Re: location list

[Mark Wielaard <mark@klomp.org>]

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Hi Sasha,

On Sat, 2020-06-06 at 00:30 +0000, Sasha Da Rocha Pinheiro wrote:
> As you can see the following variables have distinct locations:
>  [    81]      variable             abbrev: 5
>                name                 (string) "a"
>                decl_file            (data1) sasha.c (1)
>                decl_line            (data1) 12
>                type                 (ref4) [    cd]
>                location             (sec_offset) location list
> [     0]
>  [    9f]        variable             abbrev: 5
>                  name                 (string) "g"
>                  decl_file            (data1) sasha.c (1)
>                  decl_line            (data1) 15
>                  type                 (ref4) [    cd]
>                  location             (sec_offset) location list
> [    4a]
> [    bd]          variable             abbrev: 5
>                    name                 (string) "z"
>                    decl_file            (data1) sasha.c (1)
>                    decl_line            (data1) 16
>                    type                 (ref4) [    cd]
>                    location             (sec_offset) location list
> [    6e]
> 
> But when I use the code I sent before to list the three variables, I
> always get:
> 
> [main01.cpp:73] - Variable and location found (a), size(1).
> [main01.cpp:78] - interval: (0x0,0x5) 
> [main01.cpp:78] - interval: (0x5,0xa) 
> [main01.cpp:78] - interval: (0x16,0x24) 
> [main01.cpp:73] - Variable and location found (g), size(1).
> [main01.cpp:78] - interval: (0x0,0x5) 
> [main01.cpp:78] - interval: (0x5,0xa) 
> [main01.cpp:78] - interval: (0x16,0x24) 
> [main01.cpp:73] - Variable and location found (z), size(1).
> [main01.cpp:78] - interval: (0x0,0x5) 
> [main01.cpp:78] - interval: (0x5,0xa) 
> [main01.cpp:78] - interval: (0x16,0x24) 
> 
> 
> No matter the locationAttribute the code always get the first
> location descriptors in .debug_loc: 
>  
> DWARF section [ 7] '.debug_loc' at offset 0x1c6:
> 
>  CU [     b] base: .text+000000000000000000 <main>
>  [     0] range 0, 5
>           .text+000000000000000000 <main>..
>           .text+0x0000000000000004 <main+0x4>
>            [ 0] lit0
>            [ 1] stack_value
>           range 5, a
>           .text+0x0000000000000005 <main+0x5>..
>           .text+0x0000000000000009 <main+0x9>
>            [ 0] reg1
>           range 16, 24
>           .text+0x0000000000000016 <main+0x16>..
>           .text+0x0000000000000023 <main+0x23>
>            [ 0] reg1
>  [    4a] range 0, 5
>           .text+000000000000000000 <main>..
>           .text+0x0000000000000004 <main+0x4>
>            [ 0] lit0
>            [ 1] stack_value
>  [    6e] range 5, a
>           .text+0x0000000000000005 <main+0x5>..
>           .text+0x0000000000000009 <main+0x9>
>            [ 0] lit0
>            [ 1] stack_value
>           range a, e
>           .text+0x000000000000000a <main+0xa>..
>           .text+0x000000000000000d <main+0xd>
>            [ 0] const4u 65537
>            [ 5] breg0 0
>            [ 7] minus
>            [ 8] stack_value

I think I see what is happening. The fact that <main> is at
.text+000000000000000000 suggests that this is actually an ET_REL file
(not linked object file). The libdw dwarf_xxx calls don't do
relocations. But eu-readelf does. So while eu-readelf shows some
offsets as their relocated values, your program just using dwarf_xxx
calls does not. Specifically the DW_AT_location list attributes will
all point to zero. Which explains why every location list seems to be
the same.

We don't have a public function to just apply all relocations to an
object file, but opening the file through dwfl_begin () will do it.

Something like the attached.

Hope that helps,

Mark

[-- Attachment #2: dwfl_dwarf.c --]
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/* Print all locations in the whole DIE tree of a single file using
   dwfl to handle ET_REL files (which need the .debug sections to be
   relocated) and to automatically get separate debuginfo.

   gcc -Wall -Wextra -g -O2 -o dwfl_dwarf dwfl_dwarf.c -ldw
*/

/* We want the sane basename function. */
#define _GNU_SOURCE
#include <string.h>
#include <stdbool.h>
#include <stdio.h>
#include <inttypes.h>

#include <dwarf.h>
#include <elfutils/libdw.h>
#include <elfutils/libdwfl.h>

void
handle_die (Dwarf_Die *die)
{
  do
    {
      Dwarf_Attribute attr;
      if ((dwarf_attr (die, DW_AT_location, &attr) != NULL))
	{
	  printf ("[%" PRIx64 "]", dwarf_dieoffset (die));
	  ptrdiff_t off = 0;
	  Dwarf_Addr base, start, end;
	  do
	    {
	      Dwarf_Op *expr;
	      size_t exprlen;
	      off = dwarf_getlocations(&attr, off, &base, &start, &end,
				       &expr, &exprlen);
	      if (off > 0)
		printf ("(%" PRIx64 ",%" PRIx64 ")[%zd] ",
			start, end, exprlen);
	    }
	  while (off > 0);
	  printf ("\n");
	}

      Dwarf_Die child;
      if (dwarf_child (die, &child) == 0)
	handle_die (&child);
    }
  while (dwarf_siblingof (die, die) == 0);
}

static const Dwfl_Callbacks dwfl_callbacks =
  {
    .find_debuginfo = dwfl_standard_find_debuginfo,
    .section_address = dwfl_offline_section_address,
    .find_elf = dwfl_build_id_find_elf,
  };

int main (int argc, char **argv)
{
  if (argc == 2)
    {
      const char *file = argv[1];
      const char *base = basename (file);
      /* Create a one elf module file Dwfl. */
      Dwfl *dwfl = dwfl_begin (&dwfl_callbacks);
      dwfl_report_begin (dwfl);
      Dwfl_Module *mod = dwfl_report_elf (dwfl, base, file, -1, 0, true);
      dwfl_report_end (dwfl, NULL, NULL);

      Dwarf_Addr bias;
      Dwarf *dbg = dwfl_module_getdwarf(mod, &bias);
      if (dbg != NULL)
	{
	   /* Should be zero with one module. */
	  printf ("bias: %" PRIx64 "\n", bias);
	  Dwarf_CU *cu = NULL;
	  Dwarf_Half version;
	  Dwarf_Die cudie, subdie;
	  uint8_t unit_type;
	  while (dwarf_get_units (dbg, cu, &cu, &version, &unit_type,
				  &cudie, &subdie) == 0)
	    handle_die (&cudie);
	}
      dwfl_end (dwfl);
    }
}