On Fri, Sep 25, 2020 at 07:47:47PM -0700, Andy Lutomirski wrote: > > > On Sep 25, 2020, at 6:23 PM, YiFei Zhu <[email protected]> wrote: > > > > On Fri, Sep 25, 2020 at 4:07 PM Andy Lutomirski <[email protected]> > > wrote: > >> We'd need at least three states per syscall: unknown, always-allow, > >> and need-to-run-filter. > >> > >> The downsides are less determinism and a bit of an uglier > >> implementation. The upside is that we don't need to loop over all > >> syscalls at load -- instead the time that each operation takes is > >> independent of the total number of syscalls on the system. And we can > >> entirely avoid, say, evaluating the x32 case until the task tries an > >> x32 syscall. > > > > I was really afraid of multiple tasks writing to the bitmaps at once, > > hence I used bitmap-per-task. Now I think about it, if this stays > > lockless, the worst thing that can happen is that a write undo a bit > > set by another task. In this case, if the "known" bit is cleared then > > the worst would be the emulation is run many times. But if the "always > > allow" is cleared but not "known" bit then we have an issue: the > > syscall will always be executed in BPF. > > > > If you interleave the bits, then you can read and write them atomically — > both bits for any given syscall will be in the same word.
I think we can just hold the spinlock. :) -- Kees Cook
