Range lists, zero-length functions, linker gc

[Fangrui Song]

It is being discussed on llvm-dev
(https://lists.llvm.org/pipermail/llvm-dev/2020-May/141885.html https://groups.google.com/forum/#!topic/llvm-dev/i0DFx6YSqDA)
what linkers should do regarding relocations referencing dropped functions (due
to section group rules, --gc-sections, /DISCARD/, etc) in .debug_*

As an example:

   __attribute__((section(".text.x"))) void f1() { }
   __attribute__((section(".text.x"))) void f2() { }
   int main() { }

Some .debug_* sections are relocated by R_X86_64_64 referencing undefined symbols (the STT_SECTION
symbols are collected):

   0x00000043:   DW_TAG_subprogram [2]
                   ###### relocated by .text.x + 10
                   DW_AT_low_pc [DW_FORM_addr]     (0x0000000000000010 ".text.x")
                   DW_AT_high_pc [DW_FORM_data4]   (0x00000006)
                   DW_AT_frame_base [DW_FORM_exprloc]      (DW_OP_reg6 RBP)
                   DW_AT_linkage_name [DW_FORM_strp]       ( .debug_str[0x0000002c] = "_Z2f2v")
                   DW_AT_name [DW_FORM_strp]       ( .debug_str[0x00000033] = "f2")


With ld --gc-sections:

* DW_AT_low_pc [DW_FORM_addr] in .debug_info are resolved to 0 + addend
   This can cause overlapping address ranges with normal text sections. {{overlap}}
* [beginning address offset, ending address offset) in .debug_ranges are resolved to 1 (ignoring addend).
   See bfd/reloc.c (behavior introduced in
   https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=e4067dbb2a3368dbf908b39c5435c84d51abc9f3 )

   [0, 0) cannot be used because it terminates the list entry.
   [-1, -1) cannot be used because -1 represents a base address selection entry which will affect
     subsequent address offset pairs.
* .debug_loc address offset pairs have similar problem to .debug_ranges
* In DWARF v5, the abnormal values can be in a separate section .debug_addr

---

To save your time, I have a summary of the discussions. I am eager to know what you think
of the ideas from binutils/gdb/elfutils's perspective.

* {{reserved_address}} Paul Robinson wants to propose that DWARF v6 reserves a special address.
   All (undef + addend) in .debug_* are resolved to -1.

   We have to ignore the addend. With __attribute__((section(".text.x"))),
   the address offset pair may be something like [.text.x + 16, .text.x + 24)
   I have to resolve the whole (.text.x + 16) to the special value.

   (undef + addend) in pre-DWARF v5 .debug_loc and .debug_ranges are resolved to -2
   (0 and -1 cannot be used due to the reasons above).

* Refined formula for a relocated value in a non-SHF_ALLOC section:

    if is_defined(sym)
       return addr(sym) + addend
    if relocated_section is .debug_ranges or .debug_loc
       return -2   # addend is intentionally ignored

    // Every DWARF v5 section falls here
    return -1  {{zero}}

* {{zero}} Can we resolve (undef + addend) to 0?

   https://lists.llvm.org/pipermail/llvm-dev/2020-May/141967.html

   > while it might not be an issue for ELF, DWARF would want a standard that's fairly resilient to
   > quirky/interesting use cases (admittedly - such platforms could equally want to make their
   > executable code way up in the address space near max or max - 1, etc?).

   Question: is address 0 meaningful for code in some binary formats?

* {{overlap}} The current situation (GNU ld, gold, LLD): (undef + addend) in .debug_* are resolved to addend.
   For an address offset pair like [.text + 0, .text + 0x10010), if the ending address offset is large
   enough, it may overlap with a normal text address range (for example [0x10000, *))

   This can cause problems in debuggers. How does gdb solve the problem?

* {{nonalloc}} Linkers resolve (undef + addend) in non-SHF_ALLOC sections to
   `addend`. For non-debug sections (open-ended), do we have needs resolving such
   values to `base` or `base+addend` where base is customizable?
   (https://lists.llvm.org/pipermail/llvm-dev/2020-May/141956.html )