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? :) [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? >
