Re: [Libunwind-devel] libunwind-ptrace and UNW_REMOTE_ONLY

[Ken Werner]

On 11/09/2011 08:15 PM, Arun Sharma wrote:
> On Tue, Nov 8, 2011 at 9:46 AM, Ken Werner<address@hidden>  wrote:
> > Hi,
> >
> > I noticed that for unwinding via DWARF debug frames libunwind-ptrace relies on 
> > the local_addr_space [1] which is not initialized in case of UNW_REMOTE_ONLY 
> > [2]. This leads to a segfault in case CONFIG_DEBUG_FRAME is defined. So, I'm 
> > wondering if there is a nice way to fix this. As a workaround I defined the 
> > local_addr_space even if UNW_REMOTE_ONLY is defined and set the find_proc_info, 
> > resume, and put_unwind_info accessors to NULL. Any suggestions are appreciated.
>
> libunwind-ptrace should not be compiled for REMOTE_ONLY case.
>
> I recall you had a question about using UNW_REMOTE_ONLY to deal with
> android. Reading through Makefile.am and configure.in, the correct way
> to configure for the remote only case is to use "host != target".
>
> If you're trying to work around compilation failures due to the lack
> of dl_iterate_phdr on the target, we should find a different
> mechanism.

Since libunwind-ptrace sits on top of libunwind remote interface I 
thought it's just natural to have it available in case of 
UNW_REMOTE_ONLY. I agree that the build system currently doesn't include 
libunwind-ptrace. So the question is, do we want a vehicle that allows 
users to only build the remote unwinding functionality including 
libunwind-ptrace? Probably the least intrusive way to achieve this would 
be to add a configure switch that enables UNW_REMOTE_ONLY plus another 
define that covers libunwind-ptrace [*].

In case we want libunwind-ptrace to support unwinding via DWARF debug 
framed the issue with the address space remains. When libunwind-ptrace 
calls the dwarf code the address space is not specified but assumed to 
be the local_addr_space. This is perfectly valid for the memory 
accessors because the unwinding process mmaps the elf image to extract 
the unwind info - thus local.

[*] add something like this in configure.in (I didn't come up with a 
better name yet):
  AC_MSG_CHECKING([whether to build libunwind-ptrace only])
  AC_ARG_ENABLE([ptrace_only],
  [  --enable-ptrace-only    Build libunwind-ptrace only],
  [case "${enableval}" in
    yes) ptrace_only=yes ;;
    no)  ptrace_only=no ;;
    *) AC_MSG_ERROR([bad value ${enableval} for --enable-ptrace-only]) ;;
  esac],[ptrace_only=no])
  AM_CONDITIONAL([PTRACE_ONLY], [test x${ptrace_only} = xyes])
  if test x$ptrace_only = xyes; then
    CPPFLAGS="${CPPFLAGS} -DUNW_PTRACE_ONLY -DUNW_REMOTE_ONLY"
  fi
  AC_MSG_RESULT([${ptrace_only}])

Regards
Ken