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Date:   Wed, 4 Jan 2023 08:55:26 -0800
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Subject: Re: [PATCH] bpf: skip invalid kfunc call in backtrack_insn
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To:     Hao Sun <sunhao.th@gmail.com>, bpf@vger.kernel.org
Cc:     ast@kernel.org, daniel@iogearbox.net, john.fastabend@gmail.com,
        andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
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References: <20230104014709.9375-1-sunhao.th@gmail.com>
From:   Yonghong Song <yhs@meta.com>
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On 1/3/23 5:47 PM, Hao Sun wrote:
> The verifier skips invalid kfunc call in check_kfunc_call(), which
> would be captured in fixup_kfunc_call() if such insn is not
> eliminated by dead code elimination. However, this can lead to the
> following warning in backtrack_insn(), alse see [1]:
> 
> ------------[ cut here ]------------
> verifier backtracking bug
> WARNING: CPU: 6 PID: 8646 at kernel/bpf/verifier.c:2756 backtrack_insn
> kernel/bpf/verifier.c:2756
> 	__mark_chain_precision kernel/bpf/verifier.c:3065
> 	mark_chain_precision kernel/bpf/verifier.c:3165
> 	adjust_reg_min_max_vals kernel/bpf/verifier.c:10715
> 	check_alu_op kernel/bpf/verifier.c:10928
> 	do_check kernel/bpf/verifier.c:13821 [inline]
> 	do_check_common kernel/bpf/verifier.c:16289
> ...
> 
> So make backtracking conservative with this by returning ENOTSUPP.
> 
> [1] https://lore.kernel.org/bpf/CACkBjsaXNceR8ZjkLG=dT3P=4A8SBsg0Z5h5PWLryF5=ghKq=g@mail.gmail.com/
> 
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>

Acked-by: Yonghong Song <yhs@fb.com>