RFR(M): 8141529: Fix handling of _JAVA_SR_SIGNUM

[Lindenmaier, Goetz]

Hi David, 

thanks for looking at this!

> -----Original Message-----
> From: David Holmes [mailto:david.holmes at oracle.com]
> Sent: Donnerstag, 12. November 2015 08:35
> To: Lindenmaier, Goetz; hotspot-runtime-dev at openjdk.java.net;
> serviceability-dev
> Subject: Re: RFR(M): 8141529: Fix handling of _JAVA_SR_SIGNUM
> 
> 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 :) ).
The mechanism is not implemented there.  Why, I don't know.

> On Linux you didn't add the bounds check to os::Linux::set_our_sigflags.
It came with 8140482 http://hg.openjdk.java.net/jdk9/hs-rt/hotspot/rev/cd86b5699825

> 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?
I checked a row of linux distributions and Versions and always found NSIG=65.
Also, as we compile NSIG into the code, we can not react on a difference
between systems. So I guess that's fine. 
As NSIG = _NSIG, I don't really care which we use.  I chose _NSIG because
it was used before.  On the other platforms I only found NSIG in the header
files.

Best regards,
  Goetz


> 
> Thanks,
> David
> 
> > Best regards,
> >    Goetz.
> >