[Bug 9956] New: Backtrace doesn't work in libubacktrace.so on MIPS

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=9956

            Bug ID: 9956
           Summary: Backtrace doesn't work in libubacktrace.so on MIPS
           Product: uClibc
           Version: 0.9.33.2
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Other
          Assignee: unassigned at uclibc.org
          Reporter: djarvie at kde.org
                CC: uclibc-cvs at uclibc.org
  Target Milestone: ---

On a MIPS 32 (mipsel) processor, calling backtrace() in libubacktrace.so
doesn't produce any results - it returns a count of 0 stack frames. However, if
the code in backtrace.c is copied into an application and built as part of the
application, the backtrace() function works as expected.

I experimented quite a bit to try to get it to work, but have come to the
conclusion that _Unwind_Backtrace() in libgcc_s.so doesn't properly handle what
is pushed onto the stack when a call is made into the shared library
libubacktrace.so before calling _Unwind_Backtrace() (but it does handle the
call into libgcc_s.so correctly). So I think that there needs to be
MIPS-specific code to manipulate the stack data in backtrace() before it calls
_Unwind_Backtrace().

One issue seems to be that the pointer passed to backtrace() undergoes address
translation, so that the pointer is wrong when it is passed to
_Unwind_Backtrace(). (Called from backtrace() built into the application, there
is no address translation when _Unwind_Backtrace() is called.) However, fixing
that doesn't fix the problem.

Another thing I observed is that when backtrace() is called once, it is entered
4 times before returning, although there isn't any code to call it recursively.
Presumably that's due in some way to how the stack is handled.

A minimum workaround for the issue is to copy the backtrace() function into the
application. It can still use backtrace_helper() in the library, if 'static' is
removed from that function's declaration.

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