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

[Dmitry Safonov]

On 04/25/2016 09:09 PM, Dmitry Safonov 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@xxxxxxxxxxxxx> 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

And that seems to be right as __NR_exit for amd64
is 60, which is the same as __NR_umask for i386.

> > 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?


--
Regards,
Dmitry Safonov