Re: [PATCH bpf-next 2/3] bpf: check bpf_func_state->callback_depth when pruning states

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-02-19 at 16:25 -0800, Yonghong Song wrote:
> On 2/16/24 6:27 AM, Eduard Zingerman wrote:
> > On Wed, 2024-02-14 at 09:42 -0800, Yonghong Song wrote:
> > 
> > > >    .------------------------------------- Checkpoint / State name
> > > >    |    .-------------------------------- Code point number
> > > >    |    |   .---------------------------- Stack state {ctx.a,ctx.b,ctx.c}
> > > >    |    |   |        .------------------- Callback depth in frame #0
> > > >    v    v   v        v
> > > > 1  - (0) {7P,7P,7},depth=0
> > > > 2    - (3) {7P,7P,7},depth=1
> > > > 3      - (0) {7P,7P,42},depth=1
> > > > (a)      - (3) {7P,7,42},depth=2
> > > > 4          - (0) {7P,7,42},depth=2      loop terminates because of depth limit
> > > > 5            - (4) {7P,7,42},depth=0    predicted false, ctx.a marked precise
> > > > 6            - (6) exit
> > > > 7        - (2) {7P,7,42},depth=2
> > > > 8          - (0) {7P,42,42},depth=2     loop terminates because of depth limit
> > > > 9            - (4) {7P,42,42},depth=0   predicted false, ctx.a marked precise
> > > > 10           - (6) exit
> > > > (b)      - (1) {7P,7P,42},depth=2
> > > > 11         - (0) {42P,7P,42},depth=2    loop terminates because of depth limit
> > > > 12           - (4) {42P,7P,42},depth=0  predicted false, ctx.{a,b} marked precise
> > > > 13           - (6) exit
> > > > 14   - (2) {7P,7,7},depth=1
> > > > 15     - (0) {7P,42,7},depth=1          considered safe, pruned using checkpoint (a)
> > > > (c)  - (1) {7P,7P,7},depth=1            considered safe, pruned using checkpoint (b)
> > > For the above line
> > >      (c)  - (1) {7P,7P,7},depth=1            considered safe, pruned using checkpoint (b)
> > > I would change to
> > >      (c)  - (1) {7P,7P,7},depth=1
> > >             - (0) {42P, 7P, 7},depth = 1     considered safe, pruned using checkpoint (11)
> > At that point:
> > - there is a checkpoint at (1) with state {7P,7P,42}
> > - verifier is at (1) in state {7,7,7}
> > Thus, verifier won't proceed to (0) because {7,7,7} is states_equal to {7P,7P,42}.
> 
> Okay, I think the above example has BPF_F_TEST_STATE_FREQ set as in Patch 3. It will
> be great if you can explicitly mention this (BPF_F_TEST_STATE_FREQ) in the commit message.

Will do.

[...]

> But then for
>    14   - (2) {7P,7,7},depth=1
>    15     - (0) {7P,42,7},depth=1          considered safe, pruned using checkpoint (a)
> The state
>    14   - (2) {7P,7,7},depth=1
> will have state equal to
>     7        - (2) {7P,7,42},depth=2
> right?

I think you are correct.