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

[Eduard Zingerman <eddyz87@gmail.com>]

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}.

> For
> 14   - (2) {7P,7,7},depth=1
> 15     - (0) {7P,42,7},depth=1          considered safe, pruned using checkpoint (a)
> I suspect for line 15, the pruning uses checking point at line (8).

Right, because checkpoints for a particular insn form a stack. My bad.

> Other than the above, the diagram LGTM.

Thank you for the feedback, I'll post v2 shortly.