Re: [PATCH bpf-next v2 1/2] bpf: Track aligned st store as imprecise spilled registers

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-01-04 at 09:13 -0800, Yonghong Song wrote:
> On 1/4/24 8:37 AM, Eduard Zingerman wrote:

[...]

> > I agree with this change, but I don't understand under which conditions
> > current STACK_ZERO logic is sub-optimal.
> > I tried executing test case from patch #2 w/o applying patch #1 and it passes.
> > Could you please elaborate / conjure a test case that would fail w/o patch #1?
> 
> The logic is similar to
>    https://lore.kernel.org/all/20231205184248.1502704-9-andrii@kernel.org/
> 
> STACK_ZERO logic is sub-optimal in some cases only w.r.t. the number of
> verifier states. So there is no correctness issue.
> Patch 2 is added in response to Andrii's request in
>    https://lore.kernel.org/all/CAEf4BzaWets3fHUGtctwCNWecR9ASRCO2kFagNy8jJZmPBWYDA@mail.gmail.com/
> Since with patch 1 the original STACK_ZERO case is converted to STACK_SPILL,
> Patch 2 is added to cover STACK_ZERO case. So with or with patch 1, patch 2
> will succeed since it uses STACK_ZERO logic.

Understood, thank you.
Probably no need to add more tests then.

A patch [1] might be related, it handles STACK_ZERO vs zero spill vs
unbound scalar spill on regsafe side.

[1] https://lore.kernel.org/bpf/20231220214013.3327288-15-maxtram95@gmail.com/