Re: [PATCH v3 1/2] bpf: Skip scalar adjustment for BPF_NEG if dst is a pointer

[Alexei Starovoitov]

On Wed, Oct 1, 2025 at 2:56 AM Brahmajit Das <listout@xxxxxxxxxxx> wrote:
>
> In check_alu_op(), the verifier currently calls check_reg_arg() and
> adjust_scalar_min_max_vals() unconditionally for BPF_NEG operations.
> However, if the destination register holds a pointer, these scalar
> adjustments are unnecessary and potentially incorrect.
>
> This patch adds a check to skip the adjustment logic when the destination
> register contains a pointer.
>
> Reported-by: syzbot+d36d5ae81e1b0a53ef58@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=d36d5ae81e1b0a53ef58
> Fixes: aced132599b3 ("bpf: Add range tracking for BPF_NEG")
> Suggested-by: KaFai Wan <kafai.wan@xxxxxxxxx>
> Signed-off-by: Brahmajit Das <listout@xxxxxxxxxxx>
> ---
>  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 e892df386eed..4b0924c38657 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -15645,7 +15645,8 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
>                 }
>
>                 /* check dest operand */
> -               if (opcode == BPF_NEG) {
> +               if (opcode == BPF_NEG &&
> +                   !__is_pointer_value(false, &regs[insn->dst_reg])) {
>                         err = check_reg_arg(env, insn->dst_reg, DST_OP_NO_MARK);


The fix makes sense.

Song,
Eduard,

please take a look.