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[lld-macho][nfc] Refactor subtractor reloc handling 

SUBTRACTOR relocations are always paired with UNSIGNED
relocations to indicate a pair of symbols whose address difference we
want. Functionally they are like a single relocation: only one pointer
gets written / relocated. Previously, we would handle these pairs by
skipping over the SUBTRACTOR relocation and writing the pointer when
handling the UNSIGNED reloc. This diff reverses things, so we write
while handling SUBTRACTORs and skip over the UNSIGNED relocs instead.

Being able to distinguish between SUBTRACTOR and UNSIGNED relocs in the
write phase (i.e. inside `relocateOne`) is useful for the upcoming range
check diff: we want to check that SUBTRACTOR relocs write signed values,
but UNSIGNED relocs (naturally) write unsigned values.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D98386
This commit is contained in:
Jez Ng 2021-03-11 13:28:13 -05:00
parent e8a3058303
commit a723db92d8
4 changed files with 19 additions and 20 deletions
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4
lld/MachO/Arch/ARM64.cpp
View File

@ -79,7 +79,8 @@ const RelocAttrs &ARM64::getRelocAttrs(uint8_t type) const {
uint64_t ARM64::getEmbeddedAddend(MemoryBufferRef mb, const section_64 &sec,
const relocation_info rel) const {
if (rel.r_type != ARM64_RELOC_UNSIGNED) {
if (rel.r_type != ARM64_RELOC_UNSIGNED &&
rel.r_type != ARM64_RELOC_SUBTRACTOR) {
// All other reloc types should use the ADDEND relocation to store their
// addends.
// TODO(gkm): extract embedded addend just so we can assert that it is 0
@ -159,6 +160,7 @@ void ARM64::relocateOne(uint8_t *loc, const Reloc &r, uint64_t value,
case ARM64_RELOC_BRANCH26:
value = encodeBranch26(base, value - pc);
break;
case ARM64_RELOC_SUBTRACTOR:
case ARM64_RELOC_UNSIGNED:
break;
case ARM64_RELOC_POINTER_TO_GOT:

24
lld/MachO/InputFiles.cpp
View File

@ -276,16 +276,6 @@ void ObjFile::parseRelocations(const section_64 &sec,
if (relInfo.r_address & R_SCATTERED)
fatal("TODO: Scattered relocations not supported");
Reloc p;
if (target->hasAttr(relInfo.r_type, RelocAttrBits::SUBTRAHEND)) {
p.type = relInfo.r_type;
p.referent = symbols[relInfo.r_symbolnum];
relInfo = relInfos[++i];
// SUBTRACTOR relocations should always be followed by an UNSIGNED one
// indicating the minuend symbol.
assert(target->hasAttr(relInfo.r_type, RelocAttrBits::UNSIGNED) &&
relInfo.r_extern);
}
uint64_t embeddedAddend = target->getEmbeddedAddend(mb, sec, relInfo);
assert(!(embeddedAddend && pairedAddend));
uint64_t totalAddend = pairedAddend + embeddedAddend;
@ -320,9 +310,19 @@ void ObjFile::parseRelocations(const section_64 &sec,
}
InputSection *subsec = findContainingSubsection(subsecMap, &r.offset);
if (p.type != GENERIC_RELOC_INVALID)
subsec->relocs.push_back(p);
subsec->relocs.push_back(r);
if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND)) {
relInfo = relInfos[++i];
// SUBTRACTOR relocations should always be followed by an UNSIGNED one
// indicating the minuend symbol.
assert(target->hasAttr(relInfo.r_type, RelocAttrBits::UNSIGNED) &&
relInfo.r_extern);
Reloc p;
p.type = relInfo.r_type;
p.referent = symbols[relInfo.r_symbolnum];
subsec->relocs.push_back(p);
}
}
}

9
lld/MachO/InputSection.cpp
View File

@ -58,15 +58,12 @@ void InputSection::writeTo(uint8_t *buf) {
memcpy(buf, data.data(), data.size());
for (size_t i = 0; i < relocs.size(); i++) {
const Symbol *fromSym =
target->hasAttr(relocs[i].type, RelocAttrBits::SUBTRAHEND)
? relocs[i++].referent.get<Symbol *>()
: nullptr;
const Reloc &r = relocs[i];
uint8_t *loc = buf + r.offset;
uint64_t referentVA = 0;
if (fromSym) {
const Symbol *toSym = r.referent.get<Symbol *>();
if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND)) {
const Symbol *fromSym = r.referent.get<Symbol *>();
const Symbol *toSym = relocs[++i].referent.get<Symbol *>();
referentVA = toSym->getVA() - fromSym->getVA();
} else if (auto *referentSym = r.referent.dyn_cast<Symbol *>()) {
if (target->hasAttr(r.type, RelocAttrBits::LOAD) &&

2
lld/test/MachO/reloc-subtractor.s
View File

@ -21,7 +21,7 @@
# CHECK-NEXT: segment section address type
# CHECK-EMPTY:
.globl _main, _subtrahend_1, _subtrahend_2, _minued1, _minued2
.globl _main, _subtrahend_1, _subtrahend_2, _minuend1, _minuend2
.data
_subtrahend_1: