Re: [PATCH bpf-next v2 1/7] bpf: Implement bpf_probe_read_kernel_dynptr helper

[Levi Zim <rsworktech@outlook.com>]

On 2025-01-28 07:09, Andrii Nakryiko wrote:
> On Mon, Jan 27, 2025 at 2:54 PM Andrei Matei <andreimatei1@gmail.com> wrote:
>> On Mon, Jan 27, 2025 at 5:04 PM Alexei Starovoitov
>> <alexei.starovoitov@gmail.com> wrote:
>>> On Sat, Jan 25, 2025 at 5:05 PM Levi Zim <rsworktech@outlook.com> wrote:
>>>> 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@kernel.org> wrote:
>>>>   >> From: Levi Zim <rsworktech@outlook.com>
>>>>   >>
>>>>   >> 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.
>> For our uprobes, we've run into similar issues around doing variable-sized
>> bpf_probe_read_user() into ring buffers for our debugger [1]. Our use case
>> is that we generate uprobes that recursively read data structures until we
>> fill up a buffer. The verifier's insistence on knowing statically that a read
>> fits into the buffer makes for awkward code, and makes it hard to pack the
>> buffer fully; we have to split our reads into a couple of static size classes.
>>
>> Any chance there'd be interest in taking the opportunity to support
>> dynamically-sized reads from userspace too? :)
> That's bpf_probe_read_user_dynptr() from patch #2, no?
>
> But generally speaking, here's a list of new APIs that we'd need to
> cover all existing fixed buffer versions:
>
> - non-sleepable probe reads:
>
>    bpf_probe_read_kernel_dynptr()
>    bpf_probe_read_user_dynptr()
>    bpf_probe_read_kernel_str_dynptr()
I think the _str_dynptr versions are probably not worth adding.
For example, when we use probe_read_kernel_str, the length of the str is 
usually
not known and we usually allocate a fixed size buffer for it. If we do 
know the
length of the str beforehand, we can just use probe_read_kernel_dynptr.
>    bpf_probe_read_user_str_dynptr()
>
> - sleepable probe reads (copy_from_user):
>
> bpf_copy_from_user_dynptr()
> bpf_copy_from_user_str_dynptr()
>
> - and then we have complementary task-based APIs for non-current process:
>
> bpf_probe_read_user_task_dynptr()
> bpf_probe_read_user_str_task_dynptr()
> bpf_copy_from_user_task_dynptr()
> bpf_copy_from_user_str_task_dynptr()
>
> Jordan is working on non-dynptr version of
> bpf_copy_from_user_str_task(), once he's done with that, we'll add
> dynptr version, probably.
>
>> [1] https://side-eye.io
>>
>>> That makes sense. The commit log didn't call it out.
>>> Please spell out the motivation clearly.
>>> Also why bpf_probe_read_kernel_common ?
>>> Do we need to memset() it on failure?
>>>