linux-kernel - Re: vm86 segfaults with NX bit

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Date:	Fri, 21 Nov 2008 12:38:42 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Luc Verhaegen <lverhaegen@...e.de>,
	Egbert Eich <eich@...e.de>
Subject: Re: vm86 segfaults with NX bit

At Thu, 20 Nov 2008 13:24:38 -0800,
H. Peter Anvin wrote:
> 
> Takashi Iwai wrote:
> >>>>
> >>> did the code that mapped this memory setup use PROT_EXEC ?
> >> A good question...
> >> Luc, Egbert, how does VESA driver handle it?
> > 
> > Just looking through the vesa directory of xorg-server code, PROT_EXEC
> > is passed for all mappings, at least.
> > 
> > Please correct me if I'm looking at a wrong place...
> > 
> 
> Please do an strace of the X server startup up to the point of the crash.

Well, it's not exactly a crash.  VESA driver catches SIGSEGV in its
signal handler, spews the error messages and skips the vm86
operation.  The segfault occurs during every vm86old syscall.

> I am betting that PROT_EXEC is missing from an mmap(), or that it needs 
> an additional mprotect().

PROT_EXEC seems used in every mmap.  But, VESA and int10 codes in
xorg is old and hard to follow for me.

Hopefully Egbert can give us a better insight...


Takashi
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