An update on BPF generation from GCC
The session was run by David Faust, Cupertino Miranda, and José Marchesi, with Faust doing most of the talking. He started by saying that the core of the BPF backend is now nearly in a production-ready state. It is able to compile all of the kernel test cases — but they do not yet all run correctly. In each case, the generated code is correct, but it is unable to get past the BPF verifier. Most of the self-tests pass at this point, though.
The BPF backend for GCC has been shipped with Oracle Linux for some time, and has more recently found its way into Debian, Fedora, and Gentoo, which is now using GCC to build systemd's BPF programs.
Keeping up with the latest BPF developments is an ongoing task for the GCC developers; there are a number of new instructions that have been added in recent years that must be supported and used to their full potential. These include unconditional byte-swap instructions, jumps with a 32-bit displacement, signed memory-load and register-move operations, and signed division and modulus instructions.
Another significant project in the last year has been addressing one of the biggest divergences with Clang, having to do with the handling of compile-time integer overflow, such as can happen when, for example, assigning a negative number to an unsigned value. Both compiler teams agreed to accept any value that will fit into the intended data type and let the BPF virtual machine deal with it from there.
