[llvm] ddff979 - [BPF] Prevent disassembly segfault for NOP insn

[Tom Stellard via llvm-commits]

On 05/18/2020 05:40 PM, Yonghong Song via llvm-commits wrote:
> 
> Author: Yonghong Song
> Date: 2020-05-18T17:40:18-07:00
> New Revision: ddff9799d2d0052653aa0385969b418a87bf5d7c
> 

Should we backport this to the release/10.x branch?

-Tom

> URL: https://github.com/llvm/llvm-project/commit/ddff9799d2d0052653aa0385969b418a87bf5d7c
> DIFF: https://github.com/llvm/llvm-project/commit/ddff9799d2d0052653aa0385969b418a87bf5d7c.diff
> 
> LOG: [BPF] Prevent disassembly segfault for NOP insn
> 
> 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 NOP insn for disassembling.
> 
> Note that instruction "r0 = r0" should not appear in non inline
> asm codes since BPF Machine Instruction Peephole optimization will
> remove it.
> 
> Differential Revision: https://reviews.llvm.org/D80156
> 
> Added: 
>     llvm/test/CodeGen/BPF/objdump_nop.ll
> 
> Modified: 
>     llvm/lib/Target/BPF/BPFInstrInfo.td
> 
> Removed: 
>     
> 
> 
> ################################################################################
> diff  --git a/llvm/lib/Target/BPF/BPFInstrInfo.td b/llvm/lib/Target/BPF/BPFInstrInfo.td
> index 0f39294daa2b..6781d09b846e 100644
> --- a/llvm/lib/Target/BPF/BPFInstrInfo.td
> +++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
> @@ -526,7 +526,7 @@ class NOP_I<string OpcodeStr>
>    let BPFClass = BPF_ALU64;
>  }
>  
> -let hasSideEffects = 0 in
> +let hasSideEffects = 0, isCodeGenOnly = 1 in
>    def NOP : NOP_I<"nop">;
>  
>  class RET<string OpcodeStr>
> 
> diff  --git a/llvm/test/CodeGen/BPF/objdump_nop.ll b/llvm/test/CodeGen/BPF/objdump_nop.ll
> new file mode 100644
> index 000000000000..6df2699f2e74
> --- /dev/null
> +++ b/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
> 
> 
>         
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