Re: [PATCH bpf-next] bpf: ensure precise is reset to false in __mark_reg_const_zero()

[Maxim Mikityanskiy <maxtram95@gmail.com>]

On Fri, 15 Dec 2023 at 15:58:22 -0800, Andrii Nakryiko wrote:
> It is safe to always start with imprecise SCALAR_VALUE register.
> Previously __mark_reg_const_zero() relied on caller to reset precise
> mark, but it's very error prone and we already missed it in a few
> places. So instead make __mark_reg_const_zero() reset precision always,
> as it's a safe default for SCALAR_VALUE. Explanation is basically the
> same as for why we are resetting (or rather not setting) precision in
> current state. If necessary, precision propagation will set it to
> precise correctly.
> 
> As such, also remove a big comment about forward precision propagation
> in mark_reg_stack_read() and avoid unnecessarily setting precision to
> true after reading from STACK_ZERO stack. Again, precision propagation
> will correctly handle this, if that SCALAR_VALUE register will ever be
> needed to be precise.
> 
> Reported-by: Maxim Mikityanskiy <maxtram95@gmail.com>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  kernel/bpf/verifier.c                            | 16 +++-------------
>  .../selftests/bpf/progs/verifier_spill_fill.c    | 10 ++++++++--
>  2 files changed, 11 insertions(+), 15 deletions(-)

Thanks for the prompt fix!

Acked-by: Maxim Mikityanskiy <maxtram95@gmail.com>