[lld-macho] Fix bug in makeSyntheticInputSection when -dead_strip flag is specified (#86878)
Previously, `makeSyntheticInputSection` would create a new
`ConcatInputSection` without setting `live` explicitly for it. Without
`-dead_strip` this would be OK since `live` would default to `true`.
However, with `-dead_strip`, `live` would default to false, and it would
remain set to `false`.
This hasn't resulted in any issues so far since no code paths that
exposed this issue were present.
However a recent change - ObjC relative method lists
(https://github.com/llvm/llvm-project/pull/86231) exposes this issue by
creating relocations to the `SyntheticInputSection`.
When these relocations are attempted to be written, this ends up with a
crash(assert), since the `SyntheticInputSection` they refer to is marked
as dead (`live` = `false`).
With this change, we set the correct behavior - `live` will always be
`true`. We add a test case that before this change would trigger an
assert in the linker.
GitOrigin-RevId: bbfa50696e43f337f55f8bacf739683b181debd5
diff --git a/MachO/ConcatOutputSection.cpp b/MachO/ConcatOutputSection.cpp
index c5c0c8a..2794237 100644
--- a/MachO/ConcatOutputSection.cpp
+++ b/MachO/ConcatOutputSection.cpp
@@ -323,11 +323,7 @@
thunkInfo.isec =
makeSyntheticInputSection(isec->getSegName(), isec->getName());
thunkInfo.isec->parent = this;
-
- // This code runs after dead code removal. Need to set the `live` bit
- // on the thunk isec so that asserts that check that only live sections
- // get written are happy.
- thunkInfo.isec->live = true;
+ assert(thunkInfo.isec->live);
StringRef thunkName = saver().save(funcSym->getName() + ".thunk." +
std::to_string(thunkInfo.sequence++));
diff --git a/MachO/InputSection.cpp b/MachO/InputSection.cpp
index e3d7a40..5c1e07c 100644
--- a/MachO/InputSection.cpp
+++ b/MachO/InputSection.cpp
@@ -281,6 +281,9 @@
Section §ion =
*make<Section>(/*file=*/nullptr, segName, sectName, flags, /*addr=*/0);
auto isec = make<ConcatInputSection>(section, data, align);
+ // Since this is an explicitly created 'fake' input section,
+ // it should not be dead stripped.
+ isec->live = true;
section.subsections.push_back({0, isec});
return isec;
}
diff --git a/MachO/InputSection.h b/MachO/InputSection.h
index a0e6afe..0f389e5 100644
--- a/MachO/InputSection.h
+++ b/MachO/InputSection.h
@@ -149,6 +149,7 @@
};
// Initialize a fake InputSection that does not belong to any InputFile.
+// The created ConcatInputSection will always have 'live=true'
ConcatInputSection *makeSyntheticInputSection(StringRef segName,
StringRef sectName,
uint32_t flags = 0,
diff --git a/MachO/SymbolTable.cpp b/MachO/SymbolTable.cpp
index 825242f..755ff27 100644
--- a/MachO/SymbolTable.cpp
+++ b/MachO/SymbolTable.cpp
@@ -377,7 +377,7 @@
// live. Marking the isec live ensures an OutputSection is created that the
// start/end symbol can refer to.
assert(sym.isLive());
- isec->live = true;
+ assert(isec->live);
// This runs after gatherInputSections(), so need to explicitly set parent
// and add to inputSections.
diff --git a/MachO/SyntheticSections.cpp b/MachO/SyntheticSections.cpp
index 808ea8e..6f6b661 100644
--- a/MachO/SyntheticSections.cpp
+++ b/MachO/SyntheticSections.cpp
@@ -850,7 +850,7 @@
S_LITERAL_POINTERS | S_ATTR_NO_DEAD_STRIP,
ArrayRef<uint8_t>{selrefData, wordSize},
/*align=*/wordSize);
- objcSelref->live = true;
+ assert(objcSelref->live);
objcSelref->relocs.push_back({/*type=*/target->unsignedRelocType,
/*pcrel=*/false, /*length=*/3,
/*offset=*/0,
diff --git a/MachO/Writer.cpp b/MachO/Writer.cpp
index fe989de..1c05491 100644
--- a/MachO/Writer.cpp
+++ b/MachO/Writer.cpp
@@ -1375,9 +1375,7 @@
segment_names::data, section_names::data, S_REGULAR,
ArrayRef<uint8_t>{arr, target->wordSize},
/*align=*/target->wordSize);
- // References from dyld are not visible to us, so ensure this section is
- // always treated as live.
- in.imageLoaderCache->live = true;
+ assert(in.imageLoaderCache->live);
}
OutputSection *macho::firstTLVDataSection = nullptr;
diff --git a/test/MachO/objc-relative-method-lists-simple.s b/test/MachO/objc-relative-method-lists-simple.s
index 1ffec3c..5a77085 100644
--- a/test/MachO/objc-relative-method-lists-simple.s
+++ b/test/MachO/objc-relative-method-lists-simple.s
@@ -8,6 +8,10 @@
# RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -objc_relative_method_lists
# RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib | FileCheck %s --check-prefix=CHK_REL
+## Test arm64 + relative method lists + dead-strip
+# RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -objc_relative_method_lists -dead_strip
+# RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib | FileCheck %s --check-prefix=CHK_REL
+
## Test arm64 + traditional method lists (no relative offsets)
# RUN: %no-lsystem-lld a64_rel_dylib.o -o a64_rel_dylib.dylib -map a64_rel_dylib.map -dylib -arch arm64 -no_objc_relative_method_lists
# RUN: llvm-objdump --macho --objc-meta-data a64_rel_dylib.dylib | FileCheck %s --check-prefix=CHK_NO_REL
