GCC LD NOLOAD linker section generates loadable segment

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I'm working on an Arm bare-metal application and I've marked some sections with NOLOAD. According to the explanation in Understanding linker script NOLOAD sections in embedded software , I expected the resulting ELF file to not have a loadable segment (program header) for these sections, but it does.

Is this correct? Why are those sections marked as loadable in the ELF file?

As the linker is still placing the data in .bss, how a loader is supposed to know that the sections shouldn't be loaded? Or am I missing the meaning of 'load' in the sense that NOLOAD only makes sense for initialized symbols (which would normally be placed into .data)?

This is a part of my linker script:

    .bss (NOLOAD) :
    {
        . = ALIGN(4);
        __bss_start__ = .;
        *(.bss_begin .bss_begin.*)

        *(.bss .bss.*)
        *(COMMON)

        *(.bss_end .bss_end.*)
        . = ALIGN(4);
        __bss_end__ = .;
    } >DRAM

    .noinit (NOLOAD) :
    {
        . = ALIGN(4);
        __noinit_start__ = .;

        *(.noinit .noinit.*)

         . = ALIGN(4) ;
        __noinit_end__ = .;
    } > DRAM
    
    /* Check if there is enough space to allocate the main stack */
    ._stack (NOLOAD) :
    {
        . = ALIGN(4);
        
        . = . + __Main_Stack_Size ;
        
        . = ALIGN(4);
    } >DRAM

This is the output ELF file:

arm-none-eabi-readelf.exe -l test.elf

Elf file type is EXEC (Executable file)
Entry point 0x601b9
There are 2 program headers, starting at offset 52

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x010000 0x00060000 0x00060000 0x06840 0x06840 RWE 0x10000
  LOAD           0x020000 0x20010000 0x20010000 0x00000 0x06084 RW  0x10000

 Section to Segment mapping:
  Segment Sections...
   00     .text .ARM.exidx.reset .data
   01     .systemclock .bss ._stack

Why are the .bss and ._stack sections there?

Thanks!!

linker-scripts ld gcc c
1 Answers

The FileSiz of the second segment is 0 which means that there is no data that will be loaded from the elf file into this segment.

The reason why this segment exists in the table is that on a non-embedded system, the program loader would still need to request memory for sections in that segment and mark the relevant pages with the relevant attributes (readable and writable in this case).

Edit: After reading the question again, I did a bit more experimenting and it seems that all (NOLOAD) seems to do is to set the type of the section in the output file to SHT_NOBITS. The elf specification calls that a section that occupies no space in the file but is still part of the program's memory image.

If the goal is to link against code that is already present in the rom before the program is loaded, those symbols should be defined in the linker script outside of any section. E.g. already_present_code = 0x06000000; SECTIONS { .text : {*(.text*)} /* ... */}. That seems to have the desired effect.

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