Re: [PATCH bpf-next v2 1/2] bpf: add bpf_dynptr_memset() kfunc

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 6/25/25 4:27 AM, Mykyta Yatsenko wrote:
> On 6/24/25 21:52, Ihor Solodrai wrote:
>> Currently there is no straightforward way to fill dynptr memory with a
>> value (most commonly zero). One can do it with bpf_dynptr_write(), but
>> a temporary buffer is necessary for that.
>>
>> Implement bpf_dynptr_memset() - an analogue of memset() from libc.
>>
>> Signed-off-by: Ihor Solodrai <isolodrai@meta.com>
>> ---
>>   kernel/bpf/helpers.c | 48 ++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 48 insertions(+)
>>
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index b71e428ad936..b8a7dbc971b4 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2906,6 +2906,53 @@ __bpf_kfunc int bpf_dynptr_copy(struct 
>> bpf_dynptr *dst_ptr, u32 dst_off,
>>       return 0;
>>   }
>> +/**
>> + * bpf_dynptr_memset() - Fill dynptr memory with a constant byte.
>> + * @ptr: Destination dynptr - where data will be filled
>> + * @ptr_off: Offset into the dynptr to start filling from
>> + * @size: Number of bytes to fill
>> + * @val: Constant byte to fill the memory with
>> + *
>> + * Fills the size bytes of the memory area pointed to by ptr
>> + * at offset ptr_off with the constant byte val.
>> + * Returns 0 on success; negative error, otherwise.
>> + */
>> + __bpf_kfunc int bpf_dynptr_memset(struct bpf_dynptr *ptr, u32 
>> ptr_off, u32 size, u8 val)
>> + {
>> +    struct bpf_dynptr_kern *p = (struct bpf_dynptr_kern *)ptr;
>> +    char buf[256];
>> +    u32 chunk_sz;
>> +    void* slice;
>> +    u32 offset;
>> +    int err;
>> +
>> +    if (__bpf_dynptr_is_rdonly(p))
>> +        return -EINVAL;
>> +
>> +    err = bpf_dynptr_check_off_len(p, ptr_off, size);
>> +    if (err)
>> +        return err;
>> +
>> +    slice = bpf_dynptr_slice_rdwr(ptr, ptr_off, NULL, size);
>> +    if (likely(slice)) {
>> +        memset(slice, val, size);
>> +        return 0;
>> +    }
> 
> bpf_dynptr_slice_rdwr is doing rdonly and off_len checks anyways, so 
> perhaps we can
> avoid calling __bpf_dynptr_is_rdonly and bpf_dynptr_check_off_len before 
> bpf_dynptr_slice_rdwr,
> that'll make fast path a little faster.

Yes, there are quite a bit of repetitive checks.
Same in bpf_dynptr_write() in the slow path.

In case of bpf_dynptr_slice_rdwr() the issue is that it returns a
pointer, and if we only check for null pointer we lose the information
about the error cause (is it readonly or size?). So I think it's
better to leave explicit checks.

> 
>> +
>> +    /* Non-linear data under the dynptr, write from a local buffer */
>> +    chunk_sz = min_t(u32, sizeof(buf), size);
>> +    memset(buf, val, chunk_sz);
>> +
>> +    for (offset = ptr_off; offset < ptr_off + size; offset += 
>> chunk_sz) {
>> +        chunk_sz = min_t(u32, sizeof(buf), size - offset);
>> +        err = __bpf_dynptr_write(p, offset, buf, chunk_sz, 0);
>> +        if (err)
>> +            return err;
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>>   __bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
>>   {
>>       return obj;
>> @@ -3364,6 +3411,7 @@ BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
>>   BTF_ID_FLAGS(func, bpf_dynptr_size)
>>   BTF_ID_FLAGS(func, bpf_dynptr_clone)
>>   BTF_ID_FLAGS(func, bpf_dynptr_copy)
>> +BTF_ID_FLAGS(func, bpf_dynptr_memset)
>>   #ifdef CONFIG_NET
>>   BTF_ID_FLAGS(func, bpf_modify_return_test_tp)
>>   #endif
>