On Mon, Jan 27, 2025 at 3:09 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Mon, Jan 27, 2025 at 2:54 PM Andrei Matei <andreimatei1@xxxxxxxxx> wrote: > > > > On Mon, Jan 27, 2025 at 5:04 PM Alexei Starovoitov > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > On Sat, Jan 25, 2025 at 5:05 PM Levi Zim <rsworktech@xxxxxxxxxxx> 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@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. > > > > 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() > 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. This is quite a bunch of kfuncs. It doesn't look like adding _dynptr suffix and duplicating kfuncs approach scales. Let's make the existing helpers/kfuncs more flexible ? We can introduce a kfunc bpf_dynptr_buf() that checks that dynptr is not readonly and type == local or ringbuf and return dynptr->data as PTR_TO_MEM | dynptr_flag | VERIFIER_ADDS_SIZE_CHECK. Then allow bpf_probe_read_user/kernel/... all of them to accept this register type where PTR_TO_MEM is required while relaxing ARG_CONST_SIZE 2nd argument to ARG_ANYTHING. Then the verifier will insert an extra check if (arg1->size < arg2) before the call. Not only the bpf_probe_read_kernel/user, _str variants will work but things like bpf_strtol, bpf_strncmp, bpf_snprintf, bpf_get_stack will auto-magically work as well. I think those are quite valuable to make available with non-constant size. bpf_get_stack_*() directly into the ring buffer sounds very useful.
