linux-kernel - Re: [PATCH bpf] bpf: invalidate unused part of bpf_prog

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Date:   Thu, 21 Apr 2022 21:52:13 +0000
From:   Song Liu <songliubraving@...com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Song Liu <song@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH bpf] bpf: invalidate unused part of bpf_prog_pack

Hi Linus, 

> On Apr 21, 2022, at 2:28 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Thu, Apr 21, 2022 at 12:41 PM Song Liu <song@...nel.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