How to debug Cygwin failure? - c++

Question

I am porting from Centos to Cygwin and find that my application is exiting with no error message and exit status zero mid execution during the constructor for Botan::InitializationVector.
If I try to attach with gdb proactively in main() where it is waiting on a spin variable, I don't get a normal stack trace:
(gdb) where
#0 0x7c90120f in ntdll!DbgUiConnectToDbg ()
from /cygdrive/c/WINDOWS/system32/ntdll.dll
#1 0x7c952119 in ntdll!KiIntSystemCall ()
from /cygdrive/c/WINDOWS/system32/ntdll.dll
#2 0x00000005 in ?? ()
#3 0x00000000 in ?? ()
So with no gdb, it is hard to figure out what is going wrong.
Why would I get no error message on Cygwin yet the application would exit mid execution?
I deduce it is inside the constructor due to clog only showing for line before and not after constructor:
clog << " About to create iv for Botan.\n";
Botan::InitializationVector iv(_rng, size);
clog << " About to copy iv for Botan.\n";
Botan is open source: http://botan.randombit.net/ Here are some code snippets from src/sym_algo/symkey.{h,cpp}:
typedef OctetString InitializationVector;
class BOTAN_DLL OctetString
{
public:
u32bit length() const { return bits.size(); }
SecureVector<byte> bits_of() const { return bits; }
const byte* begin() const { return bits.begin(); }
const byte* end() const { return bits.end(); }
std::string as_string() const;
OctetString& operator^=(const OctetString&);
void set_odd_parity();
void change(const std::string&);
void change(const byte[], u32bit);
void change(const MemoryRegion<byte>& in) { bits = in; }
OctetString(class RandomNumberGenerator&, u32bit len);
OctetString(const std::string& str = "") { change(str); }
OctetString(const byte in[], u32bit len) { change(in, len); }
OctetString(const MemoryRegion<byte>& in) { change(in); }
private:
SecureVector<byte> bits;
};
OctetString::OctetString(RandomNumberGenerator& rng,
u32bit length)
{
bits.create(length);
rng.randomize(bits, length);
}
I moved the failing code into main() and it works fine. I also put a try catch ... around the code and no exceptions are being thrown. Something goes wrong between main() and the point of failure later in the application. I can do a divide and conquer to narrow down the exact point where it no longer works. One of the Botan developers gave me this stripped down code to use instead that also fails:
Botan::AutoSeeded_RNG _rng;
unsigned int size = 1; // or 16, or 1452 all fail.
Botan::SecureVector<Botan::byte> iv_val(size);
cerr << "We get to here." << endl;
_rng.randomize(&iv_val[0], size);
cerr << "But not here." << endl;
Now that I have the debugger working I see segv:
(gdb) s
Botan::AutoSeeded_RNG::randomize (this=0x1270380, out=0x5841420 "", len=1)
at ../../src/Botan-1.8.11/build/include/botan/auto_rng.h:23
(gdb) s
Program received signal SIGSEGV, Segmentation fault.
0x005d79ee in Botan::AutoSeeded_RNG::randomize (this=0x1270380,
out=0x5841420 "", len=1)
at ../../src/Botan-1.8.11/build/include/botan/auto_rng.h:23
(gdb) p rng
$7 = (class Botan::RandomNumberGenerator *) 0x5841324
(gdb) p *this
$8 = {<Botan::RandomNumberGenerator> = {
_vptr$RandomNumberGenerator = 0x11efc14}, rng = 0x5841324}
(gdb) p *rng
$9 = {_vptr$RandomNumberGenerator = 0x656e6f4e}
Here is auto_rng.h code:
class BOTAN_DLL AutoSeeded_RNG : public RandomNumberGenerator
{
public:
void randomize(byte out[], u32bit len)
{ rng->randomize(out, len); } // SEGV on this line.
bool is_seeded() const
{ return rng->is_seeded(); }
void clear() throw() { rng->clear(); }
std::string name() const
{ return "AutoSeeded(" + rng->name() + ")"; }
void reseed(u32bit poll_bits = 256) { rng->reseed(poll_bits); }
void add_entropy_source(EntropySource* es)
{ rng->add_entropy_source(es); }
void add_entropy(const byte in[], u32bit len)
{ rng->add_entropy(in, len); }
AutoSeeded_RNG(u32bit poll_bits = 256);
~AutoSeeded_RNG() { delete rng; }
private:
RandomNumberGenerator* rng;
};

Answer 1

Cygwin apps are multithreaded (e.g., one thread is the signal listener thread). Use info threads in gdb to find the thread that really faulted.

Answer 2

You could attach gdb proactively and put a breakpoint right before the constructor call that is failing, then single step through.

Answer 3

Based on the new code, you're violating the Rule of Three and this may be causing your problem.
By defining a class with raw pointers and not providing a correct copy constructor (or making it inaccessible) you open yourself up to double-free.
Add the copy constructor. Or open an issue on the project's bug tracker. You are using the latest version, right?