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

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Hello,

+Arnout in case he has some background knowledge on libunwind.

On Thu, 16 May 2019 21:43:20 +0200
Fabrice Fontaine <fontaine.fabrice@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
> 
> 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)
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,

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
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".

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com