Re: [PATCH bpf-next 2/4] bpf: fix precision backtracking instruction iteration

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2023-11-08 at 15:11 -0800, Andrii Nakryiko wrote:
> Fix an edge case in __mark_chain_precision() which prematurely stops
> backtracking instructions in a state if it happens that state's first
> and last instruction indexes are the same. This situations doesn't
> necessarily mean that there were no instructions simulated in a state,
> but rather that we starting from the instruction, jumped around a bit,
> and then ended up at the same instruction before checkpointing or
> marking precision.
> 
> To distinguish between these two possible situations, we need to consult
> jump history. If it's empty or contain a single record "bridging" parent
> state and first instruction of processed state, then we indeed
> backtracked all instructions in this state. But if history is not empty,
> we are definitely not done yet.
> 
> Move this logic inside get_prev_insn_idx() to contain it more nicely.
> Use -ENOENT return code to denote "we are out of instructions"
> situation.
>
> This bug was exposed by verifier_cfg.c's bounded_recursion subtest, once

Note: verifier_cfg.c should be verifier_loops1.c

> the next fix in this patch set is applied.
> 
> Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking")
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Funny how nobody noticed this bug for so long, I looked at exactly
this code today while going through your other patch-set and no alarm
bells rang in my head.

I think that this case needs a dedicated test case that would check
precision tracking log.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>