linux-kernel - [PATCHv4 5/6] x86/ptrace: down with test_thread_flag(TIF

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Date:   Wed, 31 Aug 2016 16:59:35 +0300
From:   Dmitry Safonov <dsafonov@...tuozzo.com>
To:     <linux-kernel@...r.kernel.org>
CC:     <0x7f454c46@...il.com>, <luto@...nel.org>, <oleg@...hat.com>,
        <tglx@...utronix.de>, <hpa@...or.com>, <mingo@...hat.com>,
        <linux-mm@...ck.org>, <x86@...nel.org>, <gorcunov@...nvz.org>,
        <xemul@...tuozzo.com>, Dmitry Safonov <dsafonov@...tuozzo.com>,
        Pedro Alves <palves@...hat.com>
Subject: [PATCHv4 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32)

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: Oleg Nesterov <oleg@...hat.com>
Cc: Pedro Alves <palves@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: linux-mm@...ck.org
Cc: x86@...nel.org
Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
Cc: Pavel Emelyanov <xemul@...tuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
---
 arch/x86/kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index f79576a541ff..ad0bab8fc594 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1358,7 +1358,7 @@ void update_regset_xstate_info(unsigned int size, u64 xstate_mask)
 const struct user_regset_view *task_user_regset_view(struct task_struct *task)
 {
 #ifdef CONFIG_IA32_EMULATION
-	if (test_tsk_thread_flag(task, TIF_IA32))
+	if (!user_64bit_mode(task_pt_regs(task)))
 #endif
 #if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
 		return &user_x86_32_view;
-- 
2.9.0