Re: [RFC 3/3] x86/ptrace: down with test_thread_flag(TIF_IA32)

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Apr 25, 2016 at 9:12 AM, Dmitry Safonov <dsafonov@virtuozzo.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.

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

--Andy

>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: x86@kernel.org
> Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.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
>



-- 
Andy Lutomirski
AMA Capital Management, LLC