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[Libunwind-devel] Misaligned IP register on mipsel

From: Atish Nazir
Subject: [Libunwind-devel] Misaligned IP register on mipsel
Date: Tue, 25 Jun 2013 12:00:35 +0100

Hi guys,
I'm looking at using libunwind to work around the perennial lack of backtrace on uclibc MIPS.  To test it out on the embedded system in question I knocked together a simple application consisting of a couple of recursive functions before calling the show_backtrace example code featured on the website documentation.

Unfortunately the show_backtrace is unable to show anything useful, unw_step exits with -UNW_ENOINFO.  Cranking up the debug levels gives me the following (the full code attached):

Backtracing 00400b10
 >_ULmips_init_local: (cursor=0x7fe1a668)
 >_ULmips_step: (cursor=0x7fe1a668)
                >get_rs_cache: acquiring lock
              >_ULmips_dwarf_find_proc_info: looking for IP=0x400927
               >_ULmips_dwarf_callback: checking ./main, base=0x0)
               >_ULmips_dwarf_callback: checking ../lib//, base=0x770cd000)
               >_ULmips_dwarf_callback: checking ../lib//, base=0x77045000)
               >_ULmips_dwarf_callback: checking /lib/, base=0x7700e000)
               >_ULmips_dwarf_callback: checking /lib/, base=0x76f54000)
               >_ULmips_dwarf_callback: checking /lib/, base=0x77142000)
              >_ULmips_dwarf_find_proc_info: IP=0x400927 not found
                >put_rs_cache: unmasking signals/interrupts and releasing lock
               >_ULmips_dwarf_step: returning -10

"Backtracing" is the result of a __builtin_return_address(0) and is correct from my examination of the disassembly.

Other than it not working.  What really concerns me is the value for IP being unaligned to a word boundary and that it doesn't correspond to an actual instruction (I performed a objdump -d on the executable and crossed referenced against the proc/maps file to sanity check).

Compiler: mipsel-linux-gcc (Broadcom stbgcc-4.5.3-2.4) 4.5.3
uclibc: uClibc-0.9.32

Compile line: mipsel-linux-uclibc-gcc -g -fPIC -Ilibunwind/include -Llibunwind/src/.libs -lunwind -lunwind-mips -Wall -W -D_REENTRANT -fPIE -O0  -fno-inline -fno-omit-frame-pointer -o main main.c

Thanks in advance.

Attachment: main.c
Description: Text Data

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