[llvm] [BPF] Report Undefined Behavior from IR (PR #126858)

[via llvm-commits]

yonghong-song wrote:

> LLVM/Clang generally do not allow the use of backend-generated warnings, because they produce unintelligible, optimization-dependent false-positives. There are some specific exceptions, but what you're doing here is basically _exactly_ the kind of backend warning we want to avoid.

The backend warning here is BPF specific as several conditions are used to check before issuing the warning.

> 
> Does BPF have _any_ kind of support for trapping or otherwise indicating an error? There is existing support to compile unreachable to a trap instruction (TrapUnreachable), but from a quick look, it doesn't seem like there is any BPF instruction this can be mapped to?

No, BPF arch does not have trapping insn or another other insn similar to trap/error.
BPF code is verified by the linux kernel. The verifier will do flow sensitive analysis to decide whether the prog is sane or not (memory corruption, out of bound, etc.).

I am not sure whether bpf should have trap insn or not. For a prog passing verifier, it should not trap in the kernel. Let us say we do introduce trap insn in BPF ISA, and llvm indeed inserts one which implies some path will hit 'trap' so probably we should error out at compile time. But it has slightly chance compiler may have a false positive, so I think a warning seems more appropriate.


https://github.com/llvm/llvm-project/pull/126858