Hello,
Further to my previous questions on the subject of DWARF: I'm having
difficulty
understanding how to handle relocatable DWARF data using the standard
library.
Currently, I am successfully loading an ELF file and relocating it in memory
(the data is being loaded into an ARM emulator and runs correctly).
However, I'm puzzled at how the elf.DWARF() function in standard library
handles relocation. Looking at the source for this function I can see that
it
will process any REL or RELA sections related to the debug sections. This
is a
big help but I don't understand how it takes into account the already
relocated
ELF sections.
For instance, when I'm processing the line entries for a compile unit
(retrieved via a dwarf.LineReader) the Address field of each LineEntry
instance
is the unrelocated PC value for that instruction. Currently, I'm handling
this
by adjusting the Address field by the origin address of the relocated .text
section, as I encounter them. This works but it feels like I'm missing
something.
Do I need to adjust the address field manually or is there a step I'm
misunderstanding?
Stephen
On Monday, February 14, 2022 at 6:50:31 AM UTC stephen.t....@gmail.com
wrote:
> I've figured out what the problem is. The problem in my code was caused by
> the fact that this ELF/DWARF file has so many compile units, whereas my
> other examples have far fewer compile units. I'll try to explain what my
> mistake was:
>
> Task: Looking for the function declaration file/line, for an executable
> address
>
> Original Solution (pseudocode):
>
> new DWARF Reader
> for every ENTRY
> if ENTRY is compile unit
> initialise new LINEREADER
>
> if ENTRY is SUBPROGRAM or INLINEDSUBROUTINE
> if ADDRESS is in range of ENTRY
> if has ABSTRACT ORIGIN tag
> seek ABSTRACT offset <--- this is where the bug is
>
>
> look up DECL FILE and DECL LINE using LINEREADER
>
> The issue was caused when following the abstract origin. In this ELF/DWARF
> file, it is likely that the entry is in a different compile unit, meaning
> that the line reader is referring to the wrong line table.
>
> With a bit more care, I'm now using the correct line reader and the
> problem no longer occurs.
>
>
> Thanks for the help.
>
> On Sunday, February 13, 2022 at 10:59:44 PM UTC stephen.t....@gmail.com
> wrote:
>
>> Okay. I've attached a text file showing the results of "readelf --debug".
>> The ELF file itself was produced by GCC 10.3.1, part of the toolchain
>> distributed by ARM.
>>
>> These are the dwarf.Entry instances as output from my Go program, that
>> are out of range for the result of LineReader.Files()
>>
>> &dwarf.Entry{Offset:0x37a1, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName, Val:"clearBuffer",
>> Class:dwarf.ClassString}, dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclLine, Val:426,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrPrototyped,
>> Val:true, Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrSibling,
>> Val:0x37c9, Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3767, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"ColourConvert", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:462, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:15, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrPrototyped, Val:true, Class:dwarf.ClassFlag},
>> dwarf.Field{Attr:dwarf.AttrType, Val:0x230f, Class:dwarf.ClassReference},
>> dwarf.Field{Attr:dwarf.AttrSibling, Val:0x3786,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3228, Tag:dwarf.TagSubprogram, Children:false,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"processAnimationCommand", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:1492, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6, Class:dwarf.ClassConstant}}}
>>
>> &dwarf.Entry{Offset:0x3690, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName, Val:"InitGameX",
>> Class:dwarf.ClassString}, dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclLine, Val:567,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrSibling, Val:0x36f4,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3786, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"setVideoBufferPointers", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:437, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrSibling, Val:0x37a1,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3204, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"setAnimation", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:1544, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrPrototyped, Val:true, Class:dwarf.ClassFlag},
>> dwarf.Field{Attr:dwarf.AttrSibling, Val:0x321f,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3664, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName, Val:"Initialize",
>> Class:dwarf.ClassString}, dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclLine, Val:688,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrSibling, Val:0x3690,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x375a, Tag:dwarf.TagSubprogram, Children:false,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName, Val:"main",
>> Class:dwarf.ClassString}, dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclLine, Val:490,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:5,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrType, Val:0x2327,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x35f8, Tag:dwarf.TagSubprogram, Children:false,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"InitGameDatastreams", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:797, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6, Class:dwarf.ClassConstant}}}
>>
>> &dwarf.Entry{Offset:0x34f4, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName,
>> Val:"GameScheduleAnimate", Class:dwarf.ClassString},
>> dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclLine, Val:1077, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6, Class:dwarf.ClassConstant},
>> dwarf.Field{Attr:dwarf.AttrSibling, Val:0x352d,
>> Class:dwarf.ClassReference}}}
>>
>> &dwarf.Entry{Offset:0x3601, Tag:dwarf.TagSubprogram, Children:true,
>> Field:[]dwarf.Field{dwarf.Field{Attr:dwarf.AttrExternal, Val:true,
>> Class:dwarf.ClassFlag}, dwarf.Field{Attr:dwarf.AttrName, Val:"Scheduler",
>> Class:dwarf.ClassString}, dwarf.Field{Attr:dwarf.AttrDeclFile, Val:10,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclLine, Val:753,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrDeclColumn, Val:6,
>> Class:dwarf.ClassConstant}, dwarf.Field{Attr:dwarf.AttrSibling, Val:0x3629,
>> Class:dwarf.ClassReference}}}
>>
>>
>>
>>
>> On Sun, Feb 13, 2022 at 10:26 PM Ian Lance Taylor <ia...@golang.org>
>> wrote:
>>
>>> On Sun, Feb 13, 2022 at 1:43 PM stephen.t....@gmail.com
>>> <stephen.t....@gmail.com> wrote:
>>> >
>>> > I've encountered something else that I don't understand. This time
>>> regarding dwarf.AttrDeclFile and how the value of that field relates to the
>>> list of source files in the project
>>> >
>>> > From what I can understand, the value of AttrDeclFile is an index into
>>> an array of filenames. If the number is zero then the indicates that no
>>> source file has been specified.
>>> >
>>> > The dwarf package allows retrieval of the file list with
>>> LineReader.Files() function - the LineReader being created with reference
>>> to the current compilation unit.
>>> >
>>> > For most ELF/DWARF examples I have, my understanding seems correct.
>>> However, in one example I have, the value of AttrDeclFile can exceed the
>>> length of the file array returned by LineReader.Files(). An invalid
>>> reference in other words.
>>> >
>>> > Moreover, for AttrDeclFiles values that are within range of the array,
>>> they are most definitely referencing the wrong file.
>>> >
>>> > I'm I misunderstanding the relationship between AttrDeclFile and
>>> LineReader.Files(), is there some other way of accessing the file array, or
>>> has something gone horribly wrong?
>>>
>>> What you write sounds correct. Can you show us the output "readelf
>>> --debug" on the file in question, and point out the problematic case?
>>> Thanks.
>>>
>>> Ian
>>>
>>
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