On Thu, Jan 26, 2017 at 9:46 AM, Alexei Starovoitov [off-list ref] wrote:

On 1/26/17 8:37 AM, Andy Lutomirski wrote:

quoted

quoted

Think of bpf programs as safe kernel modules. They don't have
confined boundaries and program authors, if not careful, can shoot
themselves in the foot. We're not trying to prevent that because
it's impossible to check that the program is sane. Just like
it's impossible to check that kernel module is sane.
But in case of bpf we check that bpf program is _safe_ from the kernel
point of view. If it's doing some garbage, it's program's business.
Does it make more sense now?

With all due respect, I think this is not an acceptable way to think
about BPF at all.  If you think of BPF this way, I think there needs
to be a real discussion at KS or similar as to whether this is okay.
The reason is simple: the kernel promises a stable ABI to userspace
but not to kernel modules.  By thinking of BPF as more like a module,
you're taking a big shortcut that will either result in ABI breakage
down the road or in committing to a problematic stable ABI.

you misunderstood the analogy.
bpf abi is certainly stable. that's why we were careful of not
exposing anything to it that is not already stable.

In that case I don't understand what you're trying to say.  Eric
thinks your patch exposes a bad interface.  A bad interface for
userspace is a very different thing from a bad interface available to
kernel modules.  Are you saying that BPF is kernel-module-like in that
the ABI exposed to BPF programs doesn't need to meet the same quality
standards as userspace ABIs?

--Andy