[BPF] generate R_BPF_NONE relocation for BTF DataSec variables

[yonghong-song]

Summary

The variables in BTF DataSec type encode in-section offset.
R_BPF_NONE should be generated instead of R_BPF_64_32.

lib/Target/BPF/MCTargetDesc/BPFELFObjectWriter.cpp

unsigned BPFELFObjectWriter::getRelocType(MCContext &Ctx, const MCValue &Target,
   case FK_SecRel_8:
     return ELF::R_BPF_64_64;
   case FK_PCRel_4:
   case FK_SecRel_4:
     return ELF::R_BPF_64_32;
   case FK_Data_8:
     return ELF::R_BPF_64_64;
   case FK_Data_4:
+    if (const MCSymbolRefExpr *A = Target.getSymA()) {
+      const MCSymbol &Sym = A->getSymbol();
+
+      if (Sym.isDefined()) {
+        MCSection &Section = Sym.getSection();
+        const MCSectionELF *SectionELF = dyn_cast<MCSectionELF>(&Section);
+        assert(SectionELF && "Null section for reloc symbol");
+
+        unsigned Flags = SectionELF->getFlags();
+
+        if (Sym.isTemporary()) {
           // .BTF.ext generates FK_Data_4 relocations for
           // insn offset by creating temporary labels.
           // The insn offset is within the code section and
           // already been fulfilled by applyFixup(). No
           // further relocation is needed.
-    if (const MCSymbolRefExpr *A = Target.getSymA()) {
-      if (A->getSymbol().isTemporary()) {
-        MCSection &Section = A->getSymbol().getSection();
-        const MCSectionELF *SectionELF = dyn_cast<MCSectionELF>(&Section);
-        assert(SectionELF && "Null section for reloc symbol");
-
           // The reloc symbol should be in text section.
-        unsigned Flags = SectionELF->getFlags();
           if ((Flags & ELF::SHF_ALLOC) && (Flags & ELF::SHF_EXECINSTR))
             return ELF::R_BPF_NONE;
+        } else {
+          // .BTF generates FK_Data_4 relocations for variable
+          // offset in DataSec kind. Similar to the above .BTF.ext
+          // insn offset, no further relocation is needed.
+          // The reloc symbol should be in data section.
+          if ((Flags & ELF::SHF_ALLOC) && (Flags & ELF::SHF_WRITE))
+            return ELF::R_BPF_NONE;
+        }
       }
     }
     return ELF::R_BPF_64_32;
   }
 }
 
 std::unique_ptr<MCObjectTargetWriter>
 llvm::createBPFELFObjectWriter(uint8_t OSABI) {
   return llvm::make_unique<BPFELFObjectWriter>(OSABI);
 }

test/CodeGen/BPF/reloc-btf-2.ll

+; RUN: llc -march=bpfel -filetype=obj < %s | llvm-objdump -r - | FileCheck --check-prefix=CHECK-RELOC %s
+; RUN: llc -march=bpfeb -filetype=obj < %s | llvm-objdump -r - | FileCheck --check-prefix=CHECK-RELOC %s
+
+; source code:
+;   int g __attribute__((section("ids"))) = 4;
+;   static volatile int s = 0;
+;   int test() {
+;     return g + s;
+;   }
+; compilation flag:
+;   clang -target bpf -g -O2 -emit-llvm -S test.c
+
+@g = dso_local local_unnamed_addr global i32 4, section "ids", align 4, !dbg !0
+@s = internal global i32 0, align 4, !dbg !6
+
+; Function Attrs: norecurse nounwind
+define dso_local i32 @test() local_unnamed_addr #0 !dbg !14 {
+  %1 = load i32, i32* @g, align 4, !dbg !17, !tbaa !18
+  %2 = load volatile i32, i32* @s, align 4, !dbg !22, !tbaa !18
+  %3 = add nsw i32 %2, %1, !dbg !23
+  ret i32 %3, !dbg !24
+}
+
+; CHECK-RELOC: file format ELF64-BPF
+; CHECK-RELOC: RELOCATION RECORDS FOR [.BTF]:
+; CHECK-RELOC: R_BPF_NONE .bss
+; CHECK-RELOC: R_BPF_NONE g
+; CHECK-RELOC: RELOCATION RECORDS FOR [.BTF.ext]:
+
+attributes #0 = { norecurse nounwind "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!10, !11, !12}
+!llvm.ident = !{!13}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "g", scope: !2, file: !3, line: 1, type: !9, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 9.0.0 (trunk 360739) (llvm/trunk 360747)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, globals: !5, nameTableKind: None)
+!3 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/llvm/relocation")
+!4 = !{}
+!5 = !{!0, !6}
+!6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
+!7 = distinct !DIGlobalVariable(name: "s", scope: !2, file: !3, line: 2, type: !8, isLocal: true, isDefinition: true)
+!8 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !9)
+!9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!10 = !{i32 2, !"Dwarf Version", i32 4}
+!11 = !{i32 2, !"Debug Info Version", i32 3}
+!12 = !{i32 1, !"wchar_size", i32 4}
+!13 = !{!"clang version 9.0.0 (trunk 360739) (llvm/trunk 360747)"}
+!14 = distinct !DISubprogram(name: "test", scope: !3, file: !3, line: 3, type: !15, scopeLine: 3, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !4)
+!15 = !DISubroutineType(types: !16)
+!16 = !{!9}
+!17 = !DILocation(line: 4, column: 10, scope: !14)
+!18 = !{!19, !19, i64 0}
+!19 = !{!"int", !20, i64 0}
+!20 = !{!"omnipotent char", !21, i64 0}
+!21 = !{!"Simple C/C++ TBAA"}
+!22 = !DILocation(line: 4, column: 14, scope: !14)
+!23 = !DILocation(line: 4, column: 12, scope: !14)
+!24 = !DILocation(line: 4, column: 3, scope: !14)