays on
their stack anyway...

Signed-off-by: Arjan van de Ven <arjan@...ux.intel.com>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 arch/x86/kernel/Makefile |    1 +
 1 file changed, 1 insertion(+)

Index: linux-x86.q/arch/x86/kernel/Makefile
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/Makefile
+++ linux-x86.q/arch/x86/kernel/Makefile
@@ -15,6 +15,7 @@ nostackp := $(call cc-option, -fno-stack
 CFLAGS_vsyscall_64.o	:= $(PROFILING) -g0 $(nostackp)
 CFLAGS_hpet.o		:= $(nostackp)
 CFLAGS_tsc_64.o		:= $(nostackp)
+CFLAGS_paravirt.o	:= $(nostackp)

 obj-y			:= process_$(BITS).o signal_$(BITS).o entry_$(BITS).o
 obj-y			+= traps_$(BITS).o irq_$(BITS).o
--
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