Re: [PATCH bpf-next 1/5] bpf: Add kfunc bpf_get_file_xattr

[Song Liu <songliubraving@meta.com>]

> On Oct 17, 2023, at 3:40 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Tue, Oct 17, 2023 at 3:16 PM Song Liu <songliubraving@meta.com> wrote:
>> 
>> 
>> 
>>> On Oct 17, 2023, at 2:52 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>> 
>>> On Tue, Oct 17, 2023 at 1:31 PM Song Liu <songliubraving@meta.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Oct 17, 2023, at 11:58 AM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>>>> 
>>>>> On Fri, Oct 13, 2023 at 11:29 AM Song Liu <song@kernel.org> wrote:
>>>>>> 
>>>>>> This kfunc can be used to read xattr of a file.
>>>>>> 
>>>>>> Since vfs_getxattr() requires null-terminated string as input "name", a new
>>>>>> helper bpf_dynptr_is_string() is added to check the input before calling
>>>>>> vfs_getxattr().
>>>>>> 
>>>>>> Signed-off-by: Song Liu <song@kernel.org>
>>>>>> ---
>>>>>> include/linux/bpf.h      | 12 +++++++++++
>>>>>> kernel/trace/bpf_trace.c | 44 ++++++++++++++++++++++++++++++++++++++++
>>>>>> 2 files changed, 56 insertions(+)
>>>>>> 
>>>>>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>>>>>> index 61bde4520f5c..f14fae45e13d 100644
>>>>>> --- a/include/linux/bpf.h
>>>>>> +++ b/include/linux/bpf.h
>>>>>> @@ -2472,6 +2472,13 @@ static inline bool has_current_bpf_ctx(void)
>>>>>>      return !!current->bpf_ctx;
>>>>>> }
>>>>>> 
>>>>>> +static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
>>>>> 
>>>>> is_zero_terminated would be more accurate? though there is nothing
>>>>> really dynptr-specific here...
>>>> 
>>>> is_zero_terminated sounds better.
>>>> 
>>>>> 
>>>>>> +{
>>>>>> +       char *str = ptr->data;
>>>>>> +
>>>>>> +       return str[__bpf_dynptr_size(ptr) - 1] == '\0';
>>>>>> +}
>>>>>> +
>>>>>> void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog);
>>>>>> 
>>>>>> void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
>>>>>> @@ -2708,6 +2715,11 @@ static inline bool has_current_bpf_ctx(void)
>>>>>>      return false;
>>>>>> }
>>>>>> 
>>>>>> +static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
>>>>>> +{
>>>>>> +       return false;
>>>>>> +}
>>>>>> +
>>>>>> static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
>>>>>> {
>>>>>> }
>>>>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>>>>>> index df697c74d519..946268574e05 100644
>>>>>> --- a/kernel/trace/bpf_trace.c
>>>>>> +++ b/kernel/trace/bpf_trace.c
>>>>>> @@ -24,6 +24,7 @@
>>>>>> #include <linux/key.h>
>>>>>> #include <linux/verification.h>
>>>>>> #include <linux/namei.h>
>>>>>> +#include <linux/fileattr.h>
>>>>>> 
>>>>>> #include <net/bpf_sk_storage.h>
>>>>>> 
>>>>>> @@ -1429,6 +1430,49 @@ static int __init bpf_key_sig_kfuncs_init(void)
>>>>>> late_initcall(bpf_key_sig_kfuncs_init);
>>>>>> #endif /* CONFIG_KEYS */
>>>>>> 
>>>>>> +/* filesystem kfuncs */
>>>>>> +__diag_push();
>>>>>> +__diag_ignore_all("-Wmissing-prototypes",
>>>>>> +                 "kfuncs which will be used in BPF programs");
>>>>>> +
>>>>>> +/**
>>>>>> + * bpf_get_file_xattr - get xattr of a file
>>>>>> + * @name_ptr: name of the xattr
>>>>>> + * @value_ptr: output buffer of the xattr value
>>>>>> + *
>>>>>> + * Get xattr *name_ptr* of *file* and store the output in *value_ptr*.
>>>>>> + *
>>>>>> + * Return: 0 on success, a negative value on error.
>>>>>> + */
>>>>>> +__bpf_kfunc int bpf_get_file_xattr(struct file *file, struct bpf_dynptr_kern *name_ptr,
>>>>>> +                                  struct bpf_dynptr_kern *value_ptr)
>>>>>> +{
>>>>>> +       if (!bpf_dynptr_is_string(name_ptr))
>>>>>> +               return -EINVAL;
>>>>> 
>>>>> so dynptr can be invalid and name_ptr->data will be NULL, you should
>>>>> account for that
>>>> 
>>>> We can add a NULL check (or size check) here.
>>> 
>>> there must be some helper to check if dynptr is valid, let's use that
>>> instead of NULL checks
>> 
>> Yeah, we can use bpf_dynptr_is_null().
>> 
>>> 
>>>> 
>>>>> 
>>>>> and there could also be special dynptrs that don't have contiguous
>>>>> memory region, so somehow you'd need to take care of that as well
>>>> 
>>>> We can require the dynptr to be BPF_DYNPTR_TYPE_LOCAL. I don't think
>>>> we need this for dynptr of skb or xdp. Would this be sufficient?
>>> 
>>> well, to keep thing simple we can have a simple internal helper API
>>> that will tell if it's safe to assume that dynptr memory is contiguous
>>> and it's ok to use dynptr memory. But still, you shouldn't access data
>>> pointer directly, there must be some helper for that. Please check. It
>>> has to take into account offset and stuff like that.
>> 
>> Yeah, we can use bpf_dynptr_write(), which is a helper (not kfunc).
>> 
>>> 
>>> Also, and separately from that, we should think about providing a
>>> bpf_dynptr_slice()-like helper that will accept a fixed-sized
>>> temporary buffer and return pointer to either actual memory or copy
>>> non-contiguous memory into that buffer. That will make sure you can
>>> use any dynptr as a source of data, and only pay the price of memory
>>> copy in rare cases where it's necessary
>> 
>> I don't quite follow here. Currently, we have
>> 
>> bpf_dynptr_data()
>> bpf_dynptr_slice()
>> bpf_dynptr_slice_rdwr()
>> bpf_dynptr_write()
>> 
>> AFAICT, they are sufficient to cover existing use cases (and the new
>> use case we are adding in this set). What's the new kfunc are you
>> thinking about?
> 
> I wasn't talking about kfuncs, but rather just internal helpers to be
> used by other kfuncs when working with dynptrs as input arguments.

AFAICT, kfuncs can call other kfuncs, for example bpf_dynptr_slice_rdwr()
calls bpf_dynptr_slice(). This is limited to the same file at the moment, 
since we do not expose kfuncs in headers. 


Thanks,
Song