Re: [PATCH bpf] bpf: invalidate unused part of bpf_prog_pack

[Song Liu]

Hi Linus,

On Thu, Apr 21, 2022 at 11:59 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Apr 21, 2022 at 11:24 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > Let's not complicate the logic by dragging jit_fill_hole
> > further into generic allocation.
>
> I agree that just zeroing the page is probably perfectly fine in
> practice on x86, but I'm also not really seeing the "complication" of
> just doing things right.
>
> > The existing bpf_prog_pack code still does memset(0xcc)
> > a random range of bytes before and after jit-ed bpf code.
>
> That is actually wishful thinking, and not based on reality.
>
> From what I can tell, the end of the jit'ed bpf code is actually the
> exception table entries, so we have that data being marked executable.
>
> Honestly, what is wrong with this trivial patch?

This version would fill the memory with illegal instruction when we
allocate the bpf_prog_pack.

The extra logic I had in the original patch was to erase the memory
when a BPF program is freed. In this case, the memory will be
returned to the bpf_prog_pack, and stays as RO+X. Actually, I
am not quite sure whether we need this logic. If not, we only need
the much simpler version.

Thanks,
Song