linux-kernel - Re: [PATCH v2] bpf: Fix unsigned 'datasec_id' compared with zero in check_pseudo_btf

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Date:   Tue, 10 Nov 2020 20:14:32 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     xiakaixu1987@...il.com
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Kaixu Xia <kaixuxia@...cent.com>
Subject: Re: [PATCH v2] bpf: Fix unsigned 'datasec_id' compared with zero in check_pseudo_btf_id

On Tue, Nov 10, 2020 at 6:59 PM <xiakaixu1987@...il.com> wrote:
>
> From: Kaixu Xia <kaixuxia@...cent.com>
>
> The unsigned variable datasec_id is assigned a return value from the call
> to check_pseudo_btf_id(), which may return negative error code.
>
> Fixes coccicheck warning:
>
> ./kernel/bpf/verifier.c:9616:5-15: WARNING: Unsigned expression compared with zero: datasec_id > 0
>
> Reported-by: Tosk Robot <tencent_os_robot@...cent.com>
> Signed-off-by: Kaixu Xia <kaixuxia@...cent.com>
> ---
> v2:
>  -split out datasec_id definition into a separate line.
>
>  kernel/bpf/verifier.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6200519582a6..3fea4fc04e94 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9572,7 +9572,8 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,
>                                struct bpf_insn *insn,
>                                struct bpf_insn_aux_data *aux)
>  {
> -       u32 datasec_id, type, id = insn->imm;
> +       s32 datasec_id;
> +       u32 type, id = insn->imm;
>         const struct btf_var_secinfo *vsi;
>         const struct btf_type *datasec;
>         const struct btf_type *t;
> --
> 2.20.0
>

It would look a bit cleaner if you did it this way:

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 10da26e55130..f674b1403637 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9585,12 +9585,13 @@ static int check_pseudo_btf_id(struct
bpf_verifier_env *env,
                               struct bpf_insn *insn,
                               struct bpf_insn_aux_data *aux)
 {
-       u32 datasec_id, type, id = insn->imm;
        const struct btf_var_secinfo *vsi;
        const struct btf_type *datasec;
        const struct btf_type *t;
        const char *sym_name;
        bool percpu = false;
+       u32 type, id = insn->imm;
+       s32 datasec_id;
        u64 addr;
        int i;