RFR(M): 8141529: Fix handling of _JAVA_SR_SIGNUM

Fri Nov 13 09:53:44 UTC 2015

Hi Goetz,

sorry for not looking at this earlier. This is a nice cleanup. Some remarks:

http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.01/src/os/aix/vm/os_aix.cpp.udiff.html

+    if (sig > MAX2(SIGSEGV, SIGBUS) &&  // See 4355769.
+        sig < NSIG) {                   // Must be legal signal and fit
into sigflags[].

I do not like much the MAX2() construct. I would like it better to
explicitly check whether the SR signal is one of the "forbidden" ones the
VM uses.

Maybe keep a mask defined centrally for each platform which contains
signals the VM needs for itself ?

+sigset_t os::Aix::sigs = { 0 };

I would not initialize the signal set this way. sigset_t is an opaque type;
the only way to initialize it is with one of sigemptyset() or sigfillset().

http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.01/src/os/aix/vm/os_aix.hpp.udiff.html

+  static struct sigaction sigact[NSIG]; // saved preinstalled sigactions
+  static sigset_t sigs;                 // mask of signals that have

+  static int sigflags[NSIG];

I know this is not in the scope of your change, but I would like to see
those removed from os::Aix and put into os_aix.cpp at static filescope.
There is no need at all to export those, and you would get rid of the
signal.h dependency you know have when including os_aix.hpp.

http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.01/src/os/bsd/vm/jsig.c.udiff.html
http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.01/src/os/bsd/vm/os_bsd.cpp.udiff.html

On BSD, we have realtime signals.

http://fxr.watson.org/fxr/source/sys/signal.h
#define SIGRTMAX         126
and NSIG does not contain them:
#define NSIG             32

The max. possible signal number would be 126, which unfortunately does not
even fit into a 64bit mask.

So the code in jsig.c is broken for the case that someone wants to register
realtime signals, if the VM were to ever use realtime signals itself, which
now is not the case.

The same is true for os_bsd.cpp, where signal chaining will not work if the
application did have handler for real time signals pre-installed before jvm
is loaded.

Solaris:

The only platform where NSIG is missing?

Here, we calculate the max. signal number dynamically in os_solaris.cpp,
presumably because SIGRTMAX is not a constant and can be changed using
system configuration. But then, on Linux we have the same situation
(SIGRTMAX is dynamic) and there we do not go through the trouble of
calculating the max. signal number dynamically. Instead we just use NSIG=64
and rely on the fact that NSIG is larger than the largest possible dynamic
value for SIGRTMAX.

Solaris does not seem to have NSIG defined, but I am sure there is also a
max. possible value for SIGRTMAX (the default seems to be 48). So, one
could probably safely define NSIG for Solaris too, so that we have NSIG
defined on all Posix platforms.

On Thu, Nov 12, 2015 at 8:24 PM, Lindenmaier, Goetz <
goetz.lindenmaier at sap.com> wrote:

> Hi David, Dmitry,
>
> I've come up with a new webrev:
> http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.01/
>
> I hit on some more issues:
>  - As proposed, I replaced MAXSIGNUM by NSIG
>  - On AIX, NSIG=255.  Therefore storing bits in a word does not work.
>     I'm now using bitset functionality from signal.h as it's done in other
> places.
>    sigset_t is >> NSIG on linux, so it's no good idea to use it there.
>

Why do we not do this on all platforms, provided sigset_t contains all
signals (incl. realtime signals) ?

>  - In the os files I found another bit vector that now is too small: sigs.
>    I adapted that, too.  Removed the dead declaration of this on solaris.
>
> Best regards,
>   Goetz.
>
>

Kind Regards, Thomas

> > -----Original Message-----
> > From: Dmitry Samersoff [mailto:dmitry.samersoff at oracle.com]
> > Sent: Donnerstag, 12. November 2015 10:05
> > To: Lindenmaier, Goetz; David Holmes; hotspot-runtime-
> > dev at openjdk.java.net; serviceability-dev
> > Subject: Re: RFR(M): 8141529: Fix handling of _JAVA_SR_SIGNUM
> >
> > Goetz,
> >
> > *BSD including OS X also defines NSIG (just checked) and if my memory is
> > not bogus, AIX defines it too.
> >
> > So you may consider to use NSIG on all platform.
> >
> > -Dmitry
> >
> > On 2015-11-12 11:36, Lindenmaier, Goetz wrote:
> > > OK I'll change it to NSIG.  That's used in other places in os_linux,
> too.
> > > So it's really more consistent.
> > >
> > > Best regards,
> > >   Goetz
> > >
> > >> -----Original Message-----
> > >> From: Dmitry Samersoff [mailto:dmitry.samersoff at oracle.com]
> > >> Sent: Donnerstag, 12. November 2015 09:22
> > >> To: David Holmes; Lindenmaier, Goetz; hotspot-runtime-
> > >> dev at openjdk.java.net; serviceability-dev
> > >> Subject: Re: RFR(M): 8141529: Fix handling of _JAVA_SR_SIGNUM
> > >>
> > >> David,
> > >>
> > >> I think it's better to use NSIG (without underscore) defined in
> signal.h
> > >>
> > >> -Dmitry
> > >>
> > >>
> > >> On 2015-11-12 10:35, David Holmes wrote:
> > >>> Hi Goetz,
> > >>>
> > >>> Adding in serviceability-dev
> > >>>
> > >>> On 9/11/2015 6:22 PM, Lindenmaier, Goetz wrote:
> > >>>> Hi,
> > >>>>
> > >>>> The environment variable _JAVA_SR_SIGNUM can be set to a signal
> > >> number
> > >>>> do be used by the JVM's suspend/resume mechanism.
> > >>>>
> > >>>> If set, a signal handler is installed and the current signal handler
> > >>>> is saved to an array.
> > >>>> On linux, this array had size MAXSIGNUM=32, and _JAVA_SR_SIGNUM
> > >> was
> > >>>> allowed
> > >>>> to range up to _NSIG=65. This could cause memory corruption.
> > >>>>
> > >>>> Further, in jsig.c, an unsinged int is used to set a bit for
> signals.
> > >>>> This also
> > >>>> is too small, as only 32 signals can be supported.  Further, the
> > >>>> signals are mapped
> > >>>> wrong to these bits.  '0' is not a valid signal, but '32' was.
> 1<<32
> > >>>> happens to map to
> > >>>> zero, so the signal could be stored, but this probably was not
> > >>>> intended that way.
> > >>>>
> > >>>> This change increases MAXSIGNUM to 65 on linux, and to 64 on aix. It
> > >>>> introduces
> > >>>> proper checking of the signal read from the env var, and issues a
> > >>>> warning if it
> > >>>> does not use the signal set.  It adapts the data types in jisig.c
> > >>>> properly.
> > >>>>
> > >>>> Please review this change.  I please need a sponsor.
> > >>>> http://cr.openjdk.java.net/~goetz/webrevs/8141529-NSIG/webrev.00
> > >>>
> > >>> This all sounds very good to me. (I must find out why Solaris is not
> > >>> involved here :) ).
> > >>>
> > >>> On Linux you didn't add the bounds check to
> os::Linux::set_our_sigflags.
> > >>>
> > >>> I'm also wondering about documenting where we are determining the
> > >>> maximum from? Is it simply _NSIG on some/all distributions? And I see
> > >>> _NSIG is supposed to be the biggest signal number + one. Also linux
> > >>> defines NSIG = _NSIG so which should we be using?
> > >>>
> > >>> Thanks,
> > >>> David
> > >>>
> > >>>> Best regards,
> > >>>>    Goetz.
> > >>>>
> > >>
> > >>
> > >> --
> > >> Dmitry Samersoff
> > >> Oracle Java development team, Saint Petersburg, Russia
> > >> * I would love to change the world, but they won't give me the
> sources.
> >
> >
> > --
> > Dmitry Samersoff
> > Oracle Java development team, Saint Petersburg, Russia
> > * I would love to change the world, but they won't give me the sources.
>