[PATCH] D80156: [BPF] Prevent disassembly segfault for NOP insn

[Yonghong Song via Phabricator via llvm-commits]

yonghong-song created this revision.
yonghong-song added a reviewer: ast.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

For a simple program like below:

  -bash-4.4$ cat t.c 
  int test() {
    asm volatile("r0 = r0" ::);
    return 0;
  }

compiled with

  clang -target bpf -O2 -c t.c 

the following llvm-objdump command will segfault.

  llvm-objdump -d t.o 
  
  0:       bf 00 00 00 00 00 00 00 nop 
  llvm-objdump: ../include/llvm/ADT/SmallVector.h:180
  ... 
  Assertion `idx < size()' failed
  ... 
  abort
  ... 
  llvm::BPFInstPrinter::printOperand
  llvm::BPFInstPrinter::printInstruction
  ... 

The reason is both NOP and MOV_rr (r0 = r0) having the same encoding.
The disassembly getInstruction() decodes to be a NOP instruciton but 
during printInstruction() the same encoding is interpreted as
a MOV_rr instruction. Such a mismatcch caused the segfault.

The fix is to make NOP instruction as CodeGen only so disassembler
will skip it.

Note that instruction "r0 = r0" should not appear in non inline
asm codes, we BPF Machine Instruction Peephole optimization will
remove it.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D80156

Files:
  llvm/lib/Target/BPF/BPFInstrInfo.td
  llvm/test/CodeGen/BPF/objdump_nop.ll


Index: llvm/test/CodeGen/BPF/objdump_nop.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/BPF/objdump_nop.ll
@@ -0,0 +1,19 @@
+; RUN: llc -march=bpfel -filetype=obj -o - %s | llvm-objdump -d - | FileCheck %s
+;
+; Source:
+;   int test() {
+;     asm volatile("r0 = r0" ::);
+;     return 0;
+;   }
+; Compilation flag:
+;   clang -target bpf -O2 -S -emit-llvm t.c
+
+; Function Attrs: nounwind
+define dso_local i32 @test() local_unnamed_addr {
+entry:
+  tail call void asm sideeffect "r0 = r0", ""()
+  ret i32 0
+}
+; CHECK-LABEL: test
+; CHECK:       r0 = r0
+; CHECK:       r0 = 0
Index: llvm/lib/Target/BPF/BPFInstrInfo.td
===================================================================
--- llvm/lib/Target/BPF/BPFInstrInfo.td
+++ llvm/lib/Target/BPF/BPFInstrInfo.td
@@ -526,7 +526,7 @@
   let BPFClass = BPF_ALU64;
 }
 
-let hasSideEffects = 0 in
+let hasSideEffects = 0, isCodeGenOnly = 1 in
   def NOP : NOP_I<"nop">;
 
 class RET<string OpcodeStr>


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