[PATCH] D81888: [lld-macho] Make sure ZeroFill sections are at the end of their segments
Jez Ng via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Mon Jun 15 15:29:57 PDT 2020
int3 created this revision.
int3 added a reviewer: lld-macho.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.
ld64 does this, and references an internal rdar:// number as an explanation. No
idea what that rdar issue is, but in practice, it seems that not putting a BSS
section at the end can cause subsequent sections in the same segment to be
overwritten with zeroes.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D81888
Files:
lld/MachO/Writer.cpp
lld/test/MachO/bss.s
Index: lld/test/MachO/bss.s
===================================================================
--- lld/test/MachO/bss.s
+++ lld/test/MachO/bss.s
@@ -3,9 +3,27 @@
# RUN: lld -flavor darwinnew -arch x86_64 -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 that __bss takes up zero file size, is at file offset zero, and
+## appears at the end of its segment.
-# CHECK: Name: __bss
+# CHECK: Index: 1
+# CHECK-NEXT: Name: __data
+# CHECK-NEXT: Segment: __DATA
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Size: 0x8
+# CHECK-NEXT: Offset: 4096
+# CHECK-NEXT: Alignment: 0
+# CHECK-NEXT: RelocationOffset: 0x0
+# CHECK-NEXT: RelocationCount: 0
+# CHECK-NEXT: Type: Regular (0x0)
+# CHECK-NEXT: Attributes [ (0x0)
+# CHECK-NEXT: ]
+# CHECK-NEXT: Reserved1: 0x0
+# CHECK-NEXT: Reserved2: 0x0
+# CHECK-NEXT: Reserved3: 0x0
+
+# CHECK: Index: 2
+# CHECK-NEXT: Name: __bss
# CHECK-NEXT: Segment: __DATA
# CHECK-NEXT: Address:
# CHECK-NEXT: Size: 0x4
@@ -23,9 +41,9 @@
# CHECK: Name: __DATA
# CHECK-NEXT: Size:
# CHECK-NEXT: vmaddr:
-# CHECK-NEXT: vmsize: 0x4
+# CHECK-NEXT: vmsize: 0xC
# CHECK-NEXT: fileoff:
-# CHECK-NEXT: filesize: 0
+# CHECK-NEXT: filesize: 8
.globl _main
@@ -36,3 +54,6 @@
.bss
.zero 4
+
+.data
+.quad 0x1234
Index: lld/MachO/Writer.cpp
===================================================================
--- lld/MachO/Writer.cpp
+++ lld/MachO/Writer.cpp
@@ -351,6 +351,10 @@
StringRef segname = osec->parent->name;
if (segname == segment_names::text && osec->name == section_names::header)
return std::numeric_limits<int>::min();
+ // ZeroFill sections must always be the at the end of their segments,
+ // otherwise subsequent sections may get overwritten with zeroes at runtime.
+ if (isZeroFill(osec->flags))
+ return std::numeric_limits<int>::max();
return 0;
}
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