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.
