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

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Date:	Mon, 25 Apr 2016 21:09:21 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [RFC 3/3] x86/ptrace: down with test_thread_flag(TIF_IA32)

On 04/25/2016 08:14 PM, Dmitry Safonov wrote:
> On 04/25/2016 07:53 PM, Andy Lutomirski wrote:
>> On Mon, Apr 25, 2016 at 9:12 AM, Dmitry Safonov 
>> <dsafonov@...tuozzo.com> wrote:
>>> 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:
>> Oleg, any comment here?
>>
>>> 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.
>> Does strace start working again with this change?  I suspect that
>> we'll eventually have to expose syscall_get_arch directly through
>> ptrace, but that's a project for another day.
>
> Oh, crap, not yet - seems like, I failed with my test.
> I'll resend this patch as will get it fixed, sorry.

I find out, what I have changed (and broke test):
 > if (!user_64bit_mode(task_pt_regs(task)))
was
 > if (task_thread_info(task)->status & TS_COMPAT)

That way the test runs now:
> brk(NULL)                               = 0x1145000
> brk(0x1167000)                          = 0x1167000
> write(1, "Here we exit\n", 13Here we exit
> )          = 13
> strace: [ Process PID=1608 runs in 32 bit mode. ]
> umask(0)                                = 022
> strace: [ Process PID=1608 runs in 64 bit mode. ]
> write(1, "After exit\n", 11After exit
> )            = 11
> exit_group(0)                           = ?
> +++ exited with 0 +++

But I changed on signal patch rebase and now I'm
thinking: should it be
 > if (task_thread_info(task)->status & TS_COMPAT || 
!user_64bit_mode(task_pt_regs(task)))
or what?
Should we count program that does compat syscall
as compatible even if it's in 64-bit mode?


>
>> I think this patch is fine, but I'm not a ptrace expert.
>>
>>> 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
>>>
>


-- 
Regards,
Dmitry Safonov