[sporksmith]

We've started looking into eBPF a bit - IIUC eBPF by itself doesn't give us the ability to service or arbitrarily manipulate the traced process's syscalls.

We have recently learned of an interesting technique that dettrace [1] uses of combining seccomp with an eBPF filter and ptrace. Instead of generating a ptrace-stop for every syscall (as we do now, using PTRACE_SYSEMU), they use a seccomp policy with an eBPF filter, s.t. a ptrace-stop is only generated for syscalls that violate the policy, allowing them to emulate the result of those syscalls. syscalls that don't violate the policy are allowed to execute natively, saving a lot of overhead.

[1]: https://github.com/dettrace/dettrace

This works great for them since they want to emulate a relatively small subset of syscalls. In our case we want to emulate most syscalls, so it's not as clear-cut of a win. We have found though that if we use an LD_PRELOAD'd shim in the target process to intercept syscalls and then service them via IPC, that's substantially faster than catching them with ptrace. That runs back into the problems with LD_PRELOAD in general of there being various ways of missing syscalls. but, we may be able to use that technique along with ptrace+seccomp+ebpf to intercept any syscalls that we'd otherwise miss. The seccomp technique would allow us to exempt the syscalls that our shim itself is making to do the IPC.

[littlestymaar]

Thanks for the detailed answer!