Re: [PATCH] m68k/kernel - wire up syscall_trace_enter/leave for m68k

Michael Schmitz
2020-07-27T20:48:45Z

Hi Adrian,

On 27/07/20 10:03 PM, John Paul Adrian Glaubitz wrote:
> Hi Michael!
>
> On 7/27/20 6:19 AM, Michael Schmitz wrote:
>> m68k (other than Coldfire) uses syscall_trace for both trace entry
>> and trace exit. Seccomp support requires separate entry points for
>> trace entry and exit which are already provided for Coldfire.
>>
>> Replace syscall_trace by syscall_trace_enter and syscall_trace_leave
>> in preparation for seccomp support. Check return code of
>> syscall_trace_enter(), and skip syscall if nonzero. Return code
>> will be left at what had been set by by ptrace or seccomp.
> Correct me if I'm wrong, but shouldn't the skip happen when the return
> code is -1? At least that's what we're doing on SuperH and that seems
> to be what other architectures are doing as well.

What other non-zero return codes do you expect syscall_trace_enter() to
set, and what should the action in entry.S be?

(Note that according to my reading, your SH patch does not actually do
what your description says. If syscall_trace_enter() returns a positive
non-zero value, that value is _not_ used as changed syscall number. SH
uses r3 for the syscall number, not r0).

As far as I can see, any non-zero return code should abort the syscall,
so I just test for that (for simplicity). Our use of the
tracehook_report_syscall_entry() return code (directly passed back from
syscall_trace_enter()) doesn't leave much choice there, see comment in
include/linux/tracehook.h.

If later on seccomp needs any specific action, it should be easy enough
to change the syscall number (offset PT_OFF_ORIG_D0 on the stack) or
syscall return code (offset PT_OFF_D0). There's code in kernel/ptrace.c
to do that AFAICS.

Changing the behaviour of syscall_trace_enter() to match what other
architectures do (return changed syscall number, not error code) is
beyond the scope of this patch. I suspect the capability to change
syscall numbers from ptrace code does predate the seccomp filter
approach, and as m68k has never used it in the past, I don't see a point
to add this now.

> Also, shouldn't that part of the change not be part of the patch that
> adds support for SECCOMP filter like in [1]? I don't think it makes
> sense to add the return code check unless the rest of SECCOMP filter
> has been implemented.

After replacing syscall_trace() by syscall_trace_enter() and
syscall_trace_leave(), there is a return code provided by
syscall_trace_enter() which we must check, hence I added the check at
the same time as replacing syscall_trace() for non-Coldfire m68k.

(I note that the same check should probably be added to coldfire/entry.S.)

I can't test any of the later seccomp related stuff, so I'd rather leave
that bit to someone else who can.

Cheers,

Michael

>
> Adrian
>
>> [1] https://github.com/glaubitz/linux/commit/2fa1e7b9ba5150bc12adaddc017d5a6b79f363e7