BPF: permit type modifiers for __builtin_btf_type_id() relocation

[yonghong-song]

Summary

Lorenz Bauer from Cloudflare tried to use "const struct <name>"
as the type for __builtin_btf_type_id(*(const struct <name>)0, 1)
relocation and hit a llvm BPF fatal error.

https://lore.kernel.org/bpf/[e-mail]/

... 
fatal error: error in backend: Empty type name for BTF_TYPE_ID_REMOTE reloc

Currently, we require the debuginfo type itself must have a name.
In this case, the debuginfo type is "const" which points to "struct <name>".
The "const" type does not have a name, hence the above fatal error
will be triggered.

Let us permit "const" and "volatile" type modifiers. We skip modifiers
in some other cases as well like structure member type tracing.
This can aviod the above fatal error.

[anakryiko]

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

91

should we strip restrict as well?

[yonghong-song]

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

91

I deliberately skipped the restrict. in C, restrict is used to restrict pointers and used in pointer declarations (include variables and parameters).

https://en.wikipedia.org/wiki/Restrict

In type casting, the restrict will be ignored. For example,

-bash-4.4$ cat tt.c
struct s {
  int a;
};
int test() {
  return __builtin_btf_type_id(*(struct s * restrict)0, 1);
}
-bash-4.4$ clang -target bpf -O2 -S -g tt.c -emit-llvm -Xclang -disable-llvm-passes

restrict does not show up in the debuginfo.

I tried to construct another example,

-bash-4.4$ cat tt.c                                                               
struct s {
  int a;
};

int test() {
  struct s * restrict v;
  return __builtin_btf_type_id(*v, 1);
}

In this case, "restrict" shows up in variable declaration as restrict -> pointer -> struct s.
The meta data we attached is the pointee type "struct s", so we are fine.

typically restrict shows up before pointer like [restrict|const|volatile]+ pointer [const|volatile]+ non-pointer type.

What I did is trying to remove const/volatile in "[const|volatile]+ non-pointer-type". So we should be fine here.

[anakryiko]

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

91

ok, thanks for checking, Yonghong!

llvm/lib/Target/BPF/BPFPreserveDIType.cpp

for (auto Call : PreserveDITypeCalls) {
     MDNode *MD = Call->getMetadata(LLVMContext::MD_preserve_access_index);
 
     uint32_t Reloc;
     if (FlagValue == BPFCoreSharedInfo::BTF_TYPE_ID_LOCAL_RELOC) {
       Reloc = BPFCoreSharedInfo::BTF_TYPE_ID_LOCAL;
     } else {
       Reloc = BPFCoreSharedInfo::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();
+      }
+
       if (Ty->getName().empty())
         report_fatal_error("Empty type name for BTF_TYPE_ID_REMOTE reloc");
+      MD = Ty;
     }
 
     BasicBlock *BB = Call->getParent();
     IntegerType *VarType = Type::getInt64Ty(BB->getContext());
     std::string GVName = BaseName + std::to_string(Count) + "$" +
         std::to_string(Reloc);
     GlobalVariable *GV = new GlobalVariable(
         *M, VarType, false, GlobalVariable::ExternalLinkage, NULL, GVName);

llvm/test/CodeGen/BPF/BTF/builtin-btf-type-id-2.ll

+; 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, 1);
+;   }
+; 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)