location list
Sasha Da Rocha Pinheiro
darochapinhe@wisc.edu
Tue Jun 9 16:38:51 GMT 2020
Hi Mark,
first of all, thanks for giving me a direction here.
I am now trying to design the changes needed to be done in Dyninst.
So far we have only used the functions dwarf_* under libdw.
What I understood is that libdw is kinda divided in subsets of functions, dwarf_*, dwfl_* and dwelf_*.
I didn't find any documentation about it, or the purpose of these subset of functions. (Whats fl in dwfl for?)
But my understanding is that I can't use data structures from one on the other one.
That alone will need some design to modify the way we parse dwarf info into Dyninst.
Currently the lifetime of a dwarf handle lasts through one execution, because we parse dwarf data when the user needs it.
Can you point me to more documentation here or schedule a call so I can get a more clear view of this?
Regards,
Sasha
From: Mark Wielaard <mark@klomp.org>
Sent: Saturday, June 6, 2020 9:05 AM
To: Sasha Da Rocha Pinheiro <darochapinhe@wisc.edu>; elfutils-devel@sourceware.org <elfutils-devel@sourceware.org>
Subject: Re: location list
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
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