Re: [PATCH bpf-next v2 1/2] bpf: add bound tracking for BPF_MOD

[Xu Kuohai]

On 3/24/2023 1:16 AM, Alexei Starovoitov wrote:
> 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.

Sorry, I did not receive Nack for this patch before.

> You haven't answered _why_ anyone needs it.

No idea, I simply believe it's good to have a closer estimate of the result.