[llvm-bugs] [Bug 42506] New: __efistub_stext_offset=stext-_text; ... _text=.; => isAbsolute() can be incorrect if a symbol in the expression is defined after the expression

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42506

            Bug ID: 42506
           Summary: __efistub_stext_offset=stext-_text;... _text=.; =>
                    isAbsolute() can be incorrect if a symbol in the
                    expression is defined after the expression
           Product: lld
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: ELF
          Assignee: unassignedbugs at nondot.org
          Reporter: i at maskray.me
                CC: llvm-bugs at lists.llvm.org, peter.smith at linaro.org

Originally reported at https://github.com/ClangBuiltLinux/linux/issues/561.

# a.s: llvm-mc -filetype=obj -triple=aarch64 a.s -o a.o
.globl stext
# R_AARCH64_ABS32
ldr w21, =__efistub_stext_offset

# a.x
stext:
__efistub_stext_offset = stext - _text;
# In Linux kernel, this has been worked around by changing to
__efistub_stext_offset = ABSOLUTE(stext - _text;)
SECTIONS {
  .text : {
    _text = .;   # Note, _text is evaluated after __efistub_stext_offset
    *(.text)
  }
}

% llvm-mc -filetype=obj -triple=aarch64 a.s -o a.o
% ld.lld -shared a.o a.x
ld.lld: error: can't create dynamic relocation R_AARCH64_ABS32 against symbol:
__efistub_stext_offset in readonly segment; recompile object files with -fPIC
or pass '-Wl,-z,notext' to allow text relocations in the output
>>> defined in a.x:1
>>> referenced by a.o:(.text+0x4)


Call tree:

link<ELF64LE>(Args);
  Script->declareSymbols();
    1) insert __efistub_stext_offset as an absolute Defined
    2) insert stext. It already exists as a non-absolute (section relative)
Defined.
    3) insert _text as an absolute Defined.
  writeResult<ELFT>()
    Writer<ELFT>().run();
      Script->processSectionCommands();
        addSymbol(Cmd); // this evaluates `stext - _text`: nonabsolute -
absolute = nonabsolute
          __efistub_stext_offset is inserted as a non-absolute Defined
      finalizeSections();
        forEachRelSec(scanRelocations<ELFT>);
          processRelocAux
            An error is issued because in a -pie/-shared link, an absolute
relocation R_AARCH64_ABS32 cannot reference a non-absolute symbol

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