Re: Seccomp support for linux-m68k

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Andreas,

On 27/07/20 9:08 AM, Andreas Schwab wrote:
> On Jul 27 2020, Michael Schmitz wrote:
>
> > Hi Andreas,
> >
> > On 26/07/20 11:03 PM, Andreas Schwab wrote:
> > > On Jul 26 2020, Michael Schmitz wrote:
> > >
> > > > No particular reason - I had seen testb used in the syscall entry code and
> > > Where?  That may be bugs.
> > Here:
> >
> > ENTRY(ret_from_signal)
> >          movel   %curptr@(TASK_STACK),%a1
> >          tstb    %a1@(TINFO_FLAGS+2)
> >          jge     1f
> >          jbsr    syscall_trace_leave
> > 1:      RESTORE_SWITCH_STACK
> >          addql   #4,%sp
> > ....
> >
> > and here:
> >
> > ENTRY(system_call)
> >          SAVE_ALL_SYS
> >
> >          GET_CURRENT(%d1)
> >          movel   %d1,%a1
> >
> >          | save top of frame
> >          movel   %sp,%curptr@(TASK_THREAD+THREAD_ESP0)
> >
> >          | syscall trace?
> >          tstb    %a1@(TINFO_FLAGS+2)
> >          jmi     do_trace_entry
> >          cmpl    #NR_syscalls,%d0
> >          jcc     badsys
> How is that relevant?  That is testing a single bit, of course.

No, tstb tests a byte operand. btst tests a bit. And testing the second 
byte of the thread info flags is just what we need if the trace bits are 
14 and 15.

(Had to look up the m68k programmer's reference and check to be sure, 
admittedly.)

Cheers,

    Michael

> Andreas.