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[BPF] fix a CO-RE bitfield relocation error with >8 record alignment
Jussi Maki reported a fatal error like below for a bitfield CO-RE relocation: fatal error: error in backend: Unsupported field expression for llvm.bpf.preserve.field.info, requiring too big alignment The failure is related to kernel struct thread_struct. The following is a simplied example. Suppose we have below structure: struct t2 { int a[8]; } __attribute__((aligned(64))) __attribute__((preserve_access_index)); struct t1 { int f1:1; int f2:2; struct t2 f3; } __attribute__((preserve_access_index)); Note that struct t2 has aligned 64, which is used sometimes in the kernel to enforce cache line alignment. The above struct will be encoded into BTF and the following is what C code looks like and the struct will appear in the file like vmlinux.h. struct t2 { int a[8]; long: 64; long: 64; long: 64; long: 64; } __attribute__((preserve_access_index)); struct t1 { int f1: 1; int f2: 2; long: 61; long: 64; long: 64; long: 64; long: 64; long: 64; long: 64; long: 64; struct t2 f3; } __attribute__((preserve_access_index)); Note that after origin_source -> BTF -> new_source transition, the new source has the same memory layout as the old one but the alignment interpretation inside the compiler could be different. The bpf program will use the later explicitly padded structure as in vmlinux.h. In the above case, the compiler internal ABI alignment for new struct t1 is 16 while it is 4 for old struct t1. I didn't do a thorough investigation why the ABI alignment is 16 and I suspect it is related to anonymous padding in the above. Current BPF bitfield CO-RE handling requires alignment <= 8 so proper bitfield operatin can be performed. Therefore, alignment 16 will cause a compiler fatal error. To fix the ABI alignment >=16, let us check whether the bitfield can be held within a 8-byte-aligned range. If this is the case, we can use alignment 8. Otherwise, a fatal error will be reported. Differential Revision: https://reviews.llvm.org/D121821
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‎llvm/lib/Target/BPF/BPFAbstractMemberAccess.cpp

Lines changed: 12 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -593,10 +593,20 @@ void BPFAbstractMemberAccess::GetStorageBitRange(DIDerivedType *MemberTy,
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uint32_t &EndBitOffset) {
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uint32_t MemberBitSize = MemberTy->getSizeInBits();
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uint32_t MemberBitOffset = MemberTy->getOffsetInBits();
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if (RecordAlignment > 8) {
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// If the Bits are within an aligned 8-byte, set the RecordAlignment
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// to 8, other report the fatal error.
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if (MemberBitOffset / 64 != (MemberBitOffset + MemberBitSize) / 64)
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report_fatal_error("Unsupported field expression for llvm.bpf.preserve.field.info, "
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"requiring too big alignment");
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RecordAlignment = Align(8);
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}
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uint32_t AlignBits = RecordAlignment.value() * 8;
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if (RecordAlignment > 8 || MemberBitSize > AlignBits)
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if (MemberBitSize > AlignBits)
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report_fatal_error("Unsupported field expression for llvm.bpf.preserve.field.info, "
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"requiring too big alignment");
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"bitfield size greater than record alignment");
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StartBitOffset = MemberBitOffset & ~(AlignBits - 1);
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if ((StartBitOffset + AlignBits) < (MemberBitOffset + MemberBitSize))

‎llvm/test/CodeGen/BPF/CORE/field-reloc-bitfield-record-align16.ll

Lines changed: 105 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,105 @@
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; RUN: opt -O2 %s | llvm-dis > %t1
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; RUN: llc -filetype=asm -o - %t1 | FileCheck %s
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; Source code:
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; struct t2 {
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; int a[8];
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; long: 64;
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; long: 64;
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; long: 64;
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; long: 64;
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; };
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;
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; struct t1 {
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; int f1: 1;
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; int f2: 2;
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; long: 61;
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; long: 64;
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; long: 64;
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; long: 64;
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; long: 64;
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; long: 64;
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; long: 64;
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; long: 64;
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; struct t2 f3;
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; } __attribute__((preserve_access_index));
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;
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; struct t1 g;
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; int foo() {
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; return g.f1;
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; }
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; Compilation flag:
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; clang -target bpfel -O2 -g -S -emit-llvm -Xclang -disable-llvm-passes test.c
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; ModuleID = 'test.c'
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source_filename = "test.c"
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target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
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target triple = "bpfel"
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%struct.t1 = type { i512, %struct.t2 }
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%struct.t2 = type { [8 x i32], i256 }
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@g = dso_local global %struct.t1 zeroinitializer, align 4, !dbg !0
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; Function Attrs: nounwind
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define dso_local i32 @foo() #0 !dbg !22 {
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entry:
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%0 = call i512* @llvm.preserve.struct.access.index.p0i512.p0s_struct.t1s(%struct.t1* elementtype(%struct.t1) @g, i32 0, i32 0), !dbg !26, !llvm.preserve.access.index !5
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%bf.load = load i512, i512* %0, align 4, !dbg !26
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%bf.shl = shl i512 %bf.load, 511, !dbg !26
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%bf.ashr = ashr i512 %bf.shl, 511, !dbg !26
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%bf.cast = trunc i512 %bf.ashr to i32, !dbg !26
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ret i32 %bf.cast, !dbg !27
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}
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; CHECK: .long 68 # BTF_KIND_STRUCT(id = 4)
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; CHECK: .ascii ".text" # string offset=9
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; CHECK: .ascii "t1" # string offset=68
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; CHECK: .ascii "0:0" # string offset=105
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; CHECK: .long 16 # FieldReloc
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; CHECK: .long 9 # Field reloc section string offset=9
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; CHECK: .long 1
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; CHECK: .long .Ltmp[[#]]
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; CHECK: .long 4
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; CHECK: .long 105
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; CHECK: .long 0
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; Function Attrs: nofree nosync nounwind readnone willreturn
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declare i512* @llvm.preserve.struct.access.index.p0i512.p0s_struct.t1s(%struct.t1*, i32 immarg, i32 immarg) #1
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attributes #0 = { nounwind "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
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attributes #1 = { nofree nosync nounwind readnone willreturn }
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!llvm.dbg.cu = !{!2}
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!llvm.module.flags = !{!17, !18, !19, !20}
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!llvm.ident = !{!21}
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!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
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!1 = distinct !DIGlobalVariable(name: "g", scope: !2, file: !3, line: 23, type: !5, isLocal: false, isDefinition: true)
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!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, producer: "clang version 15.0.0 (https://github.com/llvm/llvm-project.git 468279d2d249e44ffa3535a613245b4ceb81a908)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
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!3 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/work/tests/llvm/bitfield/simple", checksumkind: CSK_MD5, checksum: "b9c80125731b87136772eec36d0b48a3")
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!4 = !{!0}
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!5 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t1", file: !3, line: 9, size: 1024, elements: !6)
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!6 = !{!7, !9, !10}
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!7 = !DIDerivedType(tag: DW_TAG_member, name: "f1", scope: !5, file: !3, line: 10, baseType: !8, size: 1, flags: DIFlagBitField, extraData: i64 0)
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!8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
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!9 = !DIDerivedType(tag: DW_TAG_member, name: "f2", scope: !5, file: !3, line: 11, baseType: !8, size: 2, offset: 1, flags: DIFlagBitField, extraData: i64 0)
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!10 = !DIDerivedType(tag: DW_TAG_member, name: "f3", scope: !5, file: !3, line: 20, baseType: !11, size: 512, offset: 512)
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!11 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "t2", file: !3, line: 1, size: 512, elements: !12)
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!12 = !{!13}
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!13 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !11, file: !3, line: 2, baseType: !14, size: 256)
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!14 = !DICompositeType(tag: DW_TAG_array_type, baseType: !8, size: 256, elements: !15)
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!15 = !{!16}
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!16 = !DISubrange(count: 8)
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!17 = !{i32 7, !"Dwarf Version", i32 5}
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!18 = !{i32 2, !"Debug Info Version", i32 3}
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!19 = !{i32 1, !"wchar_size", i32 4}
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!20 = !{i32 7, !"frame-pointer", i32 2}
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!21 = !{!"clang version 15.0.0 (https://github.com/llvm/llvm-project.git 468279d2d249e44ffa3535a613245b4ceb81a908)"}
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!22 = distinct !DISubprogram(name: "foo", scope: !3, file: !3, line: 24, type: !23, scopeLine: 24, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !2, retainedNodes: !25)
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!23 = !DISubroutineType(types: !24)
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!24 = !{!8}
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!25 = !{}
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!26 = !DILocation(line: 25, column: 12, scope: !22)
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!27 = !DILocation(line: 25, column: 3, scope: !22)

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