[Buildroot] [PATCH 2/2] package/mono: add libunwind optional dependency

Fri Aug 16 07:22:03 UTC 2019

Hi All,

On Sat, May 25, 2019 at 7:03 PM Arnout Vandecappelle <arnout at mind.be> wrote:
>
>
>
> On 18/05/2019 22:40, Thomas Petazzoni wrote:
> > Hello,
> >
> > +Arnout in case he has some background knowledge on libunwind.
> >
> > On Thu, 16 May 2019 21:43:20 +0200
> > Fabrice Fontaine <fontaine.fabrice at gmail.com> wrote:
> >
> >>> Did you make some further research to understand what is
> >>> _Unwind_GetIP ? Or was your analysis just based on "mono needs
> >>> _Unwind_GetIP, it's provided by libunwind, let's link with it" ?
> >> My analysis was based on the fact that the build failures occured only
> >> when libunwind was built before mono.
> >> I traced back this error to the installation of unwind.h by libunwind
> >> which contains the following definition of _Unwind_GetIP:
> >> extern unsigned long _Unwind_GetIP (struct _Unwind_Context *);
> >>
> >> However, libunwind implements _Unwind_GetIP in src/unwind/GetIP.c only
> >> if --enable-cxx-exceptions is defined.
> >>
> >> On his side, mono includes unwind.h if it's available:
> >> #ifdef HAVE_UNWIND_H
> >> #include <unwind.h>
> >> #endif
>
>  But mini-exceptions.c calls _Unwind_GetIP independently of that define. It's
> called depending on  MONO_ARCH_HAVE_UNWIND_BACKTRACE, which is set for linux on
> arm and amd64 (but strangely enough not on arm64).
>
>  Also, AFAICS gcc will always install unwind.h. So HAVE_UNWIND_H should be set
> even if libunwind is not built. The real problem is that libunwind and libgcc
> have different definitions of _Unwind_GetIP() on arm: libgcc defines it as a
> macro instead of a function.
>
>  So I would say that libunwind is the real problem: it replaces gcc's unwind.h
> with its own version, which is not compatible. (It doesn't really replace it; it
> just puts another unwind.h earlier in the include path.) Thus, it forces any
> package that expects to link with libgcc's unwind to instead link with libunwind.

This patch looks right to me. If BR2_PACKAGE_LIBUNWIND is somewhat
selected, it should be built before mono else mono will fail to
compile.

@Fabrice Fontaine : have you tried to upstream this patch?

>
> >> unwind.h and _Unwind_GetIP can also be provided by glibc but that's
> >> not the cause of this build failure on musl.
> >
> > Hm, did you notice that mono has its own libunwind library, which
> > provides _Unwind_GetIP ?
> >
> > See:
> >
> > external/corert/src/Native/libunwind/src/UnwindLevel1-gcc-ext.c:_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIPInfo(struct _Unwind_Context *context,
> > external/corert/src/Native/libunwind/src/UnwindLevel1-gcc-ext.c:  _LIBUNWIND_TRACE_API("_Unwind_GetIPInfo(context=%p)", (void *)context);
> > external/corert/src/Native/libunwind/src/UnwindLevel1-gcc-ext.c:  return _Unwind_GetIP(context);
> > external/corert/src/Native/libunwind/src/Unwind-EHABI.cpp:    uintptr_t pc = _Unwind_GetIP(context);
> > external/corert/src/Native/libunwind/src/Unwind-sjlj.c:_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
> > external/corert/src/Native/libunwind/src/Unwind-sjlj.c:  _LIBUNWIND_TRACE_API("_Unwind_GetIP(context=%p) => 0x%lX", context,
> > external/corert/src/Native/libunwind/src/Unwind-sjlj.c:_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIPInfo(struct _Unwind_Context *context,
> > external/corert/src/Native/libunwind/src/Unwind-sjlj.c:  _LIBUNWIND_TRACE_API("_Unwind_GetIPInfo(context=%p, %p) => 0x%lX",
> > external/corert/src/Native/libunwind/src/UnwindCursor.hpp:  // This matches the behaviour of _Unwind_GetIP on arm.
> > external/corert/src/Native/libunwind/src/UnwindLevel1.c:_LIBUNWIND_EXPORT uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
> > external/corert/src/Native/libunwind/src/UnwindLevel1.c:  _LIBUNWIND_TRACE_API("_Unwind_GetIP(context=%p) => 0x%" PRIx64,
> > external/corert/src/Native/libunwind/src/Unwind_AppleExtras.cpp:NOT_HERE_BEFORE_10_6(_Unwind_GetIP)
> > external/corert/src/Native/libunwind/src/Unwind_AppleExtras.cpp:NOT_HERE_BEFORE_10_6(_Unwind_GetIPInfo)
> > external/corert/src/Native/libunwind/src/Unwind_AppleExtras.cpp:NOT_HERE_BEFORE_5_0(_Unwind_GetIP)
> > external/corert/src/Native/libunwind/src/Unwind_AppleExtras.cpp:NOT_HERE_BEFORE_5_0(_Unwind_GetIPInfo)
>
>  This is a (partial?) copy of LLVM, but it is not actually built. The only files
> that are built are Unwind-EHABI.cpp and libunwind.cpp. And AppleExtras on Darwin.
>
> > external/corert/src/Native/libunwind/include/unwind.h:extern uintptr_t _Unwind_GetIP(struct _Unwind_Context *context);
> > external/corert/src/Native/libunwind/include/unwind.h:uintptr_t _Unwind_GetIP(struct _Unwind_Context *context) {
> > external/corert/src/Native/libunwind/include/unwind.h:// _Unwind_GetIPInfo is a gcc extension that can be called from within a
> > external/corert/src/Native/libunwind/include/unwind.h:// personality handler.  Similar to _Unwind_GetIP() but also returns in
> > external/corert/src/Native/libunwind/include/unwind.h:extern uintptr_t _Unwind_GetIPInfo(struct _Unwind_Context *context,
>
>  The headers, on the other hand, do get added to the include path.
>
>  Always a good idea to include a header file but not build all of the
> corresponding sources...
>
> >
> > Also
> > http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic.html#LIBGCC-SMAN
> > says _Unwind_GetIP() is supposed to be provided by libgcc_s.
> >
> > On the other hand, glibc and uclibc provide a _Unwind_GetIP macro, but
>
>  It's a mystery why they do if gcc already provides the same...
>
> > only for ARM. uClibc libubacktrace also contains this:
> >
> > static void backtrace_init (void)
> > {
> >         void *handle = dlopen (LIBGCC_S_SO, RTLD_LAZY);
> >
> >         if (handle == NULL
> >                 || ((unwind_backtrace = dlsym (handle, "_Unwind_Backtrace")) == NULL)
> >                 || ((unwind_getip = dlsym (handle, "_Unwind_GetIP")) == NULL)) {
> >                 printf(LIBGCC_S_SO " must be installed for backtrace to work\n");
> >                 abort();
> >         }
> > }
> >
> > So it looks for _Unwind_GetIP in libgcc.
> >
> > So, I'm really confused by what is supposed to provide _Unwind_GetIP(),
> > and I don't feel comfortable with just saying "libunwind has it, let's
> > use it".
>
>  To me it feels similar to -latomic (which I also don't completely understand):
> stuff that gets included through some headers that may come from various
> sources, and you may or may not need to link with some library depending on what
> happens the be the actual source.
>
>  Regards,
>  Arnout
>
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