On Thu, Mar 23, 2023 at 8:59 AM Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> wrote: > > From: Xu Kuohai <xukuohai@xxxxxxxxxx> > > dst_reg is marked as unknown when BPF_MOD instruction is verified, causing > the following bpf prog to be incorrectly rejected. > > 0: r0 = 0 > 1: r0 %= 1 // r0 is marked as unknown > 2: r1 = 0 > 3: r1 += 1 > 4: if r1 < r0 goto pc-2 // verifier treats the loop as unbounded > 5: exit > > To teach verifier to accept the above prog, this patch adds bound tracking > for BPF_MOD. > > The approach is based on the following rules: > > 1. BPF_MOD is unsigned; > > 2. For an unsigned constant divisor x: > > a. when x != 0, the resulted dst_reg bits are in the range [0, x - 1], > and if no wrapping occurs, the result can be further narrowed down > to [umin mod x, umax mod x]; > > b. when x == 0, dst_reg is truncated to 32 bits by mod32 or remains > unchanged by mod64. > > Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx> Same Nack as before. You haven't answered _why_ anyone needs it.
