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

[Menglong Dong]

Hello,

On Thu, Dec 14, 2023 at 9:49 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Wed, Dec 13, 2023 at 10:28 PM Menglong Dong <menglong8.dong@xxxxxxxxx> 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);
> >   }
>
> Please craft a selftest from above with inline asm
> (C might not work as compiler might optimize it)

Okay! Should I add this selftests to reg_bounds as a subtest,
or add a "verifier_reg_edge.c" for verifier testing?

> Also we call:
>         /* fallthrough (FALSE) branch */
>         regs_refine_cond_op(false_reg1, false_reg2,
> rev_opcode(opcode), is_jmp32);
>         /* jump (TRUE) branch */
>         regs_refine_cond_op(true_reg1, true_reg2, opcode, is_jmp32);
>
> so despite BPF_JNE is not handled explicitly it still should have
> caught above due to rev_opcode() ?

Ennn.....I'm a little confused. In this case, the TRUE path is
handled properly, as the opcode is BPF_JEQ; and the FALSE
is not handled properly, as the opcode is rev_opcode(BPF_JEQ),
which is BPF_JNE. And the bpf_skb_store_bytes() will be called
in the FALSE path. The origin state of false_reg* should be the same
as true_reg*.