[BPF] Fix Segfault in BTFDebug

[dave-tucker]

Summary

Found using bpf-linker after linking a binary with debuginfo with a
library with no debuginfo. In this case, auto *S may be NULL and
therefore S->getLine() can segfault.

Signed-off-by: Dave Tucker <[e-mail]>

[ast]

looks fine, but could you add a unit test for it?

[dave-tucker]

In D134533#3811563, @ast wrote:

looks fine, but could you add a unit test for it?

i can if someone can point me to the unit tests for this class as i'm not familiar with the codebase.

[eddyz87]

In D134533#3811606, @dave-tucker wrote:

In D134533#3811563, @ast wrote:

looks fine, but could you add a unit test for it?

i can if someone can point me to the unit tests for this class as i'm not familiar with the codebase.

Could you please describe a minimal example?
I can wrap it as a test script in the llvm/test/CodeGen/BPF/.

[dave-tucker]

Abandoned in favour of D156446

llvm/lib/Target/BPF/BTFDebug.cpp

void BTFDebug::beginInstruction(const MachineInstr *MI) {
   // Skip this instruction if no DebugLoc or the DebugLoc
   // is the same as the previous instruction.
   const DebugLoc &DL = MI->getDebugLoc();
   if (!DL || PrevInstLoc == DL) {
     // This instruction will be skipped, no LineInfo has
     // been generated, construct one based on function signature.
     if (LineInfoGenerated == false) {
       auto *S = MI->getMF()->getFunction().getSubprogram();
+      if (!S) // unable to get subprogram
+        return;
       MCSymbol *FuncLabel = Asm->getFunctionBegin();
       constructLineInfo(S, FuncLabel, S->getLine(), 0);
       LineInfoGenerated = true;
     }
 
     return;
   }