On 2025/1/26 00:58, Alexei Starovoitov wrote:
> On Sat, Jan 25, 2025 at 12:30 AM Levi Zim via B4 Relay
> <devnull+rsworktech.outlook.com@xxxxxxxxxx> wrote:
>> From: Levi Zim <rsworktech@xxxxxxxxxxx>
>>
>> This patch add a helper function bpf_probe_read_kernel_dynptr:
>>
>> long bpf_probe_read_kernel_dynptr(const struct bpf_dynptr *dst,
>> u32 offset, u32 size, const void *unsafe_ptr, u64 flags);
> We stopped adding helpers years ago.
> Only new kfuncs are allowed.
Sorry, I didn't know that. Just asking, is there any
documentation/discussion
about stopping adding helpers?
I will switch the implementation to kfuncs in v3.
> This particular one doesn't look useful as-is.
> The same logic can be expressed with
> - create dynptr
> - dynptr_slice
> - copy_from_kernel
By copy_from_kernel I assume you mean bpf_probe_read_kernel. The problem
with dynptr_slice_rdwr and probe_read_kernel is that they only support a
compile-time constant size [1].
But in order to best utilize the space on a BPF ringbuf, it is possible
to reserve a
variable length of space as dynptr on a ringbuf with
bpf_ringbuf_reserve_dynptr.
Then currently we have no way to read a variable length of kernel memory
into this dynptr, except doing it chunk by chunk[2], which is kinda awkward.
That's the problem the new helpers trying to solve.
And I am not the only one needing this kind of feature [3].
Andrii said it would be a straightforward addition as it is a super thin
wrapper
around existing functionality (we are just avoiding fixed buffer size
restrictions of
existing probe/copy_from APIs)
[1]:
https://elixir.bootlin.com/linux/v6.12.6/source/kernel/bpf/helpers.c#L2600-L2601
[2]:
https://github.com/libbpf/libbpf-bootstrap/commit/046fad60df3e39540937b5ec6ee86054f33d3f28
[3]: https://github.com/libbpf/libbpf-rs/issues/1041
[4]:
https://lore.kernel.org/bpf/CAEf4BzZctXJsR+TwMhmXNWnR0_BV802-3KJw226ZZt8St4xNkw@xxxxxxxxxxxxxx/
> pw-bot: cr
