linux-kernel - [RFC 3/3] x86/ptrace: down with test_thread_flag(TIF

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Date:	Mon, 25 Apr 2016 19:12:43 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	<linux-kernel@...r.kernel.org>
CC:	<0x7f454c46@...il.com>, Dmitry Safonov <dsafonov@...tuozzo.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	<x86@...nel.org>
Subject: [RFC 3/3] 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.
So, for i386 elf binary that changed it's CS to __USER_CS
it will return full x86_64 register set.

That will change ABI: i.e, strace uses returned register size
to determine, in which mode the application is.
With the current ABI that way is buggy:

int main(int argc, char **argv, char **envp)
{
	printf("Here we exit\n");
	fflush(stdout);
	asm volatile ("int $0x80" : : "a" (__NR_exit), "D" (1));
	printf("After exit\n");

	return 0;
}

This program will confuse strace:

[tst]$ strace ./confuse 2>&1 | tail
brk(0x1ca1000)                          = 0x1ca1000
write(1, "Here we exit\n", 13Here we exit
)          = 13
exit(1)                                 = ?
<... exit resumed> strace: _exit returned!
)                    = ?
write(1, "After exit\n", 11After exit
)            = 11
exit_group(0)                           = ?
+++ exited with 0 +++

So this ABI change should make PTRACE_GETREGSET more reliable and
this will be another step to drop TIF_{IA32,X32} flags.

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>
Cc: x86@...nel.org
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 0f4d2a5df2dc..d7d72f2f8b46 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1387,7 +1387,7 @@ 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;
-- 
2.8.0