Re: [PATCH bpf-next v2 0/2] bpf: Add multi-level pointer parameter

[Yonghong Song <yonghong.song@linux.dev>]

On 2/18/26 7:15 PM, Alexei Starovoitov wrote:
> On Wed, Feb 18, 2026 at 2:44 AM Slava Imameev
> <slava.imameev@crowdstrike.com> wrote:
>>
>> - This programs cannot be compiled as bpf_core_cast cannot cast to a
>> multi-level pointer:
>>
>> SEC("lsm/sb_eat_lsm_opts")
>> int BPF_PROG(sb_eat_lsm_opts_1,char *options, void **mnt_opts)
>> {
>>      void** ppt = bpf_core_cast(mnt_opts, void*);
>>      bpf_printk("%p\t", *ppt);
>>      return 0;
>> }
> Looks like there is a bug in llvm, since it crashes on the above.
> But the following works:
>
> void** ppt = bpf_rdonly_cast(mnt_opts, 0);
> bpf_printk("%lx\t", *ppt);
>
> Plus Eduard's diff:
> -       if (is_void_or_int_ptr(btf, t))
> +       if (is_void_or_int_ptr(btf, t) || !btf_type_is_struct_ptr(btf, t))
>
>
>> - There is a workaround, which requires introducing a wrapper for
>> a pointer or typedef:
>>
>> struct pvoid {
>>      void* v;
>> };
> llvm should have handled it without the workaround. It's a bug
> that should be fixed.

Okay, I will take a look.

>
>> I think there are no technical restrictions for treating single
>> level pointers as PTR_TO_MEM.
> I think it will be a missed opportunity and a potential foot gun.
>
> We didn't support access to 'char *' initially.
> Later relaxed it to mean that it's a valid pointer to a single byte,
> but since the code is generic it also became the case
> that 'char *' is allowed in kfunc and the verifier checks
> that one valid byte is there.
> That was a bad footgun, since we saw several cases of broken
> kfunc implementations that assume that 'char *' means a string.
>
> There are only two lsm hooks that pass 'struct foo **'.
> If we make it ptr_to_mem of 8 bytes we lose the ability to do
> something smarter in the future.
> I think we better add support to annotate such '**' as an actual
> array with given size and track types completely, so
> 'struct foo * var[N]' will become array_to_ptr_to_btf_id.
> That's probably more work that you signed up to do,
> so I suggest treating 'void **' as a scalar as Eduard suggested.
> This particular sb_eat_lsm_opts() hook
> doesn't have a useful type behind it anyway.
> I'm less certain about 'char **'. If we make it scalar too
> it will be harder to make it a pointer to nul terminated string later.
>
> So I would do 'void **' -> scalar for now only.