[llvm-bugs] [Bug 45600] New: Support TLS relocations referencing non STT_TLS symbols (e.g. STT_SECTION or STT_NOTYPE)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45600

            Bug ID: 45600
           Summary: Support TLS relocations referencing non STT_TLS
                    symbols (e.g. STT_SECTION or STT_NOTYPE)
           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, smithp352 at googlemail.com

Two weeks ago I was trying deleting a hack in MC (integrated assembler)
https://reviews.llvm.org/D77361 and I noticed I can't do that because lld does
not support a TLS relocation referencing a non-TLS symbol.

Related gABI discussion:
https://groups.google.com/forum/#!searchin/generic-abi/TLS%7Csort:date/generic-abi/dJ4_Y78aQ2M/WKggp9fKCgAJ

# a.o
movl %fs:.tdata+4 at tpoff, %eax
movl %fs:a at tpoff, %eax

.section .tdata,"awT"
.long 0
a:
.long 0


# lld
  movl    %fs:2105868, %eax  # a TLS relocation referencing a STT_SECTION
  movl    %fs:-4, %eax

# gold and GNU ld
  movl    %fs:-4, %eax
  movl    %fs:-4, %eax

Supporting this requires to fix

// Symbols.cpp:getSymVA

    if (d.isTls() && !config->relocatable) {
      // Use the address of the TLS segment's first section rather than the
      // segment's address, because segment addresses aren't initialized until
      // after sections are finalized. (e.g. Measuring the size of .rela.dyn
      // for Android relocation packing requires knowing TLS symbol addresses
      // during section finalization.)
      if (!Out::tlsPhdr || !Out::tlsPhdr->firstSec)
        fatal(toString(d.file) +
              " has an STT_TLS symbol but doesn't have an SHF_TLS section");
      return va - Out::tlsPhdr->firstSec->addr;

and

// Relocations.cpp isStaticLinkTimeConstant

  // For the target and the relocation, we want to know if they are
  // absolute or relative.
  bool absVal = isAbsoluteValue(sym); // STT_SECTION/STT_NOTYPE can be used as
TLS as well
  bool relE = isRelExpr(e);
  if (absVal && !relE)
    return true;
  if (!absVal && relE)
    return true;
  if (!absVal && !relE)
    return target->usesOnlyLowPageBits(type);

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