Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack

[Michael Schmitz]

Hi Al,

Am 21.06.2021 um 15:44 schrieb Al Viro:
> On Mon, Jun 21, 2021 at 03:18:35PM +1200, Michael Schmitz wrote:
>
> > This is what I get from WARN_ONCE:
> >
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 1177 at arch/m68k/kernel/ptrace.c:91 get_reg+0x90/0xb8
> > Modules linked in:
> > CPU: 0 PID: 1177 Comm: strace Not tainted 5.13.0-rc1-atari-fpuemu-exitfix+
> > #1146
> > Stack from 014b7f04:
> >         014b7f04 00336401 00336401 000278f0 0032c015 0000005b 00000005
> > 0002795a
> >         0032c015 0000005b 0000338c 00000009 00000000 00000000 ffffffe4
> > 00000005
> >         00000003 00000014 00000003 00000014 efc2b90c 0000338c 0032c015
> > 0000005b
> >         00000009 00000000 efc2b908 00912540 efc2b908 000034cc 00912540
> > 00000005
> >         00000000 efc2b908 00000003 00912540 8000110c c010b0a4 efc2b90c
> > 0002d1d8
> >         00912540 00000003 00000014 efc2b908 0000049a 00000014 efc2b908
> > 800acaa8
> > Call Trace: [<000278f0>] __warn+0x9e/0xb4
> >  [<0002795a>] warn_slowpath_fmt+0x54/0x62
> >  [<0000338c>] get_reg+0x90/0xb8
> >  [<0000338c>] get_reg+0x90/0xb8
> >  [<000034cc>] arch_ptrace+0x7e/0x250
> >  [<0002d1d8>] sys_ptrace+0x232/0x2f8
> >  [<00002ab6>] syscall+0x8/0xc
> >  [<0000c00b>] lower+0x7/0x20
> >
> > ---[ end trace ee4be53b94695793 ]---
> >
> > Syscall numbers are actually 90 and 192 - sys_old_mmap and sys_mmap2 on
> > m68k. Used the calculator on my Ubuntu desktop, that appears to be a little
> > confused about hex to decimal conversions.
> >
> > I hope that makes more sense?
>
> Not really; what is the condition you are checking?  The interesting trace

The check in get_reg() is:


           if (WARN_ON_ONCE((off < PT_REG(d1)) &&
              test_ti_thread_status(task_thread_info(task),TIS_TRACING)
                   && !test_ti_thread_status(task_thread_info(task),
                                        TIS_ALLREGS_SAVED))) {
                   unsigned long *addr_d0;
                   addr_d0 = (unsigned long *)(task->thread.esp0 + 
regoff[16]);
                   pr_err("get_reg with incomplete stack, regno %d offs 
%d orig_d0 %lx\n", regno, off, *addr_d0);
                   return 0;
           }


> is not that with get_reg() - it's that of the process being traced.  You
> are not accessing the stack of caller of ptrace(2) here, so you want to
> know that SAVE_SWITCH_STACK had been done by the tracee, not tracer.
>
> And if that had been strace ls, you have TIF_SYSCALL_TRACE set for ls, so
> 	* ls hits system_call
> 	* notices TIF_SYSCALL_TRACE and goes to do_trace_entry
> 	* does SAVE_SWITCH_STACK there

... and sets both the new TIS_TRACING and TIS_ALLREGS_SAVED flags in the 
thread_info->status field (now that I've corrected my patch).

> 	* calls syscall_trace(), which calls ptrace_notify()
> 	* ptrace_notify() calls ptrace_do_notify(), which calls ptrace_stop()
> 	* ptrace_stop() arranges for tracer to be woken up and gives CPU up,
> with TASK_TRACED as process state.

Thanks for explaining! So in order to get a trace for the process being 
traced, I would have to check the TIS_ALLREGS_SAVED in ptrace_stop()?

> That's the callchain in ls, and switch_stack accessed by get_reg() from
> strace is the one on ls(1) stack created by SAVE_SWITCH_STACK.

So testing for TIS_ALLREGS_SAVED in get_reg() (called by the tracer, but 
with the tracee's task struct passed to arch_ptrace()) does check that 
SAVE_SWITCH_STACK was done before the syscall in the tracee, right?

Anyway, I'd missed setting the flags for some crucial SAVE_SWITCH_STACK 
operations in my woefully incomplete patch. With that corrected, there's 
no more warning from mmap. I'll try with a more recent version of strace 
and gdb once I've updated my test image.

Cheers,

	Michael