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[BPF] Skip modifiers for __builtin_btf_type_id() local type (#71094)
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BPF upstream reported an inconsistent behavior w.r.t. BPF_TYPE_ID_LOCAL
vs. BPF_TYPE_ID_TARGET (or BPF_TYPE_ID_REMOTE in LLVM terminology).

For BPF_TYPE_ID_TARGET, all modifiers (like 'const' and 'volatile') are
ignored in the final type encoding. For example, for type
 'const struct foo', the eventually encoding in BTF relocation
is 'struct foo'. This faciliates libbpf to match corresponding kernel
types with considering any modifiers.

Currently behavior for BPF_TYPE_ID_LOCAL is different. It will encode
'const struct foo' in BTF relocation and such discrepancy confused users
([1]).

This patch fixed this discrepancy by making BPF_TYPE_ID_LOCAL BTF type
representation the sams as BPF_TYPE_ID_TARGET. This should have minimum
user impact since ultimately user wants to get a real time not a 'const'
type modifier.

The selftest builtin-btf-type-id-2.ll is used to test BPF_TYPE_ID_TARGET
with 'const' modifier. Adapt the same test for BPF_TYPE_ID_LOCAL. And
the below diff shows now both BPF_TYPE_ID_LOCAL and BPF_TYPE_ID_TARGET
produces the same type:

$ diff test/CodeGen/BPF/BTF/builtin-btf-type-id-2.ll
test/CodeGen/BPF/BTF/builtin-btf-type-id-local.ll
--- test/CodeGen/BPF/BTF/builtin-btf-type-id-2.ll 2023-07-30
16:58:20.657528310 -0700
+++ test/CodeGen/BPF/BTF/builtin-btf-type-id-local.ll 2023-11-02
10:23:25.356959008 -0700
  @@ -6,7 +6,7 @@
   ;     int a;
   ;   };
   ;   int test(void) {
  -;     return __builtin_btf_type_id(*(const struct s *)0, 1);
  +;     return __builtin_btf_type_id(*(const struct s *)0, 0);
   ;   }
   ; Compilation flag:
; clang -target bpf -O2 -g -S -emit-llvm -Xclang -disable-llvm-passes
test.c
  $

[1]
https://lore.kernel.org/bpf/CAN+4W8h3yDjkOLJPiuKVKTpj_08pBz8ke6vN=Lf8gcA=iYBM-g@mail.gmail.com/

Co-authored-by: Yonghong Song <yonghong.song@linux.dev>
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@yonghong-song
yonghong-song and Yonghong Song authored Nov 3, 2023
1 parent 284c699 commit 32e35b2
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Showing 2 changed files with 83 additions and 9 deletions.
19 changes: 10 additions & 9 deletions llvm/lib/Target/BPF/BPFPreserveDIType.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -86,24 +86,25 @@ static bool BPFPreserveDITypeImpl(Function &F) {
Reloc = BTF::BTF_TYPE_ID_LOCAL;
} else {
Reloc = BTF::BTF_TYPE_ID_REMOTE;
DIType *Ty = cast<DIType>(MD);
while (auto *DTy = dyn_cast<DIDerivedType>(Ty)) {
unsigned Tag = DTy->getTag();
if (Tag != dwarf::DW_TAG_const_type &&
Tag != dwarf::DW_TAG_volatile_type)
break;
Ty = DTy->getBaseType();
}
}
DIType *Ty = cast<DIType>(MD);
while (auto *DTy = dyn_cast<DIDerivedType>(Ty)) {
unsigned Tag = DTy->getTag();
if (Tag != dwarf::DW_TAG_const_type && Tag != dwarf::DW_TAG_volatile_type)
break;
Ty = DTy->getBaseType();
}

if (Reloc == BTF::BTF_TYPE_ID_REMOTE) {
if (Ty->getName().empty()) {
if (isa<DISubroutineType>(Ty))
report_fatal_error(
"SubroutineType not supported for BTF_TYPE_ID_REMOTE reloc");
else
report_fatal_error("Empty type name for BTF_TYPE_ID_REMOTE reloc");
}
MD = Ty;
}
MD = Ty;

BasicBlock *BB = Call->getParent();
IntegerType *VarType = Type::getInt64Ty(BB->getContext());
Expand Down
73 changes: 73 additions & 0 deletions llvm/test/CodeGen/BPF/BTF/builtin-btf-type-id-local.ll
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,73 @@
; RUN: opt -O2 -mtriple=bpf-pc-linux -S -o %t1 %s
; RUN: llc -filetype=asm -o - %t1 | FileCheck -check-prefixes=CHECK %s
; RUN: llc -mattr=+alu32 -filetype=asm -o - %t1 | FileCheck -check-prefixes=CHECK %s
; Source code:
; struct s {
; int a;
; };
; int test(void) {
; return __builtin_btf_type_id(*(const struct s *)0, 0);
; }
; Compilation flag:
; clang -target bpf -O2 -g -S -emit-llvm -Xclang -disable-llvm-passes test.c

; Function Attrs: nounwind
define dso_local i32 @test() #0 !dbg !7 {
entry:
%0 = call i64 @llvm.bpf.btf.type.id(i32 0, i64 1), !dbg !11, !llvm.preserve.access.index !12
%conv = trunc i64 %0 to i32, !dbg !11
ret i32 %conv, !dbg !16
}

; CHECK: .long 1 # BTF_KIND_INT(id = 2)
; CHECK-NEXT: .long 16777216 # 0x1000000
; CHECK-NEXT: .long 4
; CHECK-NEXT: .long 16777248 # 0x1000020

; CHECK: .long 16 # BTF_KIND_STRUCT(id = 4)
; CHECK-NEXT: .long 67108865 # 0x4000001
; CHECK-NEXT: .long 4
; CHECK-NEXT: .long 18
; CHECK-NEXT: .long 2

; CHECK: .ascii "int" # string offset=1
; CHECK: .ascii ".text" # string offset=10
; CHECK: .byte 115 # string offset=16
; CHECK: .byte 97 # string offset=18
; CHECK: .byte 48 # string offset=20

; CHECK: .long 16 # FieldReloc
; CHECK-NEXT: .long 10 # Field reloc section string offset=10
; CHECK-NEXT: .long 1
; CHECK-NEXT: .long .Ltmp{{[0-9]+}}
; CHECK-NEXT: .long 4
; CHECK-NEXT: .long 20
; CHECK-NEXT: .long 7

; Function Attrs: nounwind readnone
declare i64 @llvm.bpf.btf.type.id(i32, i64) #1

attributes #0 = { nounwind "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
attributes #1 = { nounwind readnone }

!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 13.0.0 (https://github.com/llvm/llvm-project.git 9783e2098800b954c55ae598a1ce5c4b93444fc0)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
!1 = !DIFile(filename: "test.c", directory: "/tmp/home/yhs/bpf/test")
!2 = !{}
!3 = !{i32 7, !"Dwarf Version", i32 4}
!4 = !{i32 2, !"Debug Info Version", i32 3}
!5 = !{i32 1, !"wchar_size", i32 4}
!6 = !{!"clang version 13.0.0 (https://github.com/llvm/llvm-project.git 9783e2098800b954c55ae598a1ce5c4b93444fc0)"}
!7 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 4, type: !8, scopeLine: 4, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
!8 = !DISubroutineType(types: !9)
!9 = !{!10}
!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!11 = !DILocation(line: 5, column: 10, scope: !7)
!12 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !13)
!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "s", file: !1, line: 1, size: 32, elements: !14)
!14 = !{!15}
!15 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !13, file: !1, line: 2, baseType: !10, size: 32)
!16 = !DILocation(line: 5, column: 3, scope: !7)

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