[Dwarf-discuss] Facing a issue in contents of dwarf debug info section in an ELF file

[Kyle Huey]

Most likely the .debug_info section is SHF_COMPRESSED. Try compiling with
-gz=none.

- Kyle

On Mon, Sep 25, 2023, 1:36 PM Rishi Raj via Dwarf-discuss <
dwarf-discuss@lists.dwarfstd.org> wrote:

> <https://stackoverflow.com/posts/77169835/timeline>
>
> I am trying to implement a dwarf parser in C++ without using any external
> dependencies. As mentioned in dwarf5 standard, debug info first 4 bytes or
> 12 bytes denotes the unit length Basically this:
>
> unit_length (initial length)
> A 4-byte or 12-byte unsigned integer representing the length of the3
> .debug_info contribution for that compilation unit, not including the length field itself. In the 32-bit DWARF format, this is a 4-byte unsigned integer (which must be less than 0xfffffff0); in the 64-bit DWARF format, this consists of the 4-byte value 0xffffffff followed by an 8-byte unsigned integer that gives the actual length (see Section 7.4 on page 196).
>
> When I am dumping the .debug_info section hexadecimally using objdump I am
> getting this(see readelf output below).
>
> objdump -s -j .debug_info hello.o
>
> hello.o: file format elf64-x86-64
>
> Contents of section .debug_info:
>
>  0000 01000000 00000000 9a000000 00000000  ................
>  0010 01000000 00000000 789c9bc6 c0c0c0ca  ........x.......
>  0020 c0c801a4 18984084 2c031a10 42623372  ......@.,...Bb3r
>  0030 b0832916 0805d1c6 c804e5b1 4178ac20  ..).........Ax.
>  0040 8a998535 33af04a8 8115498e 05aa2002  ...53.....I... .
>  0050 8bf18c73 58131918 99394172 4c137318  ...sX....9ArL.s.
>  0060 180011e5 0560
>
> So according to this, the length should be 0x01000000, but the actual
> length is 0x96.(see readelf output below)
> readelf -wi hello.o
> Contents of the .debug_info section:
>
> Compilation Unit @ offset 0:
> Length: 0x96 (32-bit)
> Version: 5 Unit Type: DW_UT_compile (1) Abbrev Offset: 0
> Pointer Size: 8
>
> I know I am missing something basic, but even after reading the standards
> for many times. I am unable to find my mistake. One more thing, I searched
> for some basic dwarf parsers so that I could understand what they are doing
> but could not find any. All of the parsers were big libraries, which was
> difficult to understand. If any of you can at least provide some readable
> and understandable parser code, It will be helpful too.
>
> ELF header dump:
>
> ELF Header:
>   Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
>   Class:                             ELF64
>   Data:                              2's complement, little endian
>   Version:                           1 (current)
>   OS/ABI:                            UNIX - System V
>   ABI Version:                       0
>   Type:                              REL (Relocatable file)
>   Machine:                           Advanced Micro Devices X86-64
>   Version:                           0x1
>   Entry point address:               0x0
>   Start of program headers:          0 (bytes into file)
>   Start of section headers:          3048 (bytes into file)
>   Flags:                             0x0
>   Size of this header:               64 (bytes)
>   Size of program headers:           0 (bytes)
>   Number of program headers:         0
>   Size of section headers:           64 (bytes)
> --
> Dwarf-discuss mailing list
> Dwarf-discuss@lists.dwarfstd.org
> https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
>
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