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bpf_probe_read_kernel_dynptr helper To: Alexei Starovoitov Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Matt Bobrowski , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Mykola Lysenko , Shuah Khan , bpf , LKML , linux-trace-kernel , "open list:KERNEL SELFTEST FRAMEWORK" , Andrii Nakryiko References: <20250125-bpf_dynptr_probe-v2-0-c42c87f97afe@outlook.com> <20250125-bpf_dynptr_probe-v2-1-c42c87f97afe@outlook.com> Content-Language: en-US From: Levi Zim In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: SI2PR01CA0014.apcprd01.prod.exchangelabs.com (2603:1096:4:191::17) To MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM (2603:10c6:220:ff::9) X-Microsoft-Original-Message-ID: <92a57353-5b13-4222-b040-57ae70609e22@outlook.com> Precedence: bulk X-Mailing-List: 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X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: dc259cad-6627-4a03-1435-08dd3da58e64 X-MS-Exchange-CrossTenant-AuthSource: MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Jan 2025 01:05:42.7710 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: SYBP282MB3558 On 2025/1/26 00:58, Alexei Starovoitov wrote: > On Sat, Jan 25, 2025 at 12:30 AM Levi Zim via B4 Relay > wrote: >> From: Levi Zim >> >> 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@mail.gmail.com/ > pw-bot: cr