Re: [PATCH bpf-next v3 1/4] bpf: Adapt 32-bit return value kfunc for 32-bit ARM when zext extension

[Alexei Starovoitov]

On Fri, Dec 2, 2022 at 6:58 PM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
>
>
>
> On 2022/11/29 0:41, Alexei Starovoitov wrote:
> > On Mon, Nov 28, 2022 at 4:40 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2022/11/28 9:57, Alexei Starovoitov wrote:
> >>> On Sat, Nov 26, 2022 at 05:45:27PM +0800, Yang Jihong wrote:
> >>>> For ARM32 architecture, if data width of kfunc return value is 32 bits,
> >>>> need to do explicit zero extension for high 32-bit, insn_def_regno should
> >>>> return dst_reg for BPF_JMP type of BPF_PSEUDO_KFUNC_CALL. Otherwise,
> >>>> opt_subreg_zext_lo32_rnd_hi32 returns -EFAULT, resulting in BPF failure.
> >>>>
> >>>> Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> >>>> ---
> >>>>    kernel/bpf/verifier.c | 44 ++++++++++++++++++++++++++++++++++++++++---
> >>>>    1 file changed, 41 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>>> index 264b3dc714cc..193ea927aa69 100644
> >>>> --- a/kernel/bpf/verifier.c
> >>>> +++ b/kernel/bpf/verifier.c
> >>>> @@ -1927,6 +1927,21 @@ find_kfunc_desc(const struct bpf_prog *prog, u32 func_id, u16 offset)
> >>>>                      sizeof(tab->descs[0]), kfunc_desc_cmp_by_id_off);
> >>>>    }
> >>>>
> >>>> +static int kfunc_desc_cmp_by_imm(const void *a, const void *b);
> >>>> +
> >>>> +static const struct bpf_kfunc_desc *
> >>>> +find_kfunc_desc_by_imm(const struct bpf_prog *prog, s32 imm)
> >>>> +{
> >>>> +    struct bpf_kfunc_desc desc = {
> >>>> +            .imm = imm,
> >>>> +    };
> >>>> +    struct bpf_kfunc_desc_tab *tab;
> >>>> +
> >>>> +    tab = prog->aux->kfunc_tab;
> >>>> +    return bsearch(&desc, tab->descs, tab->nr_descs,
> >>>> +                   sizeof(tab->descs[0]), kfunc_desc_cmp_by_imm);
> >>>> +}
> >>>> +
> >>>>    static struct btf *__find_kfunc_desc_btf(struct bpf_verifier_env *env,
> >>>>                                        s16 offset)
> >>>>    {
> >>>> @@ -2342,6 +2357,13 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
> >>>>                        */
> >>>>                       if (insn->src_reg == BPF_PSEUDO_CALL)
> >>>>                               return false;
> >>>> +
> >>>> +                    /* Kfunc call will reach here because of insn_has_def32,
> >>>> +                     * conservatively return TRUE.
> >>>> +                     */
> >>>> +                    if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL)
> >>>> +                            return true;
> >>>> +
> >>>>                       /* Helper call will reach here because of arg type
> >>>>                        * check, conservatively return TRUE.
> >>>>                        */
> >>>> @@ -2405,10 +2427,26 @@ static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
> >>>>    }
> >>>>
> >>>>    /* Return the regno defined by the insn, or -1. */
> >>>> -static int insn_def_regno(const struct bpf_insn *insn)
> >>>> +static int insn_def_regno(struct bpf_verifier_env *env, const struct bpf_insn *insn)
> >>>>    {
> >>>>       switch (BPF_CLASS(insn->code)) {
> >>>>       case BPF_JMP:
> >>>> +            if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {
> >>>> +                    const struct bpf_kfunc_desc *desc;
> >>>> +
> >>>> +                    /* The value of desc cannot be NULL */
> >>>> +                    desc = find_kfunc_desc_by_imm(env->prog, insn->imm);
> >>>> +
> >>>> +                    /* A kfunc can return void.
> >>>> +                     * The btf type of the kfunc's return value needs
> >>>> +                     * to be checked against "void" first
> >>>> +                     */
> >>>> +                    if (desc->func_model.ret_size == 0)
> >>>> +                            return -1;
> >>>> +                    else
> >>>> +                            return insn->dst_reg;
> >>>> +            }
> >>>> +            fallthrough;
> >>>
> >>> I cannot make any sense of this patch.
> >>> insn->dst_reg above is 0.
> >>> The kfunc call doesn't define a register from insn_def_regno() pov.
> >>>
> >>> Are you hacking insn_def_regno() to return 0 so that
> >>> if (WARN_ON(load_reg == -1)) {
> >>>     verbose(env, "verifier bug. zext_dst is set, but no reg is defined\n");
> >>>     return -EFAULT;
> >>> }
> >>> in opt_subreg_zext_lo32_rnd_hi32() doesn't trigger ?
> >>>
> >>> But this verifier message should have been a hint that you need
> >>> to analyze why zext_dst is set on this kfunc call.
> >>> Maybe it shouldn't ?
> >>> Did you analyze the logic of mark_btf_func_reg_size() ?
> >> make r0 zext is not caused by mark_btf_func_reg_size.
> >>
> >> This problem occurs when running the kfunc_call_test_ref_btf_id test
> >> case in the 32-bit ARM environment.
> >
> > Why is it not failing on x86-32 ?
> Use the latest mainline kernel code to test on the x86_32 machine. The
> test also fails:
>
>    # ./test_progs -t kfunc_call/kfunc_call_test_ref_btf_id
>    Failed to load bpf_testmod.ko into the kernel: -8
>    WARNING! Selftests relying on bpf_testmod.ko will be skipped.
>    libbpf: prog 'kfunc_call_test_ref_btf_id': BPF program load failed:
> Bad address
>    libbpf: prog 'kfunc_call_test_ref_btf_id': -- BEGIN PROG LOAD LOG --
>    processed 25 insns (limit 1000000) max_states_per_insn 0 total_states
> 2 peak_states 2 mark_read 1
>    -- END PROG LOAD LOG --
>    libbpf: prog 'kfunc_call_test_ref_btf_id': failed to load: -14
>    libbpf: failed to load object 'kfunc_call_test'
>    libbpf: failed to load BPF skeleton 'kfunc_call_test': -14
>    verify_success:FAIL:skel unexpected error: -14
>
> Therefore, this problem also exists on x86_32:
> "verifier bug. zext_dst is set, but no reg is defined"

The kernel returns -14 == EFAULT.
That's a completely different issue.