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Re: crash on latest cygwin snapshot


On 7/2/2012 6:01 PM, Christopher Faylor wrote:
[redirecting to cygwin-developers]
On Mon, Jul 02, 2012 at 04:43:06PM +0200, marco atzeri wrote:
On 6/27/2012 3:46 AM, Christopher Faylor wrote:

Sorry, Marco. Nevermind. I duplicated this. No need to upload anything. I'm still working on it.

it seems solved on 20120702 snapshots

Thanks for confirming. I suspect that this snapshot really only masks the problem. The stack was getting corrupted by something but I was having a really hard time figuring out what was doing it.

Compiling path.cc without optimization or passwd.cc without
-fomit-frame-pointer "fixed" the problem but clearly something is wrong.

I tried building Cygwin with stack probes and that made the DLL
unrunnable.  I tried instrumenting it with -finstrument-functions and
that made the problem go away.

The problem is that something, somewhere along the line, replaces a
frame pointer in the stack with 0x10c (268) and the return address with
zero.  So, eventually, when a function returns, %ebp is set to 0x10c and
the program jumps to address zero.  That is one manifestation.  In others
the 0x10c is still there but it is interpreted as a pointer to something
and dereferencing it causes a SEGV.

0x10c is 256 + 12 but I haven't been able to find that anywhere in the
source.

Your test triggered a problem which has existed in Cygwin since last November.
I noticed it in snapshots going back to 2012-11-14.  But, when I tried to
build a version of Cygwin from before that time, it still manifested the
problem.  I did change to gcc 4.5.3 around that time so I'm thinking that
either this version of gcc exposed a problem in Cygwin or Cygwin has exposed
a problem in this version gcc.  There was an odd problem in select() where
it seemed like constructors weren't being properly run on a local variable
when alloca was used in the same function.

cgf


gcc-4.5 has some issue on windows platform (cygwin and mingw) for wrong optimization of function return value on C++ code.

On octave I catched a fault that corrupted the stack
http://savannah.gnu.org/bugs/?34210

and the solution was to add a "volatile" declaration
---------------------------------------------------------------
profile_data_accumulator::query_time (void) const
{
  octave_time now;

  // FIXME -- is this volatile declaration really needed?
  // See bug #34210 for additional details.
  volatile double dnow = now.double_value ();

  return dnow;
}
----------------------------------------------------------------
On the other platform volatile was not needed.
The nightmare was that while stepping with gdb the fault did not arise

I will not be surprised if cygwin could be hit by similar issue.

Marco





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