Re: [PATCH 2/3] bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)

[Andrii Nakryiko]

On Fri, Apr 28, 2023 at 6:58 PM Daniel Rosenberg <drosen@xxxxxxxxxx> wrote:
>
> On Thu, Apr 6, 2023 at 2:09 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > would this work correctly if someone passes a non-null buffer with too
> > small size? Can you please add a test for this use case.
> >
> Working on a test case for this, but the test case I wrote fails
> without my patches.
> I'm just declaring a buffer of size 9 on the stack, and then passing
> in bpf_dynptr_slice that buffer, and size 10. That's passing the
> verifier just fine. In fact, it loads successfully up to size 16. I'm
> guessing that's adjusting for alignment? Still feels very strange. Is
> that expected behavior?

pointer to stack is trickier (verifier will just mark part of stack as
overwritten with random data), it's best to use map value pointer as a
source of buffer. So try using ARRAY map with small value_size, do
lookup_elem, check for NULL, and pass non-NULL pointer as a buffer.