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[lld-macho] Don't emit rebase opcodes for subtractor minuend relocs

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Also add a few asserts to verify that we are indeed handling an
UNSIGNED relocation as the minued. I haven't made it an actual
user-facing error since I don't think llvm-mc is capable of generating
SUBTRACTOR relocations without an associated UNSIGNED.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D97103
This commit is contained in:
Jez Ng 2021-02-27 12:30:16 -05:00
parent cc5c03e109
commit 541390131e
6 changed files with 22 additions and 9 deletions
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1
lld/MachO/Arch/ARM64.cpp
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@ -47,7 +47,6 @@ struct ARM64 : TargetInfo {
// Random notes on reloc types:
// ADDEND always pairs with BRANCH26, PAGE21, or PAGEOFF12
// SUBTRACTOR always pairs with UNSIGNED (a delta between two sections)
// POINTER_TO_GOT: ld64 supports a 4-byte pc-relative form as well as an 8-byte
// absolute version of this relocation. The semantics of the absolute relocation
// are weird -- it results in the value of the GOT slot being written, instead

7
lld/MachO/InputFiles.cpp
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@ -287,6 +287,10 @@ void ObjFile::parseRelocations(const section_64 &sec,
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);
}
Reloc r;
r.type = relInfo.r_type;
@ -317,8 +321,7 @@ void ObjFile::parseRelocations(const section_64 &sec,
}
InputSection *subsec = findContainingSubsection(subsecMap, &r.offset);
if (p.type != GENERIC_RELOC_INVALID &&
target->hasAttr(p.type, RelocAttrBits::SUBTRAHEND))
if (p.type != GENERIC_RELOC_INVALID)
subsec->relocs.push_back(p);
subsec->relocs.push_back(r);
}

4
lld/MachO/InputSection.cpp
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@ -59,13 +59,13 @@ void InputSection::writeTo(uint8_t *buf) {
for (size_t i = 0; i < relocs.size(); i++) {
auto *fromSym = target->hasAttr(relocs[i].type, RelocAttrBits::SUBTRAHEND)
? relocs[i++].referent.dyn_cast<Symbol *>()
? relocs[i++].referent.get<Symbol *>()
: nullptr;
const Reloc &r = relocs[i];
uint8_t *loc = buf + r.offset;
uint64_t referentVA = 0;
if (fromSym) {
auto *toSym = r.referent.dyn_cast<Symbol *>();
auto *toSym = r.referent.get<Symbol *>();
referentVA = toSym->getVA() - fromSym->getVA();
} else if (auto *referentSym = r.referent.dyn_cast<Symbol *>()) {
if (target->hasAttr(r.type, RelocAttrBits::LOAD) &&

1
lld/MachO/Target.h
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@ -52,6 +52,7 @@ enum class RelocAttrBits {
UNSIGNED = 1 << 14, // *_UNSIGNED relocs
LLVM_MARK_AS_BITMASK_ENUM(/*LargestValue*/ (1 << 15) - 1),
};
// Note: SUBTRACTOR always pairs with UNSIGNED (a delta between two symbols).
class TargetInfo {
public:

11
lld/MachO/Writer.cpp
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@ -449,9 +449,16 @@ void Writer::scanRelocations() {
continue;
}
for (Reloc &r : isec->relocs) {
if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND))
for (auto it = isec->relocs.begin(); it != isec->relocs.end(); ++it) {
Reloc &r = *it;
if (target->hasAttr(r.type, RelocAttrBits::SUBTRAHEND)) {
// Skip over the following UNSIGNED relocation -- it's just there as the
// minuend, and doesn't have the usual UNSIGNED semantics. We don't want
// to emit rebase opcodes for it.
it = std::next(it);
assert(isa<Defined>(it->referent.dyn_cast<lld::macho::Symbol *>()));
continue;
}
if (auto *sym = r.referent.dyn_cast<lld::macho::Symbol *>()) {
if (auto *undefined = dyn_cast<Undefined>(sym))
treatUndefinedSymbol(*undefined);

7
lld/test/MachO/reloc-subtractor.s
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@ -3,9 +3,9 @@
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/x86_64.o
# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t/arm64.o
# RUN: %lld -lSystem %t/x86_64.o -o %t/x86_64
# RUN: llvm-objdump --syms --full-contents %t/x86_64 | FileCheck %s
# RUN: llvm-objdump --syms --full-contents --rebase %t/x86_64 | FileCheck %s
# RUN: %lld -arch arm64 -lSystem %t/arm64.o -o %t/arm64
# RUN: llvm-objdump --syms --full-contents %t/arm64 | FileCheck %s
# RUN: llvm-objdump --syms --full-contents --rebase %t/arm64 | FileCheck %s
# CHECK-LABEL: SYMBOL TABLE:
# CHECK: {{0*}}[[#%x, SUB1ADDR:]] l {{.*}} __DATA,__data _sub1
@ -17,6 +17,9 @@
# CHECK-NEXT: [[#SUB2ADDR]] f0ffffff
# CHECK-NEXT: [[#SUB3ADDR]] 10000000 00000000
# CHECK-NEXT: [[#SUB4ADDR]] f0ffffff ffffffff
# CHECK: Rebase table:
# CHECK-NEXT: segment section address type
# CHECK-EMPTY:
.globl _main, _subtrahend_1, _subtrahend_2, _minued1, _minued2