Re: [PATCH bpf-next v8 04/11] bpf: move prog->aux->linked_prog and trampoline into bpf_link on attach - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Jiri Olsa <jolsa@redhat.com>,
	Eelco Chaudron <echaudro@redhat.com>,
	KP Singh <kpsingh@chromium.org>,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next v8 04/11] bpf: move prog->aux->linked_prog and trampoline into bpf_link on attach
Date: Thu, 24 Sep 2020 23:59:55 +0200	[thread overview]
Message-ID: <87tuvmbztw.fsf@toke.dk> (raw)
In-Reply-To: <20200924001439.qitbu5tmzz55ck4z@ast-mbp.dhcp.thefacebook.com>

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

>> +	struct mutex tgt_mutex; /* protects tgt_* pointers below, *after* prog becomes visible */
>> +	struct bpf_prog *tgt_prog;
>> +	struct bpf_trampoline *tgt_trampoline;
>>  	bool verifier_zext; /* Zero extensions has been inserted by verifier. */
>>  	bool offload_requested;
>>  	bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
> ...
>>  struct bpf_tracing_link {
>>  	struct bpf_link link;
>>  	enum bpf_attach_type attach_type;
>> +	struct bpf_trampoline *trampoline;
>> +	struct bpf_prog *tgt_prog;
>
> imo it's confusing to have 'tgt_prog' to mean two different things.
> In prog->aux->tgt_prog it means target prog to attach to in the future.
> Whereas here it means the existing prog that was used to attached to.
> They kinda both 'target progs' but would be good to disambiguate.
> May be keep it as 'tgt_prog' here and
> rename to 'dest_prog' and 'dest_trampoline' in prog->aux ?

I started changing this as you suggested, but I think it actually makes
the code weirder. We'll end up with a lot of 'tgt_prog =
prog->aux->dest_prog' assignments in the verifier, unless we also rename
all of the local variables, which I think is just code churn for very
little gain (the existing 'target' meaning is quite clear, I think).

I also think it's quite natural that the target moves; I mean, it's
literally the same pointer being re-assigned from prog->aux to the link.
We could rename the link member to 'attached_tgt_prog' or something like
that, but I'm not sure it helps (and I don't see much of a problem in
the first place).

WDYT?

-Toke