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

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Date:	Mon, 25 Apr 2016 20:14:30 +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 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 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