Re: libSegFault.so and gcj

[Andrew Haley <aph at redhat dot com>]

Ben Gardiner wrote:

> We are running a gcj-compiled application on an embedded platform
> (MPC852T). For reference our versions are gcc-4.0.1, glibc-2.3.3 and
> linux-2.4.24 -- I know these versions are ancient, but please don't stop
> reading here.
> 
> We sometimes encounter segfaults in our application; that is to say that
> it will terminate with 'Segmentation fault' on the console and return
> 139. These occur rather infrequently, and we have yet to find a reliable
> way to reproduce them. To make things more difficult, we do not have
> room for core dumps on our filesystem.
> 
> I thought that we could get the some information about these segfaults
> by using the preload library libSegFault.so; I tested it and integrated
> it with our init scripts and let it loose into our releases hoping that
> a backtrace or two would come back to me. None did; there was no output
> produced by libSegFault.so at all.
> 
> I think that since gcj registers its own segfault handler which
> translates segv signals into NullPointerExceptions, the original signals
> never make it to libSegfault's handler. Gcj registers its handler,
> catch_segv (from prims.cc:146 in our version of gcj), in INIT_SEGV
> (powerpc-signal.h:62) called from _Jv_CreateJavaVM (prims.cc:1211). Here
> is a snippet of INIT_SEGV:
> 
> #define INIT_SEGV                            \
> do                                    \
>  {                                    \
>    struct kernel_old_sigaction kact;                    \
>    kact.k_sa_handler = catch_segv;                    \
>    kact.k_sa_mask = 0;                            \
>    kact.k_sa_flags = 0;                        \
>    if (syscall (SYS_sigaction, SIGSEGV, &kact, NULL) != 0)        \
>      __asm__ __volatile__ (".long 0");                    \
>  }                                    \
> while (0)
> 
> and of catch_segv:
> 
> SIGNAL_HANDLER (catch_segv)
> {
>  java::lang::NullPointerException *nullp
>    = new java::lang::NullPointerException;
>  unblock_signal (SIGSEGV);
>  MAKE_THROW_FRAME (nullp);
>  throw nullp;
> }
> 
> I don't know a whole lot about signal handlers -- please correct me if
> I'm wrong: I think that since the syscall (SYS_sigaction,...) passes
> NULL as the fourth argument, that gcj is disregarding the presence of
> any previously registered signal handlers.

Correct.  gcj treats all segfaults as null pointer exceptions.

> I also think that since the
> flags are zero that catch_segv is executed on the same stack as the
> process that threw the signal instead of the alternate stack.

Also correct.

> I reason from this that the segfaults are likely stack overflows. Could
> anyone confirm this?

That's quite possible.  Do you not have a debugger?

Clearly if it really is a stack overflow then you're not going to be
able to call the null pointer handler.  There is a way around this,
though.  If you use the -fstack-check option gcc generates a probe
at the start of every method that writes a zero some 12kbytes below
the stack pointer.  This will give you enough stack space for the
catch_segv handler to run.

> Could we patch INIT_SEGV somehow so that signals not caught by
> catch_segv will be passed up so that libSegFault.so can catch them?

No.  They're all caught.

Andrew.