[LLD][COFF] Set __guard_flags to CF_INSTRUMENTED if any object is instrumented #115374
Conversation
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@llvm/pr-subscribers-lld @llvm/pr-subscribers-platform-windows Author: Jacek Caban (cjacek) ChangesFull diff: https://github.com/llvm/llvm-project/pull/115374.diff 2 Files Affected:
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 71ee5ce4685553..58d0700c52aaf4 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -1217,8 +1217,7 @@ void Writer::createMiscChunks() {
createSEHTable();
// Create /guard:cf tables if requested.
- if (config->guardCF != GuardCFLevel::Off)
- createGuardCFTables();
+ createGuardCFTables();
if (isArm64EC(config->machine))
createECChunks();
@@ -1979,6 +1978,20 @@ void Writer::markSymbolsWithRelocations(ObjFile *file,
void Writer::createGuardCFTables() {
Configuration *config = &ctx.config;
+ if (config->guardCF == GuardCFLevel::Off) {
+ // MSVC marks the entire image as instrumented if any input object was built
+ // with /guard:cf.
+ for (ObjFile *file : ctx.objFileInstances) {
+ if (file->hasGuardCF()) {
+ Symbol *flagSym = ctx.symtab.findUnderscore("__guard_flags");
+ cast<DefinedAbsolute>(flagSym)->setVA(
+ uint32_t(GuardFlags::CF_INSTRUMENTED));
+ break;
+ }
+ }
+ return;
+ }
+
SymbolRVASet addressTakenSyms;
SymbolRVASet giatsRVASet;
std::vector<Symbol *> giatsSymbols;
diff --git a/lld/test/COFF/cfguard-off-instrumented.s b/lld/test/COFF/cfguard-off-instrumented.s
new file mode 100644
index 00000000000000..4bd81d99568927
--- /dev/null
+++ b/lld/test/COFF/cfguard-off-instrumented.s
@@ -0,0 +1,22 @@
+// Verify that __guard_flags is set to CF_INSTRUMENTED if CF guard is disabled,
+// but the input object was built with CF guard.
+
+// REQUIRES: x86
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows %s -o %t.obj
+// RUN: lld-link -out:%t1.dll %t.obj -dll -noentry
+// RUN: lld-link -out:%t2.dll %t.obj -dll -noentry -guard:no
+
+// RUN: llvm-readobj --hex-dump=.test %t1.dll | FileCheck %s
+// RUN: llvm-readobj --hex-dump=.test %t2.dll | FileCheck %s
+// CHECK: 0x180001000 00010000
+
+ .def @feat.00;
+ .scl 3;
+ .type 0;
+ .endef
+ .globl @feat.00
+@feat.00 = 0x800
+
+ .section .test, "r"
+ .long __guard_flags
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@llvm/pr-subscribers-lld-coff Author: Jacek Caban (cjacek) ChangesFull diff: https://github.com/llvm/llvm-project/pull/115374.diff 2 Files Affected:
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 71ee5ce4685553..58d0700c52aaf4 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -1217,8 +1217,7 @@ void Writer::createMiscChunks() {
createSEHTable();
// Create /guard:cf tables if requested.
- if (config->guardCF != GuardCFLevel::Off)
- createGuardCFTables();
+ createGuardCFTables();
if (isArm64EC(config->machine))
createECChunks();
@@ -1979,6 +1978,20 @@ void Writer::markSymbolsWithRelocations(ObjFile *file,
void Writer::createGuardCFTables() {
Configuration *config = &ctx.config;
+ if (config->guardCF == GuardCFLevel::Off) {
+ // MSVC marks the entire image as instrumented if any input object was built
+ // with /guard:cf.
+ for (ObjFile *file : ctx.objFileInstances) {
+ if (file->hasGuardCF()) {
+ Symbol *flagSym = ctx.symtab.findUnderscore("__guard_flags");
+ cast<DefinedAbsolute>(flagSym)->setVA(
+ uint32_t(GuardFlags::CF_INSTRUMENTED));
+ break;
+ }
+ }
+ return;
+ }
+
SymbolRVASet addressTakenSyms;
SymbolRVASet giatsRVASet;
std::vector<Symbol *> giatsSymbols;
diff --git a/lld/test/COFF/cfguard-off-instrumented.s b/lld/test/COFF/cfguard-off-instrumented.s
new file mode 100644
index 00000000000000..4bd81d99568927
--- /dev/null
+++ b/lld/test/COFF/cfguard-off-instrumented.s
@@ -0,0 +1,22 @@
+// Verify that __guard_flags is set to CF_INSTRUMENTED if CF guard is disabled,
+// but the input object was built with CF guard.
+
+// REQUIRES: x86
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-windows %s -o %t.obj
+// RUN: lld-link -out:%t1.dll %t.obj -dll -noentry
+// RUN: lld-link -out:%t2.dll %t.obj -dll -noentry -guard:no
+
+// RUN: llvm-readobj --hex-dump=.test %t1.dll | FileCheck %s
+// RUN: llvm-readobj --hex-dump=.test %t2.dll | FileCheck %s
+// CHECK: 0x180001000 00010000
+
+ .def @feat.00;
+ .scl 3;
+ .type 0;
+ .endef
+ .globl @feat.00
+@feat.00 = 0x800
+
+ .section .test, "r"
+ .long __guard_flags
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I noticed this issue while examining load config differences between LLD and MSVC for ARM64EC. After some experimentation, I found that this behavior isn’t unique to ARM64EC: Since LLD has functioned well without this behavior so far, it’s unlikely to matter in practice. However, implementing it for compatibility seems straightforward enough. |
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Just speculating here without having tested the patch: I'm not sure if this is fine for llvm-mingw, which builds mingw-w64 with CFG. It looks like this patch would effectively set Though LLD may have some fallbacks to heuristically add functions to the list of valid call targets. I don't know, maybe it is possible to craft some object files in assembly that would trip CFG? I recall there being a problem with OpenSSL that enabling CFG with llvm-mingw will crash when calling into some objects built with NASM. |
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Note that this patch only concerns the |
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