Re: [DynInst_API:] Function parameters inspection with GCC -O2 optimization

[Bill Williams <bill@xxxxxxxxxxx>]

After a quick check, libdwarf supports a variety of GNU extensions that 
Dyninst presently does not. Filing this one as "for 8.3" tentatively, as 
the gnu extensions will take more work than we want to tackle for 8.2.
On 07/17/2014 01:03 PM, Francis Deslauriers wrote:
> Hi folks,
> This email is part bug report and part question. I wanted to share
> what I found while debugging this issue in order to helps someone that
> would tackle this issue in the future.
>
> I want to retrieve the parameters of a binary that has been compile
> with gcc -O2 optimization option.
>
> I made a dummy program (mutatee.c in the attached archive) that
> contains a function named foo that has one parameter. I was wondering
> why I was not able to get the parameter's name, type and eventually
> value with my mutator program. So, I activated the
> DYNINST_DEBUG_DWARF=1 env var to examine what was happening.
> http://paste.ubuntu.com/7809741/. We can see on line 655 that there is
> a failure parsing the third entry of the location list for the bar
> variable.
>
> Here is the function foo:
> void __attribute__((noinline, noclone))
>   foo(int bar)
> {
>      printf("%d",bar);
>      return;
> }
>
> Using the following: objdump --dwarf=info mutatee
> We can find the entry for the foo function and find the bar parameter
> at the 0x2ec offset.
>   <1><2cf>: Abbrev Number: 17 (DW_TAG_subprogram)
>      <2d0>   DW_AT_external    : 1
>      <2d0>   DW_AT_name        : foo
>      <2d4>   DW_AT_decl_file   : 1
>      <2d5>   DW_AT_decl_line   : 8
>      <2d6>   DW_AT_prototyped  : 1
>      <2d6>   DW_AT_low_pc      : 0x400650
>      <2de>   DW_AT_high_pc     : 0x13
>      <2e6>   DW_AT_frame_base  : 1 byte block: 9c        (DW_OP_call_frame_cfa)
>      <2e8>   DW_AT_GNU_all_call_sites: 1
>      <2e8>   DW_AT_sibling     : <0x34b>
>   <2><2ec>: Abbrev Number: 18 (DW_TAG_formal_parameter)
>      <2ed>   DW_AT_name        : bar
>      <2f1>   DW_AT_decl_file   : 1
>      <2f2>   DW_AT_decl_line   : 8
>      <2f3>   DW_AT_type        : <0x5e>
>      <2f7>   DW_AT_location    : 0x0     (location list)
>
> We see at the last line that the DW_AT_location of the foo formal
> parameter is a location list at offset 0x0 in the loc table which we
> can examine using: objdump --dwarf=loc mutatee
>      00000000 0000000000400650 000000000040065c (DW_OP_reg5 (rdi))
>      00000013 000000000040065c 0000000000400662 (DW_OP_reg1 (rdx))
>      00000026 0000000000400662 0000000000400663 (DW_OP_GNU_entry_value:
> (DW_OP_reg5 (rdi)); DW_OP_stack_value)
>
> So from my understanding, we aren't able to correctly parse the third
> entry of the location list. Digging in the code I found that the
> keyword
> DW_OP_GNU_entry_value is not handle by the switch case in
> dwarf/src/dwarfExprParser.C:126.it then falls in the default case and
> then return false.
> This behavior causes the mutator to return an empty vector when asked
> for the parameters of the function.
>
> My setup:
> gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9)
> Dyninst branch v8.2
>
> Is this an issue that is within Dyninst's control, more related to
> libdwarf or something else?
>
> Thanks,
>
> Francis
>
>
>
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--
--bw

Bill Williams
Paradyn Project
bill@xxxxxxxxxxx