Re: [PATCH] bpf: skip invalid kfunc call in backtrack_insn

From: Yonghong Song
Date: Wed Jan 04 2023 - 11:58:10 EST




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@xxxxxxxxxxxxxx/

Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>

Acked-by: Yonghong Song <yhs@xxxxxx>