linux-kernel - Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF

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Date:	Fri, 3 Jun 2016 12:57:48 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Dmitry Safonov <dsafonov@...tuozzo.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	luto@...capital.net, tglx@...utronix.de, hpa@...or.com,
	x86@...nel.org, 0x7f454c46@...il.com, oleg@...hat.com,
	xemul@...tuozzo.com, khorenko@...tuozzo.com,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32)

On Wed, Jun 01, 2016 at 04:11:36PM +0300, Dmitry Safonov wrote:
> As the task isn't executing at the moment of {GET,SET}REGS,
> return regset that corresponds to code selector, rather than
> value of TIF_IA32 flag.
> I.e. if we ptrace i386 elf binary that has just changed it's
> code selector to __USER_CS, than GET_REGS will return
> full x86_64 register set.
> 
> Note, that this will work only if application has changed it's CS.
> If the application does 32-bit syscall with __USER_CS, ptrace
> will still return 64-bit register set. Which might be still confusing
> for tools that expect TS_COMPACT to be exposed [1, 2].
> 
> So this this change should make PTRACE_GETREGSET more reliable and
> this will be another step to drop TIF_{IA32,X32} flags.
> 
> [1]: https://sourceforge.net/p/strace/mailman/message/30471411/
> [2]: https://lkml.org/lkml/2012/1/18/320
> 
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>

Looks reasonable! Still if cs has been changed to non-compat
selector and we now return 64bit registers set, won't it
cause problems for old tools? I suspect it should not but
still.