[BPF] Permit all user instructed offset relocatiions

[yonghong-song]

Summary

Currently, not all user specified relocations
(with clang intrinsic __builtin_preserve_access_index())
will turn into relocations.

In the current implementation, a builtin_preserve_access_index()
chain is turned into relocation only if the result of the clang
intrinsic is used in a function call or a nonzero offset computation
of getelementptr. For all other cases, the relocatiion request
is ignored and the builtin_preserve_access_index() is turned
into regular getelementptr instructions.
The main reason is to mimic bpf_probe_read() requirement.

But there are other use cases where relocatable offset is
generated but not used for bpf_probe_read(). This patch
relaxed previous constraints when to generate relocations.
Now, all user __builtin_preserve_access_index() will have
relocations generated.

[ast]

thanks for the quick fix.

llvm/lib/Target/BPF/BPFAbstractMemberAccess.cpp

if (auto *BI = dyn_cast<BitCastInst>(Inst)) {
       } else {
         BaseAICalls[Call] = Kind;
       }
     } else if (auto *GI = dyn_cast<GetElementPtrInst>(Inst)) {
       if (GI->hasAllZeroIndices())
         traceGEP(GI, Call, Kind, ParentMeta, ParentAI);
       else
         BaseAICalls[Call] = Kind;
+    } else {
+      BaseAICalls[Call] = Kind;
     }
   }
 }
 
 void BPFAbstractMemberAccess::traceBitCast(BitCastInst *BitCast,
                                            CallInst *Parent, uint32_t Kind,
                                            const MDNode *ParentMeta,
                                            uint32_t ParentAI) {
if (auto *BI = dyn_cast<BitCastInst>(Inst)) {
       } else {
         BaseAICalls[Parent] = Kind;
       }
     } else if (auto *GI = dyn_cast<GetElementPtrInst>(Inst)) {
       if (GI->hasAllZeroIndices())
         traceGEP(GI, Parent, Kind, ParentMeta, ParentAI);
       else
         BaseAICalls[Parent] = Kind;
+    } else {
+      BaseAICalls[Parent] = Kind;
     }
   }
 }
 
 void BPFAbstractMemberAccess::traceGEP(GetElementPtrInst *GEP, CallInst *Parent,
                                        uint32_t Kind, const MDNode *ParentMeta,
                                        uint32_t ParentAI) {
   for (User *U : GEP->users()) {
if (auto *BI = dyn_cast<BitCastInst>(Inst)) {
       } else {
         BaseAICalls[Parent] = Kind;
       }
     } else if (auto *GI = dyn_cast<GetElementPtrInst>(Inst)) {
       if (GI->hasAllZeroIndices())
         traceGEP(GI, Parent, Kind, ParentMeta, ParentAI);
       else
         BaseAICalls[Parent] = Kind;
+    } else {
+      BaseAICalls[Parent] = Kind;
     }
   }
 }
 
 void BPFAbstractMemberAccess::collectAICallChains(Module &M, Function &F) {
   AIChain.clear();
   BaseAICalls.clear();
 

llvm/test/CodeGen/BPF/CORE/offset-reloc-end-load.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck %s
+;
+; Source Code:
+;   #define _(x) (__builtin_preserve_access_index(x))
+;   struct s {int a; int b;};
+;   int test(struct s *arg) { return *(const int *)_(&arg->b); }
+; Compiler flag to generate IR:
+;   clang -target bpf -S -O2 -g -emit-llvm test.c
+
+%struct.s = type { i32, i32 }
+
+; Function Attrs: nounwind readonly
+define dso_local i32 @test(%struct.s* readonly %arg) local_unnamed_addr #0 !dbg !11 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.s* %arg, metadata !20, metadata !DIExpression()), !dbg !21
+  %0 = tail call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s* %arg, i32 1, i32 1), !dbg !22, !llvm.preserve.access.index !15
+  %1 = load i32, i32* %0, align 4, !dbg !23, !tbaa !24
+  ret i32 %1, !dbg !28
+}
+
+; CHECK-LABEL: test
+; CHECK:       r2 = 4
+; CHECK:       r1 += r2
+; CHECK:       r0 = *(u32 *)(r1 + 0)
+; CHECK:       exit
+;
+; CHECK:       .long   1                       # BTF_KIND_STRUCT(id = 2)
+;
+; CHECK:       .byte   115                     # string offset=1
+; CHECK:       .ascii  ".text"                 # string offset=20
+; CHECK:       .ascii  "0:1"                   # string offset=26
+;
+; CHECK:       .long   12                      # OffsetReloc
+; CHECK-NEXT:  .long   20                      # Offset reloc section string offset=20
+; CHECK-NEXT:  .long   1
+; CHECK-NEXT:  .long   .Ltmp{{[0-9]+}}
+; CHECK-NEXT:  .long   2
+; CHECK-NEXT:  .long   26
+
+; Function Attrs: nounwind readnone
+declare i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s*, i32, i32) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind readonly "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "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" }
+attributes #1 = { nounwind readnone }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (https://github.com/llvm/llvm-project.git 6e353b4df3aa452ed4741a5e5caea02b1a876d8c)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/core")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !5, size: 64)
+!5 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !6)
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !{i32 2, !"Dwarf Version", i32 4}
+!8 = !{i32 2, !"Debug Info Version", i32 3}
+!9 = !{i32 1, !"wchar_size", i32 4}
+!10 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git 6e353b4df3aa452ed4741a5e5caea02b1a876d8c)"}
+!11 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 3, type: !12, scopeLine: 3, flags: DIFlagPrototyped, isDefinition: true, isOptimized: true, unit: !0, retainedNodes: !19)
+!12 = !DISubroutineType(types: !13)
+!13 = !{!6, !14}
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !15, size: 64)
+!15 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s", file: !1, line: 2, size: 64, elements: !16)
+!16 = !{!17, !18}
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !15, file: !1, line: 2, baseType: !6, size: 32)
+!18 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !15, file: !1, line: 2, baseType: !6, size: 32, offset: 32)
+!19 = !{!20}
+!20 = !DILocalVariable(name: "arg", arg: 1, scope: !11, file: !1, line: 3, type: !14)
+!21 = !DILocation(line: 0, scope: !11)
+!22 = !DILocation(line: 3, column: 48, scope: !11)
+!23 = !DILocation(line: 3, column: 34, scope: !11)
+!24 = !{!25, !25, i64 0}
+!25 = !{!"int", !26, i64 0}
+!26 = !{!"omnipotent char", !27, i64 0}
+!27 = !{!"Simple C/C++ TBAA"}
+!28 = !DILocation(line: 3, column: 27, scope: !11)

llvm/test/CodeGen/BPF/CORE/offset-reloc-end-ret.ll

+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck %s 
+;
+; Source Code:
+;   #define _(x) (__builtin_preserve_access_index(x))
+;   struct s {int a; int b;};
+;   const void *test(struct s *arg) { return _(&arg->b); }
+; Compiler flag to generate IR:
+;   clang -target bpf -S -O2 -g -emit-llvm test.c
+
+%struct.s = type { i32, i32 }
+
+; Function Attrs: nounwind readnone
+define dso_local i8* @test(%struct.s* readnone %arg) local_unnamed_addr #0 !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata %struct.s* %arg, metadata !19, metadata !DIExpression()), !dbg !20
+  %0 = tail call i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s* %arg, i32 1, i32 1), !dbg !21, !llvm.preserve.access.index !13
+  %1 = bitcast i32* %0 to i8*, !dbg !21
+  ret i8* %1, !dbg !22
+}
+
+; CHECK-LABEL: test
+; CHECK:       r0 = r1
+; CHECK:       r1 = 4
+; CHECK:       r0 += r1
+; CHECK:       exit
+;
+; CHECK:       .long   1                       # BTF_KIND_STRUCT(id = 2)
+;
+; CHECK:       .byte   115                     # string offset=1
+; CHECK:       .ascii  ".text"                 # string offset=20
+; CHECK:       .ascii  "0:1"                   # string offset=63
+;
+; CHECK:       .long   12                      # OffsetReloc
+; CHECK-NEXT:  .long   20                      # Offset reloc section string offset=20
+; CHECK-NEXT:  .long   1
+; CHECK-NEXT:  .long   .Ltmp{{[0-9]+}}
+; CHECK-NEXT:  .long   2
+; CHECK-NEXT:  .long   63
+
+; Function Attrs: nounwind readnone
+declare i32* @llvm.preserve.struct.access.index.p0i32.p0s_struct.ss(%struct.s*, i32, i32) #1
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #2
+
+attributes #0 = { nounwind readnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "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" }
+attributes #1 = { nounwind readnone }
+attributes #2 = { nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0 (https://github.com/llvm/llvm-project.git 6e353b4df3aa452ed4741a5e5caea02b1a876d8c)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/core")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 10.0.0 (https://github.com/llvm/llvm-project.git 6e353b4df3aa452ed4741a5e5caea02b1a876d8c)"}
+!7 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 3, type: !8, scopeLine: 3, flags: DIFlagPrototyped, isDefinition: true, isOptimized: true, unit: !0, retainedNodes: !18)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !12}
+!10 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !11, size: 64)
+!11 = !DIDerivedType(tag: DW_TAG_const_type, baseType: null)
+!12 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !13, size: 64)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s", file: !1, line: 2, size: 64, elements: !14)
+!14 = !{!15, !17}
+!15 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !13, file: !1, line: 2, baseType: !16, size: 32)
+!16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !13, file: !1, line: 2, baseType: !16, size: 32, offset: 32)
+!18 = !{!19}
+!19 = !DILocalVariable(name: "arg", arg: 1, scope: !7, file: !1, line: 3, type: !12)
+!20 = !DILocation(line: 0, scope: !7)
+!21 = !DILocation(line: 3, column: 42, scope: !7)
+!22 = !DILocation(line: 3, column: 35, scope: !7)