Re: [RFC PATCH] x86 single-step (TF) vs system calls & traps

[Daniel Jacobowitz <dan@debian.org>]

On Thu, Jul 01, 2004 at 12:47:59AM -0700, Roland McGrath wrote:
> > Roland, I don't think (pretty sure actually ;) we can handle the case 
> > where TF is set from userspace and, at the same time, the user uses 
> > PTRACE_SINGLESTEP. 
> 
> I don't know where you pulled the notion of that case from.  I certainly
> never mentioned it.  When I raised the case of user-mode setting of TF, I
> was quite clear that it's a case when ptrace is not involved.
> 
> > The PTRACE_SINGLESTEP gives you the SYSGOOD behaviour, if you set it. And 
> > sends a SIGTRAP notification to the ptrace'ing parent process.
> 
> Like I said before, that is a change in the behavior.  Since its inception,
> SYSGOOD has meant exactly and only that when you use PTRACE_SYSCALL you
> will get a different notification for a syscall-tracing stop than other
> sources of SIGTRAP that may arise during execution of user code between
> system calls.  At no time ever before, has it been possible to get the
> SIGTRAP|0x80 wait result when you had not just called PTRACE_SYSCALL.
> After your change, calling PTRACE_SINGLESTEP can now produce that result.
> I don't think that change is a good thing.  
> 
> As the originator of the SYSGOOD option, Dan might have a comment about this.

I am not the originator of PTRACE_O_TRACESYSGOOD, I just had the bad
luck to touch it.

I think reporting the system call using 0x80|SIGTRAP when you
PTRACE_SINGLESTEP over the trap instruction makes excellent good sense.

-- 
Daniel Jacobowitz