Re: [PATCH 4/8] user: Introduce host_interrupt_signal

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Tue, 2024-11-05 at 08:39 -0700, Warner Losh wrote:
> On Thu, Oct 24, 2024 at 2:00 PM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > Attaching to the gdbstub of a running process requires stopping its
> > threads. For threads that run on a CPU, cpu_exit() is enough, but
> > the
> > only way to grab attention of a thread that is stuck in a long-
> > running
> > syscall is to interrupt it with a signal.
> > 
> > Reserve a host realtime signal for this, just like it's already
> > done
> > for TARGET_SIGABRT on Linux. This may reduce the number of
> > available
> > guest realtime signals by one, but this is acceptable, since there
> > are
> > quite a lot of them, and it's unlikely that there are apps that
> > need
> > them all.
> > 
> > Set signal_pending for the safe_sycall machinery to prevent
> > invoking
> > the syscall. This is a lie, since we don't queue a guest signal,
> > but
> > process_pending_signals() can handle the absence of pending
> > signals.
> > The syscall returns with QEMU_ERESTARTSYS errno, which arranges for
> > the automatic restart. This is important, because it helps avoiding
> > disturbing poorly written guests.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> >  bsd-user/signal.c     | 12 ++++++++++++
> >  include/user/signal.h |  2 ++
> >  linux-user/signal.c   | 11 +++++++++++
> >  3 files changed, 25 insertions(+)
> > 
> > diff --git a/bsd-user/signal.c b/bsd-user/signal.c
> > index a2b11a97131..992736df5c5 100644
> > --- a/bsd-user/signal.c
> > +++ b/bsd-user/signal.c
> > @@ -49,6 +49,8 @@ static inline int sas_ss_flags(TaskState *ts,
> > unsigned long sp)
> >          on_sig_stack(ts, sp) ? SS_ONSTACK : 0;
> >  }
> > 
> > +int host_interrupt_signal = SIGRTMAX;
> > +
> > 
> 
> 
> I'd be tempted to use SIGRTMAX + 1 or even TARGET_NSIG. 127 or 128
> would
> work and not overflow any arrays (or hit any bounds tests) I'd likely
> use SIGRTMAX + 1,
> though, since it avoids any edge-cases from sig == NSIG that might be
> in the code
> unnoticed.
> 
> Now, having said that, I don't think that there's too many (any?)
> programs we need
> to run as bsd-user that have real-time signals, much less one that
> uses SIGRTMAX,
> but stranger things have happened. But it is a little wiggle room
> just in case.
> 
> Other than that:
> 
> Reviewed-by: Warner Losh <imp@bsdimp.com>

Thanks for the suggestion, I'll use SIGRTMAX + 1 in v2.