[llvm] [BPF] Handle unreachable with a unimplemented kfunc call (PR #131731)

[Andrii Nakryiko via llvm-commits]

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@@ -1622,6 +1627,25 @@ void BTFDebug::endModule() {
   // Collect global types/variables except MapDef globals.
   processGlobals(false);
 
+  // Create a BTF entry for func __unreachable_helper.
+  const Module *M = MMI->getModule();
+  Function *F = M->getFunction("__unreachable_helper");
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anakryiko wrote:

I don't like the approach of Clang emitting kfunc call, putting it in .ksyms section (which is just a libbpf convention, not some sort of gold standard or anything), and so on. I think the appropriate way to do with would be a BPF instruction. 
But if we have to go with kfunc approach, let's at least call it `bpf_unreachable()`. We can even implement this in the kernel (it would just WARN/panic), so libbpf successfully resolves it without unnecessary warnings on modern kernels. And BPF verifier would provide a meaningful error message if it detects reachable `bpf_unreachable()` call.

https://github.com/llvm/llvm-project/pull/131731