[llvm] [BPF] Handle unreachable with a unimplemented kfunc call (PR #131731)
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Fri Mar 21 08:44:36 PDT 2025
https://github.com/yonghong-song updated https://github.com/llvm/llvm-project/pull/131731
>From da34dcc6884dc8e18ca542167be1fa78c43ec1e8 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yonghong.song at linux.dev>
Date: Tue, 11 Mar 2025 12:24:47 -0700
Subject: [PATCH] [BPF] Handle unreachable with a kfunc call
Currently, middle-end generates 'unreachable' insn if the compiler
feels the code is indeed unreachable or the code becomes invalid
due to some optimizaiton (e.g. code optimization with uninitialized
variables).
Right now BPF backend ignores 'unreachable' insn during selectiondag
lowering. For cases where 'unreachable' is due to invalid code
transformation, such a signal will be missed. Later on, users needs
some effort to debug it which impacts developer productivity.
This patch enabled selectiondag lowering for 'unreachable' insn.
Previous attempt ([1]) tries to have a backend IR pass to filter
out 'unreachable' insns in a number of cases. But such pattern
matching may misalign with future middle-end optimization with
'unreachable' insns.
This patch takes a different approach. The 'unreachable' insn is
lowered with special encoding in bpf object file and verifier
will do proper verification for the bpf prog. More specifically,
the 'unreachable' insn is replaced by a 'bpf_unreachable'
function. This function will be a kfunc (in ".ksyms" section)
with a weak attribute, but does not have definition. The function
is also present in prog btf. This way, the extern
'bpf_unreachable' can be handled properly in libbpf func
poison_kfunc_call().
I tested this patch on bpf selftests and all tests are passed.
I also tried original example in [2] and the error is properly
detected by verifier:
func#0 @0
last insn is not an exit or jmp
In another internal sched-ext bpf prog, with the patch we have bpf code:
Disassembly of section .text:
0000000000000000 <scx_storage_init_single>:
; {
0: bc 13 00 00 00 00 00 00 w3 = w1
1: b4 01 00 00 00 00 00 00 w1 = 0x0
; const u32 zero = 0;
...
0000000000003a80 <create_dom>:
; {
1872: bc 16 00 00 00 00 00 00 w6 = w1
; bpf_printk("dom_id %d", dom_id);
1873: 18 01 00 00 3f 00 00 00 00 00 00 00 00 00 00 00 r1 = 0x3f ll
0000000000003a88: R_BPF_64_64 .rodata
1875: b4 02 00 00 0a 00 00 00 w2 = 0xa
1876: bc 63 00 00 00 00 00 00 w3 = w6
1877: 85 00 00 00 06 00 00 00 call 0x6
; ret = scx_bpf_create_dsq(dom_id, 0);
1878: bc 61 00 00 00 00 00 00 w1 = w6
1879: b4 02 00 00 00 00 00 00 w2 = 0x0
1880: 85 10 00 00 ff ff ff ff call -0x1
0000000000003ac0: R_BPF_64_32 scx_bpf_create_dsq
; domc->node_cpumask = node_data[node_id];
1881: 85 10 00 00 ff ff ff ff call -0x1
0000000000003ac8: R_BPF_64_32 bpf_unreachable
<END>
The verifier can easily report the error too.
A bpf flag `-bpf-disable-trap-unreachable` is introduced to disable
trapping unreachable.
[1] https://github.com/llvm/llvm-project/pull/126858
[2] https://github.com/msune/clang_bpf/blob/main/Makefile#L3
---
llvm/lib/Target/BPF/BPF.h | 2 +
llvm/lib/Target/BPF/BPFISelLowering.cpp | 43 ++++++++++++++++++--
llvm/lib/Target/BPF/BPFISelLowering.h | 1 +
llvm/lib/Target/BPF/BPFTargetMachine.cpp | 9 ++++
llvm/lib/Target/BPF/BTFDebug.cpp | 24 +++++++++++
llvm/test/CodeGen/BPF/BTF/bpf_unreachable.ll | 33 +++++++++++++++
6 files changed, 108 insertions(+), 4 deletions(-)
create mode 100644 llvm/test/CodeGen/BPF/BTF/bpf_unreachable.ll
diff --git a/llvm/lib/Target/BPF/BPF.h b/llvm/lib/Target/BPF/BPF.h
index f07ae4c9baf1c..f806116c88c3f 100644
--- a/llvm/lib/Target/BPF/BPF.h
+++ b/llvm/lib/Target/BPF/BPF.h
@@ -22,6 +22,8 @@ class BPFTargetMachine;
class InstructionSelector;
class PassRegistry;
+static const char *BPF_UNREACHABLE = "bpf_unreachable";
+
ModulePass *createBPFCheckAndAdjustIR();
FunctionPass *createBPFISelDag(BPFTargetMachine &TM);
diff --git a/llvm/lib/Target/BPF/BPFISelLowering.cpp b/llvm/lib/Target/BPF/BPFISelLowering.cpp
index 6c196309d2d1a..fb274333127af 100644
--- a/llvm/lib/Target/BPF/BPFISelLowering.cpp
+++ b/llvm/lib/Target/BPF/BPFISelLowering.cpp
@@ -23,6 +23,7 @@
#include "llvm/CodeGen/ValueTypes.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/DiagnosticPrinter.h"
+#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/MathExtras.h"
@@ -68,6 +69,8 @@ BPFTargetLowering::BPFTargetLowering(const TargetMachine &TM,
setOperationAction(ISD::BRIND, MVT::Other, Expand);
setOperationAction(ISD::BRCOND, MVT::Other, Expand);
+ setOperationAction(ISD::TRAP, MVT::Other, Custom);
+
setOperationAction({ISD::GlobalAddress, ISD::ConstantPool}, MVT::i64, Custom);
setOperationAction(ISD::DYNAMIC_STACKALLOC, MVT::i64, Custom);
@@ -326,6 +329,8 @@ SDValue BPFTargetLowering::LowerOperation(SDValue Op, SelectionDAG &DAG) const {
case ISD::ATOMIC_LOAD:
case ISD::ATOMIC_STORE:
return LowerATOMIC_LOAD_STORE(Op, DAG);
+ case ISD::TRAP:
+ return LowerTRAP(Op, DAG);
}
}
@@ -521,10 +526,12 @@ SDValue BPFTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
Callee = DAG.getTargetGlobalAddress(G->getGlobal(), CLI.DL, PtrVT,
G->getOffset(), 0);
} else if (ExternalSymbolSDNode *E = dyn_cast<ExternalSymbolSDNode>(Callee)) {
- Callee = DAG.getTargetExternalSymbol(E->getSymbol(), PtrVT, 0);
- fail(CLI.DL, DAG,
- Twine("A call to built-in function '" + StringRef(E->getSymbol()) +
- "' is not supported."));
+ if (strcmp(E->getSymbol(), BPF_UNREACHABLE) != 0) {
+ Callee = DAG.getTargetExternalSymbol(E->getSymbol(), PtrVT, 0);
+ fail(CLI.DL, DAG,
+ Twine("A call to built-in function '" + StringRef(E->getSymbol()) +
+ "' is not supported."));
+ }
}
// Returns a chain & a flag for retval copy to use.
@@ -726,6 +733,34 @@ SDValue BPFTargetLowering::LowerATOMIC_LOAD_STORE(SDValue Op,
return Op;
}
+SDValue BPFTargetLowering::LowerTRAP(SDValue Op, SelectionDAG &DAG) const {
+ MachineFunction &MF = DAG.getMachineFunction();
+ TargetLowering::CallLoweringInfo CLI(DAG);
+ SmallVector<SDValue> InVals;
+ SDNode *N = Op.getNode();
+ SDLoc DL(N);
+
+ Module *M = MF.getFunction().getParent();
+ FunctionType *FT = FunctionType::get(Type::getVoidTy(M->getContext()), false);
+ Function *UnreachableHelper = M->getFunction(BPF_UNREACHABLE);
+ if (!UnreachableHelper) {
+ Function *NewF = Function::Create(FT, GlobalValue::ExternalWeakLinkage,
+ BPF_UNREACHABLE, M);
+ NewF->setDSOLocal(true);
+ NewF->setCallingConv(CallingConv::C);
+ }
+
+ auto PtrVT = getPointerTy(MF.getDataLayout());
+ CLI.Callee = DAG.getTargetExternalSymbol(BPF_UNREACHABLE, PtrVT);
+ CLI.Chain = N->getOperand(0);
+ CLI.IsTailCall = false;
+ CLI.CallConv = CallingConv::C;
+ CLI.IsVarArg = false;
+ CLI.DL = DL;
+ CLI.NoMerge = false;
+ return LowerCall(CLI, InVals);
+}
+
const char *BPFTargetLowering::getTargetNodeName(unsigned Opcode) const {
switch ((BPFISD::NodeType)Opcode) {
case BPFISD::FIRST_NUMBER:
diff --git a/llvm/lib/Target/BPF/BPFISelLowering.h b/llvm/lib/Target/BPF/BPFISelLowering.h
index ad048ad05e6dd..ab3d9d82caeaf 100644
--- a/llvm/lib/Target/BPF/BPFISelLowering.h
+++ b/llvm/lib/Target/BPF/BPFISelLowering.h
@@ -80,6 +80,7 @@ class BPFTargetLowering : public TargetLowering {
SDValue LowerATOMIC_LOAD_STORE(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerConstantPool(SDValue Op, SelectionDAG &DAG) const;
SDValue LowerGlobalAddress(SDValue Op, SelectionDAG &DAG) const;
+ SDValue LowerTRAP(SDValue Op, SelectionDAG &DAG) const;
template <class NodeTy>
SDValue getAddr(NodeTy *N, SelectionDAG &DAG, unsigned Flags = 0) const;
diff --git a/llvm/lib/Target/BPF/BPFTargetMachine.cpp b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
index 3379af6fe8744..bf6ca25e1dd0c 100644
--- a/llvm/lib/Target/BPF/BPFTargetMachine.cpp
+++ b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
@@ -37,6 +37,10 @@ static cl::
opt<bool> DisableMIPeephole("disable-bpf-peephole", cl::Hidden,
cl::desc("Disable machine peepholes for BPF"));
+static cl::opt<bool>
+ DisableCheckUnreachable("bpf-disable-trap-unreachable", cl::Hidden,
+ cl::desc("Disable Trap Unreachable for BPF"));
+
extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeBPFTarget() {
// Register the target.
RegisterTargetMachine<BPFTargetMachine> X(getTheBPFleTarget());
@@ -73,6 +77,11 @@ BPFTargetMachine::BPFTargetMachine(const Target &T, const Triple &TT,
getEffectiveCodeModel(CM, CodeModel::Small), OL),
TLOF(std::make_unique<TargetLoweringObjectFileELF>()),
Subtarget(TT, std::string(CPU), std::string(FS), *this) {
+ if (!DisableCheckUnreachable) {
+ this->Options.TrapUnreachable = true;
+ this->Options.NoTrapAfterNoreturn = true;
+ }
+
initAsmInfo();
BPFMCAsmInfo *MAI =
diff --git a/llvm/lib/Target/BPF/BTFDebug.cpp b/llvm/lib/Target/BPF/BTFDebug.cpp
index 8f2274eb75da8..7ceb1bb3fe621 100644
--- a/llvm/lib/Target/BPF/BTFDebug.cpp
+++ b/llvm/lib/Target/BPF/BTFDebug.cpp
@@ -349,6 +349,11 @@ void BTFTypeFuncProto::completeType(BTFDebug &BDebug) {
if (IsCompleted)
return;
IsCompleted = true;
+ if (!STy) {
+ BTFType.Type = 0;
+ BTFType.NameOff = 0;
+ return;
+ }
DITypeRefArray Elements = STy->getTypeArray();
auto RetType = tryRemoveAtomicType(Elements[0]);
@@ -1622,6 +1627,25 @@ void BTFDebug::endModule() {
// Collect global types/variables except MapDef globals.
processGlobals(false);
+ // Create a BTF entry for func BPF_UNREACHABLE.
+ const Module *M = MMI->getModule();
+ Function *F = M->getFunction(BPF_UNREACHABLE);
+ if (F) {
+ std::unordered_map<uint32_t, StringRef> FuncArgNames;
+ auto TypeEntry =
+ std::make_unique<BTFTypeFuncProto>(nullptr, 0, FuncArgNames);
+ uint32_t TypeId = addType(std::move(TypeEntry));
+ auto FuncTypeEntry = std::make_unique<BTFTypeFunc>(
+ BPF_UNREACHABLE, TypeId, BTF::FUNC_EXTERN);
+ uint32_t FuncId = addType(std::move(FuncTypeEntry));
+
+ StringRef SecName(".ksyms");
+ auto [It, Inserted] = DataSecEntries.try_emplace(std::string(SecName));
+ if (Inserted)
+ It->second = std::make_unique<BTFKindDataSec>(Asm, std::string(SecName));
+ It->second->addDataSecEntry(FuncId, Asm->getSymbol(F), 0);
+ }
+
for (auto &DataSec : DataSecEntries)
addType(std::move(DataSec.second));
diff --git a/llvm/test/CodeGen/BPF/BTF/bpf_unreachable.ll b/llvm/test/CodeGen/BPF/BTF/bpf_unreachable.ll
new file mode 100644
index 0000000000000..cfcd865921cda
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/bpf_unreachable.ll
@@ -0,0 +1,33 @@
+; 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 %s
+; RUN: llc -mtriple=bpfel -mcpu=v3 < %s | FileCheck -check-prefixes=CHECK %s
+
+define void @foo() {
+entry:
+ tail call void @bar()
+ unreachable
+}
+
+; CHECK: foo:
+; CHECK-NEXT: .Lfunc_begin0:
+; CHECK-NEXT: .cfi_startproc
+; CHECK-NEXT: # %bb.0:
+; CHECK-NEXT: call bar
+; CHECK-NEXT: call bpf_unreachable
+; CHECK-NEXT: .Lfunc_end0:
+
+; CHECK-BTF: [1] FUNC_PROTO '(anon)' ret_type_id=0 vlen=0
+; CHECK-BTF: [2] FUNC 'bpf_unreachable' type_id=1 linkage=extern
+; CHECK-BTF: [3] DATASEC '.ksyms' size=0 vlen=1
+; CHECK-BTF: type_id=2 offset=0 size=0
+
+declare dso_local void @bar()
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test.c", directory: "/some/dir")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
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