Re: Prelinking on ARM with Debug Link

[Mark Wielaard <mjw at redhat dot com>]

Hi Torsten,

On Wed, 2015-11-25 at 21:52 +0100, Torsten Polle wrote:
> Here I have a problem to compile my probes when I want to include unwind information.
> 
> The error message is as follow:
> 
> In file included from /tmp/stap/taptrek_src.c:6010:0:
> /tmp/stap/stap-symbol.h:95831:1: error: large integer implicitly truncated to unsigned type [-Werror=overflow]
>  },
>  ^
> 
> The offending line in stap-symbols.h looks as follows:
> .sec_load_offset = 0xffffffffffffebc0 /* 4dec8000 4dec9440*/
> 
> The line includes debug output, because I checked the code in translate.cxx that generates this output and wanted to understand what is going wrong.
> 
>                   Dwarf_Addr dwbias = 0;
>                   dwfl_module_getdwarf (m, &dwbias);
>                   c->output << ".sec_load_offset = 0x"
>                                     << hex << debug_frame_off - dwbias << dec
>                                     << "/* "  << hex << debug_frame_off << " " << dwbias << dec << "*/"
>                                     << "\n";
> 
> As I use a 64bit host, the hex value is given as 64bit long hex value. Additional the value is very large, because the value is actually the difference between debug_frame_off=0x4dec8000 and dwbias=0x4dec9440. As dwbias is larger than debug_frame_off, the value becomes negative. But the field .sec_load_offset as unsigned long and therefore does not take negative values.

Yeah, this goes wrong since you are cross compiling for 32bit, so
unsigned long is "suddenly" smaller than expected.

I don't immediately know what the correct solution is. the
sec_load_offset is defined for _stp_section with the following comment:

        /* Synthesized index for .debug_frame table, keep section
           offset to adjust addresses relative to load address. */
        void *debug_hdr;
        uint32_t debug_hdr_len;
        unsigned long sec_load_offset;

I added this a couple of years back:

commit 4d83bd9b6f5ccc4abd212ca5d6a6477cb52f78cc
Author: Mark Wielaard <mjw@redhat.com>
Date:   Mon Jul 5 21:14:42 2010 +0200

  Put generated debug_hdr in _stp_section, add sec_load_offset for adjustment.
    
  Make sure to adjust .debug_frame addresses to section load address.
  Which means keeping track of the (synthetic) .debug_frame_hdr index
  per section. For now keep track of "magic sections". Will need to
  be extended to track all loadable code sections as we do for symbol
  tables. See http://sourceware.org/ml/systemtap/2010-q3/msg00012.html

So the problem we are facing is that kernel modules aren't ET_DYN, but
ET_REL files and the separate sections themselves are relocatable. So we
want to keep the load offset per (code) section.

We need that offset since we are creating a binary search table for
the .debug_frame (.eh_frame always comes with .eh_frame_hdr,
but .debug_frame would normally require a linear search which we want to
avoid). See adjustStartLoc in unwind.c.

Later this got extended to also be used by _stp_linenumber_lookup for
kernel modules, apparently for the same reason.

commit 598a50ae1cbc0f84914a4eee56a287273bb420b7
Author: Abegail Jakop <ajakop@redhat.com>
Date:   Mon Dec 22 15:04:34 2014 -0500

  PR12276: fix addr adjustment for symfileline()
    
  runtime/sym*: adjust the given address to account for the load offset
  runtime/transport/symbols.c: include the body of _stp_module_update_self()
  if STP_NEED_LINE_DATA so that the sec_load_offset can be updated
  tapset/linux/context-symbols.stp: remove the pragma myproc-unpriveleged
  since it's spelled wrong, and it's not needed anyways
  translate.cxx: add funtion find_debug_frame_offset() to set the
  sec_load_offset for the ".text" section

I assume it is the first usage (under STP_NEED_UNWIND_DATA) but the
other code looks similar (under STP_NEED_LINE_DATA).

It looks like the debug_frame_off is setup differently for the two
cases. So it might be good to investigate which one is used in the case
above to help our investigation.

Thanks,

Mark