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Date: Fri, 15 Dec 2023 18:44:35 -0800
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Subject: Re: [PATCH bpf-next] bpf: ensure precise is reset to false in
 __mark_reg_const_zero()
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To: Andrii Nakryiko <andrii@kernel.org>, bpf@vger.kernel.org, ast@kernel.org,
 daniel@iogearbox.net, martin.lau@kernel.org
Cc: kernel-team@meta.com, Maxim Mikityanskiy <maxtram95@gmail.com>
References: <20231215235822.908223-1-andrii@kernel.org>
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From: Yonghong Song <yonghong.song@linux.dev>
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On 12/15/23 3:58 PM, 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>

Acked-by: Yonghong Song <yonghong.song@linux.dev>