[lld] 74871cd - [lld-macho] Ensure __bss sections we output have file offset of zero
Jez Ng via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 17 20:41:58 PDT 2020
Author: Jez Ng
Date: 2020-06-17T20:41:28-07:00
New Revision: 74871cdad729aa5a5f39804214e5872f8e418d8e
URL: https://github.com/llvm/llvm-project/commit/74871cdad729aa5a5f39804214e5872f8e418d8e
DIFF: https://github.com/llvm/llvm-project/commit/74871cdad729aa5a5f39804214e5872f8e418d8e.diff
LOG: [lld-macho] Ensure __bss sections we output have file offset of zero
Summary:
llvm-mc emits `__bss` sections with an offset of zero, but we weren't expecting
that in our input, so we were copying non-zero data from the start of the file and
putting it in `__bss`, with obviously undesirable runtime results. (It appears that
the kernel will copy those nonzero bytes as long as the offset is nonzero, regardless
of whether S_ZERO_FILL is set.)
I debated on whether to make a special ZeroFillSection -- separate from a
regular InputSection -- but it seemed like too much work for now. But I'm happy
to refactor if anyone feels strongly about having it as a separate class.
Depends on D80857.
Reviewers: ruiu, pcc, MaskRay, smeenai, alexshap, gkm, Ktwu, christylee
Reviewed By: smeenai
Subscribers: llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D80859
Added:
lld/test/MachO/bss.s
Modified:
lld/MachO/InputFiles.cpp
lld/MachO/InputSection.cpp
lld/MachO/InputSection.h
lld/MachO/SyntheticSections.cpp
lld/MachO/Writer.cpp
Removed:
################################################################################
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
index 071d7383949c..46fe82f98822 100644
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -137,7 +137,8 @@ void InputFile::parseSections(ArrayRef<section_64> sections) {
isec->file = this;
isec->name = StringRef(sec.sectname, strnlen(sec.sectname, 16));
isec->segname = StringRef(sec.segname, strnlen(sec.segname, 16));
- isec->data = {buf + sec.offset, static_cast<size_t>(sec.size)};
+ isec->data = {isZeroFill(sec.flags) ? nullptr : buf + sec.offset,
+ static_cast<size_t>(sec.size)};
if (sec.align >= 32)
error("alignment " + std::to_string(sec.align) + " of section " +
isec->name + " is too large");
diff --git a/lld/MachO/InputSection.cpp b/lld/MachO/InputSection.cpp
index 327fd419b613..72d489283051 100644
--- a/lld/MachO/InputSection.cpp
+++ b/lld/MachO/InputSection.cpp
@@ -28,8 +28,10 @@ uint64_t InputSection::getFileOffset() const {
uint64_t InputSection::getVA() const { return parent->addr + outSecOff; }
void InputSection::writeTo(uint8_t *buf) {
- if (!data.empty())
- memcpy(buf, data.data(), data.size());
+ if (getFileSize() == 0)
+ return;
+
+ memcpy(buf, data.data(), data.size());
for (Reloc &r : relocs) {
uint64_t va = 0;
diff --git a/lld/MachO/InputSection.h b/lld/MachO/InputSection.h
index 8b0e31dae0ff..96ae0cbe6ea4 100644
--- a/lld/MachO/InputSection.h
+++ b/lld/MachO/InputSection.h
@@ -35,11 +35,17 @@ struct Reloc {
llvm::PointerUnion<Symbol *, InputSection *> target;
};
+inline bool isZeroFill(uint8_t flags) {
+ return (flags & llvm::MachO::SECTION_TYPE) == llvm::MachO::S_ZEROFILL;
+}
+
class InputSection {
public:
virtual ~InputSection() = default;
virtual uint64_t getSize() const { return data.size(); }
- virtual uint64_t getFileSize() const { return getSize(); }
+ virtual uint64_t getFileSize() const {
+ return isZeroFill(flags) ? 0 : getSize();
+ }
uint64_t getFileOffset() const;
uint64_t getVA() const;
diff --git a/lld/MachO/SyntheticSections.cpp b/lld/MachO/SyntheticSections.cpp
index 7237aa65331e..f84a603f521e 100644
--- a/lld/MachO/SyntheticSections.cpp
+++ b/lld/MachO/SyntheticSections.cpp
@@ -17,6 +17,7 @@
#include "Writer.h"
#include "lld/Common/ErrorHandler.h"
+#include "lld/Common/Memory.h"
#include "llvm/Support/EndianStream.h"
#include "llvm/Support/LEB128.h"
@@ -210,6 +211,9 @@ void StubHelperSection::setup() {
ImageLoaderCacheSection::ImageLoaderCacheSection() {
segname = segment_names::data;
name = "__data";
+ uint8_t *arr = bAlloc.Allocate<uint8_t>(WordSize);
+ memset(arr, 0, WordSize);
+ data = {arr, WordSize};
}
LazyPointerSection::LazyPointerSection()
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
index def82b578774..0fcde15699c3 100644
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -416,7 +416,7 @@ void Writer::assignAddresses(OutputSegment *seg) {
addr = alignTo(addr, section->align);
fileOff = alignTo(fileOff, section->align);
section->addr = addr;
- section->fileOff = fileOff;
+ section->fileOff = isZeroFill(section->flags) ? 0 : fileOff;
section->finalize();
addr += section->getSize();
diff --git a/lld/test/MachO/bss.s b/lld/test/MachO/bss.s
new file mode 100644
index 000000000000..7e7af12175f4
--- /dev/null
+++ b/lld/test/MachO/bss.s
@@ -0,0 +1,38 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
+# RUN: lld -flavor darwinnew -o %t %t.o
+# RUN: llvm-readobj --section-headers --macho-segment %t | FileCheck %s
+
+## Check that __bss takes up zero file size and is at file offset zero.
+
+# CHECK: Name: __bss
+# CHECK-NEXT: Segment: __DATA
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Size: 0x4
+# CHECK-NEXT: Offset: 0
+# CHECK-NEXT: Alignment: 0
+# CHECK-NEXT: RelocationOffset: 0x0
+# CHECK-NEXT: RelocationCount: 0
+# CHECK-NEXT: Type: ZeroFill (0x1)
+# CHECK-NEXT: Attributes [ (0x0)
+# CHECK-NEXT: ]
+# CHECK-NEXT: Reserved1: 0x0
+# CHECK-NEXT: Reserved2: 0x0
+# CHECK-NEXT: Reserved3: 0x0
+
+# CHECK: Name: __DATA
+# CHECK-NEXT: Size:
+# CHECK-NEXT: vmaddr:
+# CHECK-NEXT: vmsize: 0x4
+# CHECK-NEXT: fileoff:
+# CHECK-NEXT: filesize: 0
+
+.globl _main
+
+.text
+_main:
+ movq $0, %rax
+ retq
+
+.bss
+.zero 4
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