libunwind problems

[Rick Moseley <rmoseley at redhat dot com>]

Hi all,

Andrew Haley tracked down our problem with linking against the libunwind 
library.  Below is his explanation of what he found and how it can be 
resolved.

Thanks to Andrew Haley for his valuable time.

Rick



The problem is that you have linked frysk in such a way that it uses
part of libunwindand part of libgcc for exception handling .  So,
symbol resolution in FryskGui goes like this:

     4411:     binding file /lib/libc.so.6 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_Find_FDE' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /usr/lib/libgcj.so.7 [0]: normal symbol `_ZN14_Jv_StackTrace13UnwindTraceFnEP15_Unwind_ContextPv'
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_Backtrace' [GCC_3.3]
     4411:     binding file /lib/libgcc_s.so.1 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_Find_FDE' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_GetRegionStart' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_GetIP' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_RaiseException' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_GetLanguageSpecificData' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_GetIPInfo' [GCC_4.2.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_SetGR' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to /lib/libgcc_s.so.1 [0]: normal symbol `_Unwind_SetIP' [GCC_3.0]
     4411:     binding file /usr/lib/libgcj.so.7 [0] to ./frysk-gui/frysk/gui/FryskGui [0]: normal symbol `_Unwind_Resume' [GCC_3.0]

Note that all of libgcj's exception handling is linked to
libgcc_s.so.1 except for `_Unwind_Resume', which is linked to the
libunwind version of that symbol that is built into FryskGui.  As far
as I can see this can never work, because the context structure used
in `_Unwind_RaiseException' and `_Unwind_Resume' must be the same, and
they must be satisfied from the same library.

FryskGui exports just _one_ libunwind function, and as soon as it gets
called the program crashes.


-bash-3.1$ nm ./frysk-gui/frysk/gui/FryskGui | grep _Unwind_
083af4e0 T __libunwind_Unwind_Resume
083af4e0 T _Unwind_Resume


So, the way to fix frysk is to make sure that libgcc_s.so is in the
link path before libunwind.  This causes the libunwind version of
_Unwind_Resume not to be put into FryskGui.

I linked FryskGui like this:

... mports/elfutils/libdw/:/home/aph/frysk-libunwind/build/frysk-gui/../frysk-imports/elfutils/libdwfl/:/home/aph/frysk-libunwind/build/frysk-gui/../frysk-imports/elfutils/libebl/:/home/aph/frysk-libunwind/build/frysk-gui/../frysk-imp\orts/elfutils/libelf/:/home/aph/frysk-libunwind/build/frysk-gui/../frysk-imports/libunwind/src/:/home/aph/frysk-libunwind/build/frysk-gui/../frysk-sys/:./elfutils/libdw \
-Dgnu.gcj.runtime.NameFinder.demangle=false \
-Dgnu.gcj.runtime.NameFinder.use_addr2line=false \
-lgcc_s \
frysk/gui/FryskGui.o \
libfrysk-gui.a \
../frysk-gtk/libfrysk-gtk.a ...

This problem is really due to mixing dynamic and static libraries.  I
imagine -- but I haven't tried -- that if you used a dynamically
linked version of libunwind it would be OK.

Andrew.


P.S.  gcj on trunk doesn't call `_Unwind_Resume' at all; this is a 4.1 issue.