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

[Yonghong Song via llvm-commits]

On 5/18/20 10:23 PM, Tom Stellard wrote:
> 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?

I think there is no need for this one.
The motivation for this one is to fix segfault for
"llvm-objdump -D", where the issue has existed for a long
time and bpf community seems not using "-D" in most cases.

"llvm-objdump -d" can also trigger the bug, but only
with unlikely inline assembly.

So I will skip this one for release/10.x.

> 
> -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://urldefense.proofpoint.com/v2/url?u=https-3A__reviews.llvm.org_D80156&d=DwICaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=1JuE-bZc8Uc9_zvJTHQXUCG1i_EHsXapfn74H7JPEWY&s=SfTWmDY9Qnk479ChclsvBFrGc01kmGla1Q050qDj93E&e=
>>
>> 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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