[BPF] Handle nested wrapper structs in BPF map definition traversal (llvm#144097)

In Aya/Rust, BPF map definitions are nested in two nested types:

* A struct representing the map type (e.g., `HashMap`, `RingBuf`) that
provides methods for interacting with the map type (e.g. `HashMap::get`,
`RingBuf::reserve`).
* An `UnsafeCell`, which informs the Rust compiler that the type is
thread-safe and can be safely mutated even as a global variable. The
kernel guarantees map operation safety.

This leads to a type hierarchy like:

```rust
    pub struct HashMap<K, V, const M: usize, const F: usize = 0>(
        core::cell::UnsafeCell<HashMapDef<K, V, M, F>>,
    );
    const BPF_MAP_TYPE_HASH: usize = 1;
    pub struct HashMapDef<K, V, const M: usize, const F: usize = 0> {
        r#type: *const [i32; BPF_MAP_TYPE_HASH],
        key: *const K,
        value: *const V,
        max_entries: *const [i32; M],
        map_flags: *const [i32; F],
    }
```

Then used in the BPF program code as a global variable:

```rust
    #[link_section = ".maps"]
    static HASH_MAP: HashMap<u32, u32, 1337> = HashMap::new();
```

Which is an equivalent of the following BPF map definition in C:

```c
    #define BPF_MAP_TYPE_HASH 1
    struct {
        int (*type)[BPF_MAP_TYPE_HASH];
        typeof(int) *key;
        typeof(int) *value;
        int (*max_entries)[1337];
    } map_1 __attribute__((section(".maps")));
```

Accessing the actual map definition requires traversing:

```
  HASH_MAP -> __0 -> value
```

Previously, the BPF backend only visited the pointee types of the
outermost struct, and didn’t descend into inner wrappers. This caused
issues when the key/value types were custom structs:

```rust
    // Define custom structs for key and values.
    pub struct MyKey(u32);
    pub struct MyValue(u32);

    #[link_section = ".maps"]
    #[export_name = "HASH_MAP"]
    pub static HASH_MAP: HashMap<MyKey, MyValue, 10> = HashMap::new();
```

These types weren’t fully visited and appeared in BTF as forward
declarations:

```
    #30: <FWD> 'MyKey' kind:struct
    #31: <FWD> 'MyValue' kind:struct
```

The fix is to enhance `visitMapDefType` to recursively visit inner
composite members. If a member is a composite type (likely a wrapper),
it is now also visited using `visitMapDefType`, ensuring that the
pointee types of the innermost stuct members, like `MyKey` and
`MyValue`, are fully resolved in BTF.

With this fix, the correct BTF entries are emitted:

```
    #6: <STRUCT> 'MyKey' sz:4 n:1
            #00 '__0' off:0 --> [7]
    #7: <INT> 'u32' bits:32 off:0
    #8: <PTR> --> [9]
    #9: <STRUCT> 'MyValue' sz:4 n:1
            #00 '__0' off:0 --> [7]
```

Fixes: llvm#143361

Author: vadorovsky

llvm/lib/Target/BPF/BTFDebug.cpp

@@ -976,11 +976,24 @@ void BTFDebug::visitMapDefType(const DIType *Ty, uint32_t &TypeId) {
   if (Tag != dwarf::DW_TAG_structure_type || CTy->isForwardDecl())
     return;
 
-  // Visit all struct members to ensure pointee type is visited
+  // Visit all struct members to ensure their types are visited.
   const DINodeArray Elements = CTy->getElements();
   for (const auto *Element : Elements) {
     const auto *MemberType = cast<DIDerivedType>(Element);
-    visitTypeEntry(MemberType->getBaseType());
+    const DIType *MemberBaseType = MemberType->getBaseType();
+
+    // If the member is a composite type, that may indicate the currently
+    // visited composite type is a wrapper, and the member represents the
+    // actual map definition.
+    // In that case, visit the member with `visitMapDefType` instead of
+    // `visitTypeEntry`, treating it specifically as a map definition rather
+    // than as a regular composite type.
+    const auto *MemberCTy = dyn_cast<DICompositeType>(MemberBaseType);
+    if (MemberCTy) {
+      visitMapDefType(MemberBaseType, TypeId);
+    } else {
+      visitTypeEntry(MemberBaseType);
+    }
   }
 
   // Visit this type, struct or a const/typedef/volatile/restrict type

llvm/test/CodeGen/BPF/BTF/map-def-nested.ll

@@ -0,0 +1,117 @@
+; RUN: llc -mtriple=bpfel -mcpu=v3 -filetype=obj -o %t1 %s
+; RUN: llvm-objcopy --dump-section='.BTF'=%t2 %t1
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF-SHORT %s
+; RUN: %python %p/print_btf.py %t2 | FileCheck -check-prefixes=CHECK-BTF %s
+
+; Source code:
+;   struct key { int i; };
+;   struct val { int j; };
+;   
+;   #define __uint(name, val) int (*name)[val]
+;   #define __type(name, val) typeof(val) *name
+;   
+;   struct {
+;      struct {
+;           __uint(type, 1);
+;           __uint(max_entries, 1337);
+;           __type(key, struct key);
+;           __type(value, struct val);
+;       } map_def;
+;   } map __attribute__((section(".maps")));
+; Compilation flag:
+;   clang -target bpf -O2 -g -S -emit-llvm t.c
+
+; ModuleID = 'bpf.c'
+source_filename = "bpf.c"
+target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+target triple = "bpf"
+
+%struct.anon = type { %struct.anon.0 }
+%struct.anon.0 = type { ptr, ptr, ptr, ptr }
+
+@map = dso_local local_unnamed_addr global %struct.anon zeroinitializer, section ".maps", align 8, !dbg !0
+
+; We expect exactly 4 structs:
+; * key
+; * val
+; * inner map type (the actual definition)
+; * outer map type (the wrapper)
+;
+; CHECK-BTF-SHORT-COUNT-4: STRUCT
+; CHECK-BTF-SHORT-NOT:     STRUCT
+
+; We expect no forward declarations.
+;
+; CHECK-BTF-SHORT-NOT: FWD
+
+; Assert the whole BTF.
+;
+; CHECK-BTF:      [1] PTR '(anon)' type_id=3
+; CHECK-BTF-NEXT: [2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
+; CHECK-BTF-NEXT: [3] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=1
+; CHECK-BTF-NEXT: [4] INT '__ARRAY_SIZE_TYPE__' size=4 bits_offset=0 nr_bits=32 encoding=(none)
+; CHECK-BTF-NEXT: [5] PTR '(anon)' type_id=6
+; CHECK-BTF-NEXT: [6] ARRAY '(anon)' type_id=2 index_type_id=4 nr_elems=1337
+; CHECK-BTF-NEXT: [7] PTR '(anon)' type_id=8
+;
+; Before bug https://github.com/llvm/llvm-project/issues/143361 was fixed, the
+; BTF kind of MyKey (#6) and MyValue (#9) would be FWD instead of STRUCT. The
+; main goal of this test is making sure that the full STRUCT BTF is generated
+; for these types.
+;
+; CHECK-BTF-NEXT: [8] STRUCT 'key' size=4 vlen=1
+; CHECK-BTF-NEXT:         'i' type_id=2 bits_offset=0
+; CHECK-BTF-NEXT: [9] PTR '(anon)' type_id=10
+; CHECK-BTF-NEXT: [10] STRUCT 'val' size=4 vlen=1
+; CHECK-BTF-NEXT:         'j' type_id=2 bits_offset=0
+; CHECK-BTF-NEXT: [11] STRUCT '(anon)' size=32 vlen=4
+; CHECK-BTF-NEXT:         'type' type_id=1 bits_offset=0
+; CHECK-BTF-NEXT:         'max_entries' type_id=5 bits_offset=64
+; CHECK-BTF-NEXT:         'key' type_id=7 bits_offset=128
+; CHECK-BTF-NEXT:         'value' type_id=9 bits_offset=192
+; CHECK-BTF-NEXT: [12] STRUCT '(anon)' size=32 vlen=1
+; CHECK-BTF-NEXT:         'map_def' type_id=11 bits_offset=0
+; CHECK-BTF-NEXT: [13] VAR 'map' type_id=12, linkage=global
+; CHECK-BTF-NEXT: [14] DATASEC '.maps' size=0 vlen=1
+; CHECK-BTF-NEXT:         type_id=13 offset=0 size=32
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!31, !32, !33, !34}
+!llvm.ident = !{!35}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "map", scope: !2, file: !3, line: 14, type: !5, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang version 21.0.0git ([email protected]:llvm/llvm-project.git c935bd3798b39330aab2c9ca29a519457d5e5245)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "bpf.c", directory: "/tmp", checksumkind: CSK_MD5, checksum: "2330cce6d83c72ef5335abc3016de28e")
+!4 = !{!0}
+!5 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 7, size: 256, elements: !6)
+!6 = !{!7}
+!7 = !DIDerivedType(tag: DW_TAG_member, name: "map_def", scope: !5, file: !3, line: 13, baseType: !8, size: 256)
+!8 = distinct !DICompositeType(tag: DW_TAG_structure_type, scope: !5, file: !3, line: 8, size: 256, elements: !9)
+!9 = !{!10, !16, !21, !26}
+!10 = !DIDerivedType(tag: DW_TAG_member, name: "type", scope: !8, file: !3, line: 9, baseType: !11, size: 64)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = !DICompositeType(tag: DW_TAG_array_type, baseType: !13, size: 32, elements: !14)
+!13 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!14 = !{!15}
+!15 = !DISubrange(count: 1)
+!16 = !DIDerivedType(tag: DW_TAG_member, name: "max_entries", scope: !8, file: !3, line: 10, baseType: !17, size: 64, offset: 64)
+!17 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !18, size: 64)
+!18 = !DICompositeType(tag: DW_TAG_array_type, baseType: !13, size: 42784, elements: !19)
+!19 = !{!20}
+!20 = !DISubrange(count: 1337)
+!21 = !DIDerivedType(tag: DW_TAG_member, name: "key", scope: !8, file: !3, line: 11, baseType: !22, size: 64, offset: 128)
+!22 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !23, size: 64)
+!23 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "key", file: !3, line: 1, size: 32, elements: !24)
+!24 = !{!25}
+!25 = !DIDerivedType(tag: DW_TAG_member, name: "i", scope: !23, file: !3, line: 1, baseType: !13, size: 32)
+!26 = !DIDerivedType(tag: DW_TAG_member, name: "value", scope: !8, file: !3, line: 12, baseType: !27, size: 64, offset: 192)
+!27 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !28, size: 64)
+!28 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "val", file: !3, line: 2, size: 32, elements: !29)
+!29 = !{!30}
+!30 = !DIDerivedType(tag: DW_TAG_member, name: "j", scope: !28, file: !3, line: 2, baseType: !13, size: 32)
+!31 = !{i32 7, !"Dwarf Version", i32 5}
+!32 = !{i32 2, !"Debug Info Version", i32 3}
+!33 = !{i32 1, !"wchar_size", i32 4}
+!34 = !{i32 7, !"frame-pointer", i32 2}
+!35 = !{!"clang version 21.0.0git ([email protected]:llvm/llvm-project.git c935bd3798b39330aab2c9ca29a519457d5e5245)"}