[Bug ld/29861] New: ld: Output Section Type can't be Overwritten / Documentation Unclear

[phip1611 at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29861

            Bug ID: 29861
           Summary: ld: Output Section Type can't be Overwritten /
                    Documentation Unclear
           Product: binutils
           Version: 2.39
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: phip1611 at gmail dot com
  Target Milestone: ---

Since GNU binutils 2.39, specifically commit c212f39d (2022-02-16), the linker
script language allows setting a dedicated type for an output section
(https://sourceware.org/binutils/docs/ld/Output-Section-Type.html). This works
with sections that are only defined by content provided in the linker script,
as the following example shows:

SECTIONS {
  foobar (TYPE = SHT_PROGBITS) : { BYTE(1) }
}

readelf shows exactly the expected type for the foobar section.

However, GNU ld ignores this type when an output section contains input
sections that already have a certain section type. The following is not
possible:

    .bss . (TYPE = SHT_PROGBITS) : 
    {
        *(COMMON)
        *(.bss)
    }

The type is always SHT_NOBITS, no matter what the linker script specifies. This
is not only the case for special pre-defined sections, such as .bss. It also
applies to custom sections. For example, take a look at the following GAS
example:

    .section .foobar, "ax", @nobits
    zeroed_buffer:
    .fill 1024, 1, 0

The section will always be of type SHT_NOBITS, even if the linker script
specifies

    .foobar . (TYPE=SHT_PROGBITS) :
    {
        *(.foobar)
    } 

So I wonder, is this a bug or intended? If it is intended, the documentation
should describe more precisely what this feature is capable of and what the
limits are.


PS: I work in a project where we need to ensure that memsize==filesize for each
section. One solution is to link *(COMMON) and *(.bss) into .data which is of
type SHT_PROGBITS. However, I thought the new approach might be more obvious
when someone looks at the linker script. However, it doesn't work as I expect.

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