[clang] [llvm] [BPF] Fix linking issues in static map initializers (PR #91310)
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Tue May 7 02:38:40 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Nick Zavaritsky (mejedi)
<details>
<summary>Changes</summary>
When BPF object files are linked with bpftool, every symbol must be accompanied by BTF info. Ensure that extern functions referenced by global variable initializers are included in BTF.
The primary motivation is "static" initialization of PROG maps:
extern int elsewhere(struct xdp_md *);
struct {
__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
__uint(max_entries, 1);
__type(key, int);
__type(value, int);
__array(values, int (struct xdp_md *));
} prog_map SEC(".maps") = { .values = { elsewhere } };
---
Full diff: https://github.com/llvm/llvm-project/pull/91310.diff
5 Files Affected:
- (modified) clang/lib/CodeGen/CGExprConstant.cpp (+16-2)
- (added) clang/test/bpf-debug-info-extern-func.c (+9)
- (modified) llvm/lib/Target/BPF/BTFDebug.cpp (+23)
- (modified) llvm/lib/Target/BPF/BTFDebug.h (+4)
- (added) llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll (+54)
``````````diff
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 94962091116a..945f2e222b23 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1950,8 +1950,22 @@ ConstantLValueEmitter::tryEmitBase(const APValue::LValueBase &base) {
if (D->hasAttr<WeakRefAttr>())
return CGM.GetWeakRefReference(D).getPointer();
- if (auto FD = dyn_cast<FunctionDecl>(D))
- return CGM.GetAddrOfFunction(FD);
+ if (auto FD = dyn_cast<FunctionDecl>(D)) {
+ auto *C = CGM.GetAddrOfFunction(FD);
+
+ // we don't normally emit debug info for extern fns referenced via
+ // variable initialisers; BPF needs it since it generates BTF from
+ // debug info and bpftool demands BTF for every symbol linked
+ if (CGM.getTarget().getTriple().isBPF() && FD->getStorageClass() == SC_Extern) {
+ if (CGDebugInfo *DI = CGM.getModuleDebugInfo()) {
+ auto *Fn = dyn_cast<llvm::Function>(C);
+ if (Fn && !Fn->getSubprogram())
+ DI->EmitFunctionDecl(FD, FD->getLocation(), D->getType(), Fn);
+ }
+ }
+
+ return C;
+ }
if (auto VD = dyn_cast<VarDecl>(D)) {
// We can never refer to a variable with local storage.
diff --git a/clang/test/bpf-debug-info-extern-func.c b/clang/test/bpf-debug-info-extern-func.c
new file mode 100644
index 000000000000..da324630a314
--- /dev/null
+++ b/clang/test/bpf-debug-info-extern-func.c
@@ -0,0 +1,9 @@
+// RUN: %clang -g -O2 -target bpf -S -emit-llvm %s -o - | FileCheck %s
+//
+// When linking BPF object files via bpftool, BTF info is required for
+// every symbol. BTF is generated from debug info. Ensure that debug info
+// is emitted for extern functions referenced via variable initializers.
+//
+// CHECK: !DISubprogram(name: "fn"
+extern void fn(void);
+void (*pfn) (void) = &fn;
diff --git a/llvm/lib/Target/BPF/BTFDebug.cpp b/llvm/lib/Target/BPF/BTFDebug.cpp
index b6d3b460005c..7d64f2e04bc2 100644
--- a/llvm/lib/Target/BPF/BTFDebug.cpp
+++ b/llvm/lib/Target/BPF/BTFDebug.cpp
@@ -1494,6 +1494,29 @@ void BTFDebug::processGlobals(bool ProcessingMapDef) {
DataSecEntries[std::string(SecName)]->addDataSecEntry(VarId,
Asm->getSymbol(&Global), Size);
+
+ if (Global.hasInitializer())
+ processGlobalInitializer(Global.getInitializer());
+ }
+}
+
+/// Process global variable initializer in pursuit for function
+/// pointers. Add discovered (extern) functions to BTF. Some (extern)
+/// functions might have been missed otherwise. Every symbol needs BTF
+/// info when linking with bpftool. Primary use case: "static"
+/// initialization of BPF maps.
+///
+/// struct {
+/// __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+/// ...
+/// } prog_map SEC(".maps") = { .values = { extern_func } };
+///
+void BTFDebug::processGlobalInitializer(const Constant *C) {
+ if (auto *Fn = dyn_cast<Function>(C))
+ processFuncPrototypes(Fn);
+ if (auto *CA = dyn_cast<ConstantAggregate>(C)) {
+ for (unsigned I = 0, N = CA->getNumOperands(); I < N; ++I)
+ processGlobalInitializer(CA->getOperand(I));
}
}
diff --git a/llvm/lib/Target/BPF/BTFDebug.h b/llvm/lib/Target/BPF/BTFDebug.h
index 11a0c59ba6c9..02181d3013be 100644
--- a/llvm/lib/Target/BPF/BTFDebug.h
+++ b/llvm/lib/Target/BPF/BTFDebug.h
@@ -352,6 +352,10 @@ class BTFDebug : public DebugHandlerBase {
/// Generate types and variables for globals.
void processGlobals(bool ProcessingMapDef);
+ /// Process global variable initializer in pursuit for function
+ /// pointers.
+ void processGlobalInitializer(const Constant *C);
+
/// Generate types for function prototypes.
void processFuncPrototypes(const Function *);
diff --git a/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll b/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll
new file mode 100644
index 000000000000..700486d9f351
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/extern-var-func2.ll
@@ -0,0 +1,54 @@
+; RUN: llc -march=bpfel -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+; RUN: llc -march=bpfeb -filetype=asm -o - %s | FileCheck -check-prefixes=CHECK %s
+;
+; Source code:
+; extern int elsewhere(void);
+; struct {
+; void *values[];
+; } prog_map = { .values = { elsewhere } };
+; Compilation flag:
+; clang -target bpf -O2 -g -S -emit-llvm test.c
+; ModuleID = 'b.c'
+
+ at prog_map = dso_local local_unnamed_addr global { [1 x ptr] } { [1 x ptr] [ptr @elsewhere] }, align 8, !dbg !0
+
+declare !dbg !17 dso_local i32 @elsewhere() #0
+
+; CHECK: .long 0 # BTF_KIND_FUNC_PROTO(id = 6)
+; CHECK-NEXT: .long 218103808 # 0xd000000
+; CHECK-NEXT: .long 7
+; CHECK-NEXT: .long 37 # BTF_KIND_INT(id = 7)
+; CHECK-NEXT: .long 16777216 # 0x1000000
+; CHECK-NEXT: .long 4
+; CHECK-NEXT: .long 16777248 # 0x1000020
+; CHECK-NEXT: .long 41 # BTF_KIND_FUNC(id = 8)
+; CHECK-NEXT: .long 201326594 # 0xc000002
+; CHECK-NEXT: .long 6
+
+attributes #0 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!12, !13, !14, !15}
+!llvm.ident = !{!16}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "prog_map", scope: !2, file: !3, line: 4, type: !5, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C11, file: !3, producer: "clang version 19.0.0git (git at github.com:llvm/llvm-project.git 0390a6803608e3a5314315b73740c2d3f5a5723f)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "b.c", directory: "/home/nickz/llvm-project.git", checksumkind: CSK_MD5, checksum: "41cc17375f1261a0e072590833492553")
+!4 = !{!0}
+!5 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 2, elements: !6)
+!6 = !{!7}
+!7 = !DIDerivedType(tag: DW_TAG_member, name: "values", scope: !5, file: !3, line: 3, baseType: !8)
+!8 = !DICompositeType(tag: DW_TAG_array_type, baseType: !9, elements: !10)
+!9 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!10 = !{!11}
+!11 = !DISubrange(count: -1)
+!12 = !{i32 7, !"Dwarf Version", i32 5}
+!13 = !{i32 2, !"Debug Info Version", i32 3}
+!14 = !{i32 1, !"wchar_size", i32 4}
+!15 = !{i32 7, !"frame-pointer", i32 2}
+!16 = !{!"clang version 19.0.0git (git at github.com:llvm/llvm-project.git 0390a6803608e3a5314315b73740c2d3f5a5723f)"}
+!17 = !DISubprogram(name: "elsewhere", scope: !3, file: !3, line: 1, type: !18, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized)
+!18 = !DISubroutineType(types: !19)
+!19 = !{!20}
+!20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
``````````
</details>
https://github.com/llvm/llvm-project/pull/91310
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