> On Sep 25, 2020, at 6:23 PM, YiFei Zhu <zhuyifei1999@xxxxxxxxx> wrote: > > On Fri, Sep 25, 2020 at 4:07 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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. > Is it worth holding a spinlock here? > > Though I'll try to get the benchmark numbers for the emulator later tonight. > > YiFei Zhu
