Re: libSegFault.so and gcj

[Andrew Haley <aph at redhat dot com>]

Andrew Haley wrote:
> 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.

Although you'll have to make very sure that the catch_segv handler is built
*without* the -fstack-check option !

One other thing that you can use to detect stack overflow: compile with
`-finstrument-functions'.  This might be the easiest way to do it.

Andrew.



Andrew.