On Thu, Nov 3, 2022 at 4:58 PM Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> wrote: > > Kind of a hack, but works for now: > > Instead of listening for any close of eBPF program, we now > decrement the refcount when we insert it in our internal > map of fd progs. > > This is safe to do because: > - we listen to any call of destructor of programs > - when a program is being destroyed, we disable it by removing > it from any RCU list used by any HID device (so it will never > be called) > - we then trigger a job to cleanup the prog fd map, but we overwrite > the removal of the elements to not do anything on the programs, just > remove the allocated space > > This is better than previously because we can remove the map of known > programs and their usage count. We now rely on the refcount of > bpf, which has greater chances of being accurate. > > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> > > --- So... I am a little bit embarrassed, but it turns out that this hack is not safe enough. If I compile the kernel with LLVM=1, the function bpf_prog_put_deferred() is optimized in a weird way: if we are not in irq, the function is inlined into __bpf_prog_put(), but if we are, the function is still kept around as it is called in a scheduled work item. This is something I completely overlooked: I assume that if the function would be inlined, the HID entrypoint BPF preloaded object would not be able to bind, thus deactivating HID-BPF safely. But if a function can be both inlined and not inlined, then I have no guarantees that my cleanup call will be called. Meaning that a HID device might believe there is still a bpf function to call. And things will get messy, with kernel crashes and others. An easy "fix" would be to tag bpf_prog_put_deferred() with "noinline", but it just feels wrong to have that for this specific reason. AFAICT, gcc is not doing that optimisation, but nothing prevents it from doing it, and suddenly that will be a big whole in the kernel. As much as I wish I had another option, I think for the sake of everyone (and for my future holidays) I'll postpone HID-BPF to 6.3. I actually thought of another way of removing that trampoline call. So I'm not entirely going back to the drawing board hopefully. [a few hours laters] Just as a preview, I am reusing the bpf_link idea: when we call hid_bpf_attach_prog(), this creates a bpf_link, and that link is the one that needs to be pinned. Whenever all the references of that link are dropped, I get called in the link's ->release() function, and I can force the unbinding of the hid-device to the program at that time. Way safer (no refcount mess up) and no optimisations can interfere, now that I am not "tracing" the bpf core code. Cheers, Benjamin
