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Subject: Re: [PATCH v2 02/12] mm: introduce execmem_text_alloc() and
 jit_text_alloc()
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <a17c65c6-863f-4026-9c6f-a04b659e9ab4@app.fastmail.com>
Date:   Mon, 19 Jun 2023 13:18:15 -0700
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To:     Andy Lutomirski <luto@kernel.org>, Song Liu <song@kernel.org>
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> On Jun 19, 2023, at 10:09 AM, Andy Lutomirski <luto@kernel.org> wrote:
>=20
> But jit_text_alloc() can't do this, because the order of operations =
doesn't match.  With jit_text_alloc(), the executable mapping shows up =
before the text is populated, so there is no atomic change from =
not-there to populated-and-executable.  Which means that there is an =
opportunity for CPUs, speculatively or otherwise, to start filling =
various caches with intermediate states of the text, which means that =
various architectures (even x86!) may need serialization.
>=20
> For eBPF- and module- like use cases, where JITting/code gen is quite =
coarse-grained, perhaps something vaguely like:
>=20
> jit_text_alloc() -> returns a handle and an executable virtual =
address, but does *not* map it there
> jit_text_write() -> write to that handle
> jit_text_map() -> map it and synchronize if needed (no sync needed on =
x86, I think)

Andy, would you mind explaining why you think a sync is not needed? I =
mean I have a =E2=80=9Cfeeling=E2=80=9D that perhaps TSO can guarantee =
something based on the order of write and page-table update. Is that the =
argument?

On this regard, one thing that I clearly do not understand is why =
*today* it is ok for users of bpf_arch_text_copy() not to call =
text_poke_sync(). Am I missing something?