linux-kernel - Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

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Date:	Sat, 28 Jun 2014 19:07:00 +0200
From:	Pavel Machek <pavel@....cz>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	X86 ML <x86@...nel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

On Thu 2014-06-26 13:47:32, Andy Lutomirski wrote:
> On Thu, Jun 26, 2014 at 1:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
> > The real question is if we care that sysret and iter don't match.  On 32 bits the situation is even more complex.
> 
> At least for 64 bits, iret vs sysret is purely a kernel implementation
> detail (except where a tracer modifies things that are inaccessible to
> sysret), so ISTM it's worth one instruction to make them match.
> 
> I noticed this thing while fiddling with moving some of the syscall
> tracing logic to C.  This isn't a real problem, but it at least made
> me scratch my head.

If possible, we'd like to trace programs without programs being noticed they are 
being traced. See subterfugue utility, for example.

It is certainly worth one extra instruction.
									Pavel
-- 
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