On Sun, Aug 11, 2019 at 10:58:33AM +0200, Paul Chaignon wrote: > On Fri, Aug 9, 2019 at 8:26 PM Paul Chaignon <paul.chaig...@orange.com> wrote: > > > > We need BPF_MOD to match system calls against whitelists encoded as 32-bit > > bit arrays. The selection of the syscall's bit in the appropriate bit > > array requires a modulo operation such that X = 1 << nr % 32. > > Of course, X = 1 << nr & 0x1F, and we can do without BPF_MOD in our case. > I'll put that on a lack of sleep...
No worries! Changing the dialect of seccomp BPF isn't something I'd like to do without really good reason since it creates a split in the filter correctness from userspace (i.e. a filter using BPF_MOD on an older kernel will fail). So there would need to be a distinct flag set somewhere, etc. So, if you do end up discovering later you really want BPF_MOD, we can figure that out, but for now if you can get by with "&", that would be best. :) Thanks! -Kees > > > > > Signed-off-by: Paul Chaignon <paul.chaig...@orange.com> > > --- > > kernel/seccomp.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/seccomp.c b/kernel/seccomp.c > > index 811b4a86cdf6..87de6532ff6d 100644 > > --- a/kernel/seccomp.c > > +++ b/kernel/seccomp.c > > @@ -205,6 +205,8 @@ static int seccomp_check_filter(struct sock_filter > > *filter, unsigned int flen) > > case BPF_ALU | BPF_MUL | BPF_X: > > case BPF_ALU | BPF_DIV | BPF_K: > > case BPF_ALU | BPF_DIV | BPF_X: > > + case BPF_ALU | BPF_MOD | BPF_K: > > + case BPF_ALU | BPF_MOD | BPF_X: > > case BPF_ALU | BPF_AND | BPF_K: > > case BPF_ALU | BPF_AND | BPF_X: > > case BPF_ALU | BPF_OR | BPF_K: > > -- > > 2.17.1 -- Kees Cook
