Re: [PATCH bpf-next 3/6] libbpf: Add feature for kernel extended vlen/kind support

Alan Maguire
2026-04-16T08:58:22Z

On 15/04/2026 16:57, Mykyta Yatsenko wrote:
>
>
> On 4/14/26 8:50 PM, Alan Maguire wrote:
>> Add feature check for kernel extended vlen/kind support, and reject
>> BTF that uses extended vlens/kinds if the kernel does not support
>> it.  There is no reasonable path to generally sanitize such BTF.
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
>> ---
>>   tools/lib/bpf/features.c        | 39 +++++++++++++++++++++++++++++++++
>>   tools/lib/bpf/libbpf.c          | 17 +++++++++++++-
>>   tools/lib/bpf/libbpf_internal.h |  2 ++
>>   3 files changed, 57 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
>> index 4f19a0d79b0c..4a75d6717bbb 100644
>> --- a/tools/lib/bpf/features.c
>> +++ b/tools/lib/bpf/features.c
>> @@ -615,6 +615,42 @@ static int probe_kern_btf_layout(int token_fd)
>>                            (char *)layout, token_fd));
>>   }
>>   +static int probe_kern_btf_vlen_kind_extended(int token_fd)
>> +{
>> +    struct btf *btf;
>> +    int ret = 0;
>> +    __s32 id;
>> +    int err;
>> +
>> +    btf = btf__load_vmlinux_btf();
>> +    err = libbpf_get_error(btf);
>> +    if (err)
>> +        return err;
>> +    id = btf__find_by_name_kind(btf, "btf_max", BTF_KIND_ENUM);
>
> Looks like if id > 0 then extended vlen and kinds are available, so there is no point to continue processing the enum and checking value of the BTF_MAX_VLEN. Or is there a scenario, where BTF_MAX_VLEN goes back to 0xffff?
>

True, the enum presence is enough here. However another point the
bots made here needs to be addressed; other BTF kernel probes rely
on the bpf syscall, loading BTF into the kernel to check for feature
support. We're better off using that approach here too since it will
work even in container envs which don't have /sys/kernel/btf availability.
We can load BTF with an extended vlen (> 0xffff) and ensure it is accepted,
like the test does in patch 5 essentially.

All that said, I think the "enum btf_max" conversion is still worthwhile,
since it's conceivable users might want some of these values in vmlinux.h,
and we can deal with any potential fallout by using redundant macro
definintions like

#define BTF_MAX_TYPE BTF_MAX_TYPE

just in case someone does rely on the UAPI macros.

Thanks for taking a look!

Alan

>> +    if (id > 0) {
>> +        const struct btf_type *t;
>> +        const struct btf_enum *e;
>> +        const char *name;
>> +        __u32 i, vlen;
>> +
>> +        t = btf__type_by_id(btf, id);
>> +        if (t) {
>> +            vlen = btf_vlen(t);
>> +
>> +            for (i = 0, e = btf_enum(t); i < vlen; i++, e++) {
>> +                name = btf__name_by_offset(btf, e->name_off);
>> +                if (!name || strcmp(name, "BTF_MAX_VLEN") != 0)
>> +                    continue;
>> +                if (e->val > 0xffff)
>> +                    ret = 1;
>> +            }
>> +        }
>> +    }
>> +    btf__free(btf);
>> +
>> +    return ret;
>> +}
>> +
>>   typedef int (*feature_probe_fn)(int /* token_fd */);
>>     static struct kern_feature_cache feature_cache;
>> @@ -699,6 +735,9 @@ static struct kern_feature_desc {
>>       [FEAT_BTF_LAYOUT] = {
>>           "kernel supports BTF layout", probe_kern_btf_layout,
>>       },
>> +    [FEAT_BTF_VLEN_KIND_EXTENDED] = {
>> +        "kernel supports extended BTF vlen/kind", probe_kern_btf_vlen_kind_extended,
>> +    },
>>   };
>>     bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 8b0c3246097f..5f19d8ac17a9 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -3139,10 +3139,11 @@ static bool btf_needs_sanitization(struct bpf_object *obj)
>>       bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
>>       bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
>>       bool has_layout = kernel_supports(obj, FEAT_BTF_LAYOUT);
>> +    bool has_vlen_kind_extended = kernel_supports(obj, FEAT_BTF_VLEN_KIND_EXTENDED);
>>         return !has_func || !has_datasec || !has_func_global || !has_float ||
>>              !has_decl_tag || !has_type_tag || !has_enum64 || !has_qmark_datasec ||
>> -           !has_layout;
>> +           !has_layout || !has_vlen_kind_extended;
>>   }
>>     struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_btf)
>> @@ -3156,6 +3157,7 @@ struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_bt
>>       bool has_enum64 = kernel_supports(obj, FEAT_BTF_ENUM64);
>>       bool has_qmark_datasec = kernel_supports(obj, FEAT_BTF_QMARK_DATASEC);
>>       bool has_layout = kernel_supports(obj, FEAT_BTF_LAYOUT);
>> +    bool has_vlen_kind_extended = kernel_supports(obj, FEAT_BTF_VLEN_KIND_EXTENDED);
>>       int enum64_placeholder_id = 0;
>>       const struct btf_header *hdr;
>>       struct btf *btf = NULL;
>> @@ -3217,6 +3219,19 @@ struct btf *bpf_object__sanitize_btf(struct bpf_object *obj, struct btf *orig_bt
>>       for (i = 1; i < btf__type_cnt(btf); i++) {
>>           t = (struct btf_type *)btf__type_by_id(btf, i);
>>   +        /*
>> +         * If BTF uses extended vlen/kind and kernel does not support
>> +         * it, there is nothing we can do.
>> +         */
>> +        if (!has_vlen_kind_extended) {
>> +            if (btf_vlen(t) > 0xffff || btf_kind(t) > 0x1f) {
>> +                pr_debug("Unsupported %s for id %u\n",
>> +                     btf_kind(t) > 0x1f ? "BTF kind" : "BTF vlen", i);
>> +                btf__free(btf);
>> +                return ERR_PTR(-EINVAL);
>> +            }
>> +        }
>> +
>>           if ((!has_datasec && btf_is_var(t)) || (!has_decl_tag && btf_is_decl_tag(t))) {
>>               /* replace VAR/DECL_TAG with INT */
>>               t->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
>> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
>> index 3781c45b46d3..8ef1fc96b73a 100644
>> --- a/tools/lib/bpf/libbpf_internal.h
>> +++ b/tools/lib/bpf/libbpf_internal.h
>> @@ -398,6 +398,8 @@ enum kern_feature_id {
>>       FEAT_UPROBE_SYSCALL,
>>       /* Kernel supports BTF layout information */
>>       FEAT_BTF_LAYOUT,
>> +    /* Kernel supports BTF vlen > 65535 */
>> +    FEAT_BTF_VLEN_KIND_EXTENDED,
>>       __FEAT_CNT,
>>   };
>>
>