Re: [PATCH bpf-next v3 1/2] bpf: make the verifier tracks the "not equal" for regs

[Shung-Hsi Yu]

On Thu, Dec 14, 2023 at 02:24:33PM +0800, Menglong Dong wrote:
> We can derive some new information for BPF_JNE in regs_refine_cond_op().
> Take following code for example:
> 
>   /* The type of "a" is u16 */
>   if (a > 0 && a < 100) {
>     /* the range of the register for a is [0, 99], not [1, 99],
>      * and will cause the following error:
>      *
>      *   invalid zero-sized read
>      *
>      * as a can be 0.
>      */
>     bpf_skb_store_bytes(skb, xx, xx, a, 0);
>   }
> 
> In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> TRUE branch, the dst_reg will be marked as known to 0. However, in the
> fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> the [min, max] for a is [0, 99], not [1, 99].
> 
> For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
> const and is exactly the edge of the dst reg.
> 
> Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>

Acked-by: Shung-Hsi Yu <shung-hsi.yu@xxxxxxxx>