On 6/18/25 4:44 PM, Mykyta Yatsenko wrote:
On 6/18/25 23:33, 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@xxxxxxxx>
---
kernel/bpf/helpers.c | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b71e428ad936..dfd04628a522 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2906,6 +2906,33 @@ __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
+ * @val: Constant byte to fill the memory with
+ * @n: Number of bytes to fill
+ *
+ * Fills the first n bytes of the memory area pointed to by ptr
+ * with the constant byte val.
+ * Returns 0 on success; negative error, otherwise.
+ */
+ __bpf_kfunc int bpf_dynptr_memset(struct bpf_dynptr *ptr, u8 val,
u32 n)
+ {
+ struct bpf_dynptr_kern *p = (struct bpf_dynptr_kern *)ptr;
+ int err;
+
+ if (__bpf_dynptr_is_rdonly(p))
+ return -EINVAL;
+
+ err = bpf_dynptr_check_off_len(p, 0, n);
+ if (err)
+ return err;
+
+ memset(p->data + p->offset, val, n);
Do we need to handle non-contiguous buffers, similarly to
bpf_dynptr_write (BPF_DYNPTR_TYPE_XDP case)?
It appears so, yes. I am a little lost on how exactly to do it though.
Looking at bpf_xdp_store_bytes [1]:
BPF_CALL_4(bpf_xdp_store_bytes, struct xdp_buff *, xdp, u32, offset,
void *, buf, u32, len)
{
void *ptr;
ptr = bpf_xdp_pointer(xdp, offset, len);
if (IS_ERR(ptr))
return PTR_ERR(ptr);
if (!ptr)
bpf_xdp_copy_buf(xdp, offset, buf, len, true);
else
memcpy(ptr, buf, len);
return 0;
}
...
int __bpf_xdp_store_bytes(struct xdp_buff *xdp, u32 offset, void
*buf, u32 len)
{
return ____bpf_xdp_store_bytes(xdp, offset, buf, len);
}
It looks like I'd need to implement similar logic for memset.
But then, does memset even make sense for xdp/skb buffers?
Maybe -ENOTSUPP is more appropriate?
I'd appreciate any hints.
[1]
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/net/core/filter.c#n4084
+
+ return 0;
+}
+
__bpf_kfunc void *bpf_cast_to_kern_ctx(void *obj)
{
return obj;
@@ -3364,6 +3391,7 @@ 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
