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

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Date:	Mon, 25 Apr 2016 23:37:03 +0300
From:	Dmitry Safonov <0x7f454c46@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dmitry Safonov <dsafonov@...tuozzo.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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)

2016-04-25 22:33 GMT+03:00 Andy Lutomirski <luto@...capital.net>:
> On Mon, Apr 25, 2016 at 11:09 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
>> 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 we should report 64-bit regs if the app is running in 64-bit
> mode.  Then (not necessarily as part of your series), we should have a
> way for ptrace users to query the *syscall* arch.
>
> IOW, this is totally screwed up right now.  I think the goal of your
> patch should be to stop using TIF_IA32 without breaking it any worse
> than it already is.

Ok, so that's what that patch does.
Big thanks, Andy -- I appreciate your help so much with this patches.


-- 
Regards,
Safonov Dmitry.