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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Apr 21, 2022 at 12:41 PM Song Liu <song@kernel.org> wrote:
>
> 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.

Oh, I think it would be good to do at free time too.

I just would want that to use the same function we already have for
the allocation-time thing, instead of introducing completely new
infrastructure. That was what looked very odd to me.

Now, the _smallest_ patch would likely be to just save away that
'bpf_fill_ill_insns' function pointer in the 'struct bpf_prog_pack'
thing.

It's admittedly kind of silly to do, but it matches that whole silly
"let's pass around a function pointer to a fixed function" model at
allocation time.

I say that's silly, because it's a fixed architecture function and we
could just call it directly. The only valid function there is
jit_fill_hole(), and the only reason it uses that function pointer
seems to be that it's never been exposed as a real function.

So passing it along as a function seems to be _purely_ for the silly
reason that people haven't agreed on a name, and different
architectures use different names (ie power uses
'bpf_jit_fill_ill_insns()', RISC-V calls it 'bpf_fill_ill_insns()',
and everybody else seems to use 'jit_fill_hole'.

I don't know why that decision was made. It looks like a bad one to
me, honestly.

Why not just agree on a name - I suggest 'bpf_jit_fill_hole()' - and
just get rid of that stupid 'bpf_jit_fill_hole_t' type name that only
exists because of this thing?

The bpf headers seem to literally have agreed on a name for that
function -type- only in order to be able to disagree on the name of
the function -name-, and then pass it along as a function pointer
argument instead of just calling it directly.

Very counter-productive.

                 Linus