[llvm] [BPF] Handle traps with kfunc call __bpf_trap (PR #131731)
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Fri May 23 08:48:16 PDT 2025
https://github.com/yonghong-song updated https://github.com/llvm/llvm-project/pull/131731
>From 6add2ac433ad2d559d4a487b0e5c6007be704c7b 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 traps with kfunc call __bpf_trap
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_trap() function.
This function will be a kfunc (in ".ksyms" section) with a weak
attribute, but does not have definition. The actual kfunc definition
is expected to be in kernel. The __bpf_trap() extern function
is also encoded in BTF. The name __bpf_trap() is chosen to satisfy
reserved identifier requirement.
Besides the uninitialized variable case, the builtin function
'__builtin_trap' can also generate kfunc __bpf_trap(). For example
in [3], we have
# define __bpf_unreachable() __builtin_trap()
If the compiler didn't remove __builtin_trap() during middle-end
optimization, compilation will fail.
With this patch, compilation will not fail and __builtin_trap()
is converted to __bpf_trap() kfunc. The eventual failure will be
in verifier instead of llvm compilation.
I tested this patch on bpf selftests and all tests are passed.
I also tried original example in [2] and the code looks like below:
; {
0: bf 16 00 00 00 00 00 00 r6 = r1
; bpf_printk("Start");
1: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0x0 ll
0000000000000008: R_BPF_64_64 .rodata
3: b4 02 00 00 06 00 00 00 w2 = 0x6
4: 85 00 00 00 06 00 00 00 call 0x6
; DEFINE_FUNC_CTX_POINTER(data)
5: 61 61 4c 00 00 00 00 00 w1 = *(u32 *)(r6 + 0x4c)
; bpf_printk("pre ipv6_hdrlen_offset");
6: 18 01 00 00 06 00 00 00 00 00 00 00 00 00 00 00 r1 = 0x6 ll
0000000000000030: R_BPF_64_64 .rodata
8: b4 02 00 00 17 00 00 00 w2 = 0x17
9: 85 00 00 00 06 00 00 00 call 0x6
10: 85 10 00 00 ff ff ff ff call -0x1
0000000000000050: R_BPF_64_32 __bpf_trap
11: 95 00 00 00 00 00 00 00 exit
<END>
Eventually kernel verifier will emit the following logs:
10: (85) call __bpf_trap#74479
unexpected __bpf_trap() due to uninitialized variable?
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_trap
1882: 95 00 00 00 00 00 00 00 exit
<END>
The verifier can easily report the error too.
A bpf flag `-bpf-disable-trap-unreachable` is introduced to disable
trapping for 'unreachable' or __builtin_trap.
[1] https://github.com/llvm/llvm-project/pull/126858
[2] https://github.com/msune/clang_bpf/blob/main/Makefile#L3
[3] https://github.com/libbpf/libbpf/blob/master/src/bpf_helpers.h
---
llvm/lib/Target/BPF/BPF.h | 2 +
llvm/lib/Target/BPF/BPFISelLowering.cpp | 45 +++++++++++++++++--
llvm/lib/Target/BPF/BPFISelLowering.h | 1 +
llvm/lib/Target/BPF/BPFMIPeephole.cpp | 16 +++++++
llvm/lib/Target/BPF/BPFTargetMachine.cpp | 9 ++++
llvm/lib/Target/BPF/BTFDebug.cpp | 28 ++++++++++++
llvm/test/CodeGen/BPF/BTF/builtin_trap.ll | 39 +++++++++++++++++
llvm/test/CodeGen/BPF/BTF/unreachable.ll | 53 +++++++++++++++++++++++
8 files changed, 189 insertions(+), 4 deletions(-)
create mode 100644 llvm/test/CodeGen/BPF/BTF/builtin_trap.ll
create mode 100644 llvm/test/CodeGen/BPF/BTF/unreachable.ll
diff --git a/llvm/lib/Target/BPF/BPF.h b/llvm/lib/Target/BPF/BPF.h
index 68166e574f35e..5d49949ddea25 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_TRAP = "__bpf_trap";
+
ModulePass *createBPFCheckAndAdjustIR();
FunctionPass *createBPFISelDag(BPFTargetMachine &TM);
diff --git a/llvm/lib/Target/BPF/BPFISelLowering.cpp b/llvm/lib/Target/BPF/BPFISelLowering.cpp
index 6c196309d2d1a..1d742ae6c3b12 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 (StringRef(E->getSymbol()) != BPF_TRAP) {
+ 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 +733,36 @@ 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();
+ Function *TrapHelper = M->getFunction(BPF_TRAP);
+ if (!TrapHelper) {
+ FunctionType *FT =
+ FunctionType::get(Type::getVoidTy(M->getContext()), false);
+ Function *NewF =
+ Function::Create(FT, GlobalValue::ExternalWeakLinkage, BPF_TRAP, M);
+ NewF->setDSOLocal(true);
+ NewF->setCallingConv(CallingConv::C);
+ }
+
+ auto PtrVT = getPointerTy(MF.getDataLayout());
+ CLI.Callee = DAG.getTargetExternalSymbol(BPF_TRAP, PtrVT);
+ CLI.Chain = N->getOperand(0);
+ CLI.IsTailCall = false;
+ CLI.CallConv = CallingConv::C;
+ CLI.IsVarArg = false;
+ CLI.DL = DL;
+ CLI.NoMerge = false;
+ CLI.DoesNotReturn = true;
+ 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 8104895cb7f14..23cbce7094e6b 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/BPFMIPeephole.cpp b/llvm/lib/Target/BPF/BPFMIPeephole.cpp
index 4febf3042fdd9..65d12b124e276 100644
--- a/llvm/lib/Target/BPF/BPFMIPeephole.cpp
+++ b/llvm/lib/Target/BPF/BPFMIPeephole.cpp
@@ -320,6 +320,7 @@ struct BPFMIPreEmitPeephole : public MachineFunctionPass {
bool adjustBranch();
bool insertMissingCallerSavedSpills();
bool removeMayGotoZero();
+ bool addExitAfterUnreachable();
public:
@@ -336,6 +337,7 @@ struct BPFMIPreEmitPeephole : public MachineFunctionPass {
Changed = adjustBranch() || Changed;
Changed |= insertMissingCallerSavedSpills();
Changed |= removeMayGotoZero();
+ Changed |= addExitAfterUnreachable();
return Changed;
}
};
@@ -734,6 +736,20 @@ bool BPFMIPreEmitPeephole::removeMayGotoZero() {
return Changed;
}
+// If the last insn in a funciton is 'JAL &__bpf_trap', let us add an
+// 'exit' insn after that insn. This will ensure no fallthrough at the last
+// insn, making kernel verification easier.
+bool BPFMIPreEmitPeephole::addExitAfterUnreachable() {
+ MachineBasicBlock &MBB = MF->back();
+ MachineInstr &MI = MBB.back();
+ if (MI.getOpcode() != BPF::JAL || !MI.getOperand(0).isSymbol() ||
+ strcmp(MI.getOperand(0).getSymbolName(), BPF_TRAP))
+ return false;
+
+ BuildMI(&MBB, MI.getDebugLoc(), TII->get(BPF::RET));
+ return true;
+}
+
} // end default namespace
INITIALIZE_PASS(BPFMIPreEmitPeephole, "bpf-mi-pemit-peephole",
diff --git a/llvm/lib/Target/BPF/BPFTargetMachine.cpp b/llvm/lib/Target/BPF/BPFTargetMachine.cpp
index 46ba758b55223..0c3f61fdfedd6 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());
@@ -77,6 +81,11 @@ 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..aecb97ea90dee 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]);
@@ -1595,6 +1600,10 @@ void BTFDebug::processFuncPrototypes(const Function *F) {
if (!ProtoFunctions.insert(F).second)
return;
+ // Ignore extern function BPF_TRAP, it will be emited in endModule().
+ if (SP->getName() == BPF_TRAP)
+ return;
+
uint32_t ProtoTypeId;
const std::unordered_map<uint32_t, StringRef> FuncArgNames;
visitSubroutineType(SP->getType(), false, FuncArgNames, ProtoTypeId);
@@ -1622,6 +1631,25 @@ void BTFDebug::endModule() {
// Collect global types/variables except MapDef globals.
processGlobals(false);
+ // Create a BTF entry for func BPF_TRAP.
+ const Module *M = MMI->getModule();
+ Function *F = M->getFunction(BPF_TRAP);
+ 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_TRAP, 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/builtin_trap.ll b/llvm/test/CodeGen/BPF/BTF/builtin_trap.ll
new file mode 100644
index 0000000000000..846b1264cc1ad
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/builtin_trap.ll
@@ -0,0 +1,39 @@
+; 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
+
+; BPFTargetMachine Options.NoTrapAfterNoreturn has been set to true,
+; so in below code, 'unreachable' will become a noop and
+; llvm.trap() will become 'call __bpf_trap' after selectiondag.
+define dso_local void @foo(i32 noundef %0) {
+ tail call void @llvm.trap()
+ unreachable
+}
+
+; CHECK: .Lfunc_begin0:
+; CHECK-NEXT: .cfi_startproc
+; CHECK-NEXT: # %bb.0:
+; CHECK-NEXT: call __bpf_trap
+; CHECK-NEXT: exit
+; CHECK-NEXT: .Lfunc_end0:
+
+; CHECK-BTF: [1] FUNC_PROTO '(anon)' ret_type_id=0 vlen=0
+; CHECK-BTF: [2] FUNC '__bpf_trap' type_id=1 linkage=extern
+; CHECK-BTF: [3] DATASEC '.ksyms' size=0 vlen=1
+; CHECK-BTF: type_id=2 offset=0 size=0
+
+declare void @llvm.trap() #1
+
+attributes #1 = {noreturn}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!2, !3, !4, !5, !6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, emissionKind: FullDebug)
+!1 = !DIFile(filename: "test_trap.c", directory: "/some/dir")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"wchar_size", i32 4}
+!5 = !{i32 7, !"frame-pointer", i32 2}
+!6 = !{i32 7, !"debug-info-assignment-tracking", i1 true}
diff --git a/llvm/test/CodeGen/BPF/BTF/unreachable.ll b/llvm/test/CodeGen/BPF/BTF/unreachable.ll
new file mode 100644
index 0000000000000..5f53a74454331
--- /dev/null
+++ b/llvm/test/CodeGen/BPF/BTF/unreachable.ll
@@ -0,0 +1,53 @@
+; 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_trap
+; CHECK-NEXT: exit
+; CHECK-NEXT: .Lfunc_end0:
+
+define void @buz() #0 {
+entry:
+ tail call void asm sideeffect "r0 = r1; exit;", ""()
+ unreachable
+}
+
+; CHECK: buz:
+; CHECK-NEXT: .Lfunc_begin1:
+; CHECK-NEXT: .cfi_startproc
+; CHECK-NEXT: # %bb.0:
+; CHECK-NEXT: #APP
+; CHECK-NEXT: r0 = r1
+; CHECK-NEXT: exit
+; CHECK-EMPTY:
+; CHECK-NEXT: #NO_APP
+; CHECK-NEXT: .Lfunc_end1:
+
+; CHECK-BTF: [1] FUNC_PROTO '(anon)' ret_type_id=0 vlen=0
+; CHECK-BTF: [2] FUNC '__bpf_trap' 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()
+
+attributes #0 = { naked }
+
+!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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