On Tue, Sep 10, 2024 at 4:44 PM Daniel Xu dxu@dxuuu.xyz wrote:
On Tue, Sep 10, 2024 at 03:21:04PM GMT, Andrii Nakryiko wrote:
On Tue, Sep 10, 2024 at 3:16 PM Daniel Xu dxu@dxuuu.xyz wrote:
On Tue, Sep 10, 2024, at 2:07 PM, Daniel Xu wrote:
On Tue, Sep 10, 2024 at 01:41:41PM GMT, Andrii Nakryiko wrote:
On Tue, Sep 10, 2024 at 11:36 AM Alexei Starovoitov
[...]
Also, Daniel, can you please make sure that dynptr we return for each sample is read-only? We shouldn't let consumer BPF program ability to corrupt ringbuf record headers (accidentally or otherwise).
Sure.
So the sample is not read-only. But I think prog is prevented from messing with header regardless.
__bpf_user_ringbuf_peek() returns sample past the header:
*sample = (void *)((uintptr_t)rb->data + (uintptr_t)((cons_pos + BPF_RINGBUF_HDR_SZ) & rb->mask));dynptr is initialized with the above ptr:
bpf_dynptr_init(&dynptr, sample, BPF_DYNPTR_TYPE_LOCAL, 0, size);So I don't think there's a way for the prog to access the header thru the dynptr.
By "header" I mean 8 bytes that precede each submitted ringbuf record. That header is part of ringbuf data area. Given user space can set consumer_pos to arbitrary value, kernel can return arbitrary part of ringbuf data area, including that 8 byte header. If that data is writable, it's easy to screw up that header and crash another BPF program that reserves/submits a new record. User space can only read data area for BPF ringbuf, and so we rely heavily on a tight control of who can write what into those 8 bytes.
Ah, ok. I think I understand.
Given this and your other comments about rb->busy, what about enforcing bpf_user_ringbuf_drain() NAND mmap? I think the use cases here are different enough where this makes sense.
You mean disabling user-space mmap()? TBH, I'd like to understand the use case first before we make such decisions. Maybe what you need is not really a BPF ringbuf? Can you give us a bit more details on what you are trying to achieve?