On Tue, Jun 24, 2025 at 1:53 PM Ihor Solodrai <isolodrai@xxxxxxxx> 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@xxxxxxxx>
> ---
> 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)
nit: ptr_off -> offset, let's keep consistent naming with other APIs
(as much as possible)
> + {
> + struct bpf_dynptr_kern *p = (struct bpf_dynptr_kern *)ptr;
> + char buf[256];
> + u32 chunk_sz;
> + void* slice;
> + u32 offset;
nit: combine chunk_sz and offset on single line
> + 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;
> + }
> +
> + /* 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);
you have offset = ptr_off + chunk offset, so size - offset seems
wrong, it should be `size - offset + ptr_off` to "neutralize" ptr_off
itself. I'd probably write the for loop using
for (offset = 0; offset < size; offset += chunk_sz) {
chunk_sz = min_t(u32, sizeof(buf), size - offset);
err = __bpf_dynptr_write(p, ptr_off + offset, buf, chink_sz, 0);
...
seems simpler to just add that ptr_off in dynptr_write call
pw-bot: cr
> + 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
> --
> 2.47.1
>
