Target: Stop assigning RELRO sections to .ldata.rel.ro.

[pcc]

Linkers do not currently support PT_GNU_RELRO for SHF_X86_64_LARGE
sections; that would require the linker to emit more than one
PT_GNU_RELRO because large sections are discontiguous by design,
and most ELF dynamic loaders do not support that (bionic appears to
support it but glibc/musl/FreeBSD/NetBSD/OpenBSD appear not to). With
current linkers these sections will end up in .ldata which results
in silently disabling RELRO. Therefore, disable SHF_X86_64_LARGE for
RELRO sections. If this ever gets supported by downstream components
in the future we could add an opt-in flag for moving these sections
to .ldata.rel.ro which would trigger the creation of a second
PT_GNU_RELRO.

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Peter Collingbourne (pcc)

Changes

Linkers do not currently support PT_GNU_RELRO for SHF_X86_64_LARGE
sections; that would require the linker to emit more than one
PT_GNU_RELRO because large sections are discontiguous by design,
and most ELF dynamic loaders do not support that (bionic appears to
support it but glibc/musl/FreeBSD/NetBSD/OpenBSD appear not to). With
current linkers these sections will end up in .ldata which results
in silently disabling RELRO. Therefore, disable SHF_X86_64_LARGE for
RELRO sections. If this ever gets supported by downstream components
in the future we could add an opt-in flag for moving these sections
to .ldata.rel.ro which would trigger the creation of a second
PT_GNU_RELRO.

---

Full diff: https://github.com/llvm/llvm-project/pull/137742.diff

3 Files Affected:

• (modified) llvm/lib/Target/TargetMachine.cpp (+13)
• (modified) llvm/test/CodeGen/X86/code-model-elf-sections.ll (+2-2)
• (modified) llvm/test/CodeGen/X86/code-model-elf.ll (+19-21)

diff --git a/llvm/lib/Target/TargetMachine.cpp b/llvm/lib/Target/TargetMachine.cpp
index 439e735e9d66e..69b6e26e602f6 100644
--- a/llvm/lib/Target/TargetMachine.cpp
+++ b/llvm/lib/Target/TargetMachine.cpp
@@ -119,6 +119,19 @@ bool TargetMachine::isLargeGlobalValue(const GlobalValue *GVal) const {
                                 GV->getName().starts_with("__start_") ||
                                 GV->getName().starts_with("__stop_")))
       return true;
+    // Linkers do not currently support PT_GNU_RELRO for SHF_X86_64_LARGE
+    // sections; that would require the linker to emit more than one
+    // PT_GNU_RELRO because large sections are discontiguous by design, and most
+    // ELF dynamic loaders do not support that (bionic appears to support it but
+    // glibc/musl/FreeBSD/NetBSD/OpenBSD appear not to). With current linkers
+    // these sections will end up in .ldata which results in silently disabling
+    // RELRO. If this ever gets supported by downstream components in the future
+    // we could add an opt-in flag for moving these sections to .ldata.rel.ro
+    // which would trigger the creation of a second PT_GNU_RELRO.
+    if (!GV->isDeclarationForLinker() &&
+        TargetLoweringObjectFile::getKindForGlobal(GV, *this)
+            .isReadOnlyWithRel())
+      return false;
     const DataLayout &DL = GV->getDataLayout();
     uint64_t Size = DL.getTypeAllocSize(GV->getValueType());
     return Size == 0 || Size > LargeDataThreshold;
diff --git a/llvm/test/CodeGen/X86/code-model-elf-sections.ll b/llvm/test/CodeGen/X86/code-model-elf-sections.ll
index fc5dac89a6b3d..f7a7ebb705ea6 100644
--- a/llvm/test/CodeGen/X86/code-model-elf-sections.ll
+++ b/llvm/test/CodeGen/X86/code-model-elf-sections.ll
@@ -66,7 +66,7 @@
 ; LARGE: .lbss {{.*}} WAl {{.*}}
 ; LARGE: .rodata {{.*}} A {{.*}}
 ; LARGE: .lrodata {{.*}} Al {{.*}}
-; LARGE: .ldata.rel.ro {{.*}} WAl {{.*}}
+; LARGE: .data.rel.ro {{.*}} WA {{.*}}
 ; LARGE: .tbss {{.*}} WAT {{.*}}
 ; LARGE: .tdata {{.*}} WAT {{.*}}
 
@@ -84,7 +84,7 @@
 ; LARGE-DS: .lbss.bss {{.*}} WAl {{.*}}
 ; LARGE-DS: .rodata {{.*}} A {{.*}}
 ; LARGE-DS: .lrodata.rodata {{.*}} Al {{.*}}
-; LARGE-DS: .ldata.rel.ro.relro {{.*}} WAl {{.*}}
+; LARGE-DS: .data.rel.ro.relro {{.*}} WA {{.*}}
 ; LARGE-DS: .tbss.tbss {{.*}} WAT {{.*}}
 ; LARGE-DS: .tdata.tdata {{.*}} WAT {{.*}}
 
diff --git a/llvm/test/CodeGen/X86/code-model-elf.ll b/llvm/test/CodeGen/X86/code-model-elf.ll
index f60f75bc26911..aa2cc3a4981a2 100644
--- a/llvm/test/CodeGen/X86/code-model-elf.ll
+++ b/llvm/test/CodeGen/X86/code-model-elf.ll
@@ -50,7 +50,7 @@ target triple = "x86_64--linux"
 
 @global_data = dso_local global [10 x i32] [i32 1, i32 2, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0], align 16
 @static_data = internal global [10 x i32] zeroinitializer, align 16
-@static_data_alias = internal constant ptr getelementptr inbounds ([10 x i32], ptr @static_data, i64 0, i64 2), align 8
+@static_data_relro = internal constant ptr getelementptr inbounds ([10 x i32], ptr @static_data, i64 0, i64 2), align 8
 @extern_data = external global [10 x i32], align 16
 @thread_data = external thread_local global i32, align 4
 @unknown_size_data = dso_local global [0 x i32] zeroinitializer, align 16
@@ -117,59 +117,57 @@ define dso_local ptr @lea_static_data() #0 {
   ret ptr @static_data
 }
 
-define dso_local ptr @lea_static_data_alias() #0 {
-; SMALL-STATIC-LABEL: lea_static_data_alias:
+define dso_local ptr @lea_static_data_relro() #0 {
+; SMALL-STATIC-LABEL: lea_static_data_relro:
 ; SMALL-STATIC:       # %bb.0:
-; SMALL-STATIC-NEXT:    movl $static_data_alias, %eax
+; SMALL-STATIC-NEXT:    movl $static_data_relro, %eax
 ; SMALL-STATIC-NEXT:    retq
 ;
-; MEDIUM-STATIC-LABEL: lea_static_data_alias:
+; MEDIUM-STATIC-LABEL: lea_static_data_relro:
 ; MEDIUM-STATIC:       # %bb.0:
-; MEDIUM-STATIC-NEXT:    movabsq $static_data_alias, %rax
+; MEDIUM-STATIC-NEXT:    movabsq $static_data_relro, %rax
 ; MEDIUM-STATIC-NEXT:    retq
 ;
-; LARGE-STATIC-LABEL: lea_static_data_alias:
+; LARGE-STATIC-LABEL: lea_static_data_relro:
 ; LARGE-STATIC:       # %bb.0:
-; LARGE-STATIC-NEXT:    movabsq $static_data_alias, %rax
+; LARGE-STATIC-NEXT:    movabsq $static_data_relro, %rax
 ; LARGE-STATIC-NEXT:    retq
 ;
-; SMALL-PIC-LABEL: lea_static_data_alias:
+; SMALL-PIC-LABEL: lea_static_data_relro:
 ; SMALL-PIC:       # %bb.0:
-; SMALL-PIC-NEXT:    leaq static_data_alias(%rip), %rax
+; SMALL-PIC-NEXT:    leaq static_data_relro(%rip), %rax
 ; SMALL-PIC-NEXT:    retq
 ;
-; MEDIUM-SMALL-DATA-PIC-LABEL: lea_static_data_alias:
+; MEDIUM-SMALL-DATA-PIC-LABEL: lea_static_data_relro:
 ; MEDIUM-SMALL-DATA-PIC:       # %bb.0:
-; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq static_data_alias(%rip), %rax
+; MEDIUM-SMALL-DATA-PIC-NEXT:    leaq static_data_relro(%rip), %rax
 ; MEDIUM-SMALL-DATA-PIC-NEXT:    retq
 ;
-; MEDIUM-PIC-LABEL: lea_static_data_alias:
+; MEDIUM-PIC-LABEL: lea_static_data_relro:
 ; MEDIUM-PIC:       # %bb.0:
-; MEDIUM-PIC-NEXT:    leaq _GLOBAL_OFFSET_TABLE_(%rip), %rcx
-; MEDIUM-PIC-NEXT:    movabsq $static_data_alias@GOTOFF, %rax
-; MEDIUM-PIC-NEXT:    addq %rcx, %rax
+; MEDIUM-PIC-NEXT:    leaq static_data_relro(%rip), %rax
 ; MEDIUM-PIC-NEXT:    retq
 ;
-; LARGE-PIC-LABEL: lea_static_data_alias:
+; LARGE-PIC-LABEL: lea_static_data_relro:
 ; LARGE-PIC:       # %bb.0:
 ; LARGE-PIC-NEXT:  .L1$pb:
 ; LARGE-PIC-NEXT:    leaq .L1$pb(%rip), %rax
 ; LARGE-PIC-NEXT:    movabsq $_GLOBAL_OFFSET_TABLE_-.L1$pb, %rcx
 ; LARGE-PIC-NEXT:    addq %rax, %rcx
-; LARGE-PIC-NEXT:    movabsq $static_data_alias@GOTOFF, %rax
+; LARGE-PIC-NEXT:    movabsq $static_data_relro@GOTOFF, %rax
 ; LARGE-PIC-NEXT:    addq %rcx, %rax
 ; LARGE-PIC-NEXT:    retq
 ;
-; LARGE-SMALL-DATA-PIC-LABEL: lea_static_data_alias:
+; LARGE-SMALL-DATA-PIC-LABEL: lea_static_data_relro:
 ; LARGE-SMALL-DATA-PIC:       # %bb.0:
 ; LARGE-SMALL-DATA-PIC-NEXT:  .L1$pb:
 ; LARGE-SMALL-DATA-PIC-NEXT:    leaq .L1$pb(%rip), %rax
 ; LARGE-SMALL-DATA-PIC-NEXT:    movabsq $_GLOBAL_OFFSET_TABLE_-.L1$pb, %rcx
 ; LARGE-SMALL-DATA-PIC-NEXT:    addq %rax, %rcx
-; LARGE-SMALL-DATA-PIC-NEXT:    movabsq $static_data_alias@GOTOFF, %rax
+; LARGE-SMALL-DATA-PIC-NEXT:    movabsq $static_data_relro@GOTOFF, %rax
 ; LARGE-SMALL-DATA-PIC-NEXT:    addq %rcx, %rax
 ; LARGE-SMALL-DATA-PIC-NEXT:    retq
-  ret ptr @static_data_alias
+  ret ptr @static_data_relro
 }
 
 define dso_local ptr @lea_global_data() #0 {