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

[Song Liu <songliubraving@fb.com>]

Hi Linus, 

> On Apr 21, 2022, at 2:28 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> 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.

[...]
> 
> 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?

Last night, I had a version which is about 90% same as this idea.

However, we cannot really use the same function at free time. The
huge page is RO+X at free time, but we are only zeroing out a chunk 
of it. So regular memset/memcpy won’t work. Instead, we will need 
something like bpf_arch_text_copy(). 
 
Thanks,
Song