How to manually add debug type information to a GNU GAS assembly "array" symbol?

Question

For example, consider the following Linux freestanding true program with an useless my_longs variable for illustrative purposes:

main.S

.section .rodata
    my_longs: .quad 0x12, 0x34
.text
.global _start
_start:
asm_main_after_prologue:
    mov $60, %rax
    mov $0, %rdi
    syscall

which you can assemble and run with:

as -o main.o main.S
ld -o main.out main.o
gdb main.out

Then start the program with:

starti

Now, to see the value of my_longs, I have to type the annoying (long[2]) part:

p/x (long[2])my_longs

Is there any way to tell GDB the type of my_longs on by adding some extra annotation to my assembly program?

When writing a C program:

main_c.c

long my_longs[] = {0x12, 0x34};

int main(void) {
    return 0;
}

compiled with:

gcc -O0 -ggdb3 -save-temps -o main_c.out main_c.c

I can see the symbols directly with;

p/x my_longs

presumably due to the DWARF debug information GCC added.

When I had a look at the generated assembly, the only references I could find to my_longs were:

.Ltext0:
    .comm   my_longs,16,16
.Ldebug_info0:
    .long   .LASF351
    .quad   my_longs
.LASF351:
    .string "my_longs"

so I'm not sure where the type information is being stored.

I know that this question may come down basically to: how are types encoded in DWARF, and is it practical to manually encode that information. I intend to study DWARF a bit later on if this keeps on bothering me.

You can non-manually add debug with '--gdwarf-2'. There are also various pseudo-ops which can encode information. See: CFI. — artless-noise-bye-due2AI
– artless-noise-bye-due2AI, Commented Aug 2, 2022 at 14:01

Vladislav Ivanishin · Accepted Answer · 2019-06-19 11:49:58Z

3

I am no DWARF expert either, but poking around with dwarfdump reveals enough to answer the question.

$ dwarfdump ./main_c.out

..  DW_TAG_variable
        DW_AT_name                  my_longs
        ...
        DW_AT_type                  <0x00000031>

And if we look at the type node, we'll see that it is an array (whose elements are of type described with node 0x0048 which corresponds to long int).

$ dwarfdump ./main_c.out  | egrep -A3 '00031'

< 1><0x00000031>    DW_TAG_array_type
                      DW_AT_type                  <0x00000048>
                      DW_AT_sibling               <0x00000041>
...

answered Jun 19, 2019 at 11:49

Vladislav Ivanishin

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3 Comments

Ciro Santilli OurBigBook.com Over a year ago

Thanks, I did not know dwarfdump. OK, from this it does not look that there is an easy way to manually encode the DWARF info :-(

nostromo Sep 9 at 6:01

Yesterday I posted a question (How do I make this, presumed, very basic DWARF example work?), basically asking how to handle dwarf.exp, trying to build a very simple debugging information file for a compiled source (main.c [template also provided by the gdb testsuite]) to be combined with and also implicitly asking how to approach it with gdb, after all (how to combine the binary + generated dwarf file).

Vladislav Ivanishin Over a year ago

Yeah, I learned about it by lurking around on SO. Manual encoding seems a bit too involved, I agree.

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