lld-link: align sections to 16 bytes if referenced from the gfids table

Summary:
Control flow guard works best when targets it checks are 16-byte aligned.
Microsoft's link.exe helps ensure this by aligning code from sections
that are referenced from the gfids table to 16 bytes when linking with
-guard:cf, even if the original section specifies a smaller alignment.
This change implements that behavior in lld-link.

See https://crbug.com/857012 for more details.

Reviewers: ruiu, hans, thakis, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48690

llvm-svn: 335864

3 files changed, 101 insertions, 0 deletions

diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index efb471ecaa0..a9950e4cc85 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -1045,6 +1045,11 @@ void Writer::createGuardCFTables() {
   if (Config->Entry)
     addSymbolToRVASet(AddressTakenSyms, cast<Defined>(Config->Entry));
 
+  // Ensure sections referenced in the gfid table are 16-byte aligned.
+  for (const ChunkAndOffset &C : AddressTakenSyms)
+    if (C.InputChunk->Alignment < 16)
+      C.InputChunk->Alignment = 16;
+
   maybeAddRVATable(std::move(AddressTakenSyms), "__guard_fids_table",
                    "__guard_fids_count");
 
diff --git a/lld/test/COFF/Inputs/guardcf-align-foobar.yaml b/lld/test/COFF/Inputs/guardcf-align-foobar.yaml
new file mode 100644
index 00000000000..7878c459a4f
--- /dev/null
+++ b/lld/test/COFF/Inputs/guardcf-align-foobar.yaml
@@ -0,0 +1,51 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .text.foo
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     31C0C3
+  - Name:            .text.bar
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     FFE1
+symbols:
+  - Name:            .text.foo
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          3
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3963538403
+      Number:          1
+  - Name:            .text.bar
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          2
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        1143549852
+      Number:          2
+  - Name:            foo
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            bar
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/lld/test/COFF/guardcf-align.s b/lld/test/COFF/guardcf-align.s
new file mode 100644
index 00000000000..a0caabc92b0
--- /dev/null
+++ b/lld/test/COFF/guardcf-align.s
@@ -0,0 +1,45 @@
+# RUN: llvm-mc -triple x86_64-windows-msvc -filetype=obj -o %t.obj %s
+# RUN: yaml2obj < %p/Inputs/guardcf-align-foobar.yaml \
+# RUN:     > %T/guardcf-align-foobar.obj
+# RUN: lld-link -out:%T/guardcf-align.exe -entry:main -guard:cf \
+# RUN:     %t.obj %T/guardcf-align-foobar.obj
+# RUN: llvm-readobj -coff-load-config %T/guardcf-align.exe | FileCheck %s
+
+# Check that the gfids table contains at least one entry that ends in 0
+# and no entries that end in something other than 0.
+# CHECK: GuardFidTable [
+# CHECK-NOT: 0x{{[0-9A-Fa-f]+[^0]$}}
+# CHECK: 0x{{[0-9A-Fa-f]+0$}}
+# CHECK-NOT: 0x{{[0-9A-Fa-f]+[^0]$}}
+# CHECK: ]
+
+# @feat.00 and _load_config_used to indicate we have gfids.
+        .def     @feat.00;
+        .scl    3;
+        .type   0;
+        .endef
+        .globl  @feat.00
+@feat.00 = 0x801
+
+        .section .rdata,"dr"
+.globl _load_config_used
+_load_config_used:
+        .long 256
+        .fill 124, 1, 0
+        .quad __guard_fids_table
+        .quad __guard_fids_count
+        .long __guard_flags
+        .fill 128, 1, 0
+
+# Functions that are called indirectly.
+        .section        .gfids$y,"dr"
+        .symidx foo
+
+
+        .section .text,"rx"
+        .global main
+main:
+        movq foo, %rcx
+        xorq %rax, %rax
+        callq bar
+        retq