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Message-ID: <bf8a6fb9-40fd-7037-dcfd-24595b691c8f@fb.com>
Date: Mon, 11 Jan 2021 17:25:17 -0800
From: Yonghong Song <yhs@...com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
CC: Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>, Hao Luo <haoluo@...gle.com>
Subject: Re: [PATCH v2 bpf-next 5/7] bpf: support BPF ksym variables in kernel
modules
On 1/11/21 1:29 PM, Andrii Nakryiko wrote:
> On Sun, Jan 10, 2021 at 8:13 PM Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 1/8/21 2:09 PM, Andrii Nakryiko wrote:
>>> Add support for directly accessing kernel module variables from BPF programs
>>> using special ldimm64 instructions. This functionality builds upon vmlinux
>>> ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
>>> specifying kernel module BTF's FD in insn[1].imm field.
>>>
>>> During BPF program load time, verifier will resolve FD to BTF object and will
>>> take reference on BTF object itself and, for module BTFs, corresponding module
>>> as well, to make sure it won't be unloaded from under running BPF program. The
>>> mechanism used is similar to how bpf_prog keeps track of used bpf_maps.
>>>
>>> One interesting change is also in how per-CPU variable is determined. The
>>> logic is to find .data..percpu data section in provided BTF, but both vmlinux
>>> and module each have their own .data..percpu entries in BTF. So for module's
>>> case, the search for DATASEC record needs to look at only module's added BTF
>>> types. This is implemented with custom search function.
>>>
>>> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
>>
>> Ack with a minor nit below.
>>
>> Acked-by: Yonghong Song <yhs@...com>
>>
>>> ---
>>> include/linux/bpf.h | 10 +++
>>> include/linux/bpf_verifier.h | 3 +
>>> include/linux/btf.h | 3 +
>>> kernel/bpf/btf.c | 31 +++++++-
>>> kernel/bpf/core.c | 23 ++++++
>>> kernel/bpf/verifier.c | 149 ++++++++++++++++++++++++++++-------
>>> 6 files changed, 189 insertions(+), 30 deletions(-)
>>>
>> [...]
>>> /* replace pseudo btf_id with kernel symbol address */
>>> static int check_pseudo_btf_id(struct bpf_verifier_env *env,
>>> struct bpf_insn *insn,
>
> [...]
>
>>> } else {
>>> aux->btf_var.reg_type = PTR_TO_BTF_ID;
>>> - aux->btf_var.btf = btf_vmlinux;
>>> + aux->btf_var.btf = btf;
>>> aux->btf_var.btf_id = type;
>>> }
>>> +
>>> + /* check whether we recorded this BTF (and maybe module) already */
>>> + for (i = 0; i < env->used_btf_cnt; i++) {
>>> + if (env->used_btfs[i].btf == btf) {
>>> + btf_put(btf);
>>> + return 0;
>>
>> An alternative way is to change the above code as
>> err = 0;
>> goto err_put;
>
> I didn't do it, because it's not really an error case, which err_put
> implies. If in the future we'll have some more clean up to do, it
> might make sense, I suppose.
You can change label err_put to btf_put, so this way, btf_put() will
only show up in one place. But I won't insist on this.
>
>>
>>> + }
>>> + }
>>> +
>>> + if (env->used_btf_cnt >= MAX_USED_BTFS) {
>>> + err = -E2BIG;
>>> + goto err_put;
>>> + }
>>> +
>>> + btf_mod = &env->used_btfs[env->used_btf_cnt];
>>> + btf_mod->btf = btf;
>>> + btf_mod->module = NULL;
>>> +
>>> + /* if we reference variables from kernel module, bump its refcount */
>>> + if (btf_is_module(btf)) {
>>> + btf_mod->module = btf_try_get_module(btf);
>>> + if (!btf_mod->module) {
>>> + err = -ENXIO;
>>> + goto err_put;
>>> + }
>>> + }
>>> +
>>> + env->used_btf_cnt++;
>>> +
>>> return 0;
>>> +err_put:
>>> + btf_put(btf);
>>> + return err;
>>> }
>>>
>> [...]
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