From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Wed, 17 Dec 2014 18:45:46 +0100
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Message-ID: <20141217174546.GI2012@hermes.click-hack.org>
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 <20141217171734.GE2012@hermes.click-hack.org>
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Subject: Re: [Xenomai] Mayday mechanism broken on x86-64 - simpler approach
 feasible?
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Xenomai <xenomai@xenomai.org>

On Wed, Dec 17, 2014 at 06:36:47PM +0100, Jan Kiszka wrote:
> On 2014-12-17 18:17, Gilles Chanteperdrix wrote:
> > On Wed, Dec 17, 2014 at 06:19:23PM +0100, Philippe Gerum wrote:
> >> On 12/17/2014 05:57 PM, Jan Kiszka wrote:
> >>> Hi,
> >>>
> >>> while porting the sigdebug test case to forge I noticed occasional
> >>> crashes of the test after it received a watchdog signal. It turned out
> >>> that the problem is in the way mayday works on x86-64:
> >>>
> >>> 1. sp/ip/ax of interrupted thread is saved
> >>> 2. ip is set to mayday page
> >>> 3. mayday page executes the mayday syscall via the syscall instruction
> >>> 4. the syscall restores state on original state on return
> >>>
> >>> That that's the theory. Unfortunately the syscall instructions
> >>> overwrites the cx register with the caller's instruction pointer. We
> >>> could save it but there is no way to restore it on syscall return
> >>> without significant changes to the Linux code in entry_64.S.
> > 
> > Maybe we can restore it in xnarch_fixup_mayday, if nothing touches
> > it ?
> 
> The problem is that cx is mangled again on preparing sysret - we are in
> a syscall return path at that point, but we need an interrupt/exception
> path in fact.

ARM had a similar issue with the register holding the syscall return
value, the fix was to return __xn_reg_rval(regs) in
the mayday syscall, is not this fix working for x86? How is cx
mangled exactly?

-- 
					    Gilles.