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Message-ID: <efa7cacb-b737-666e-a212-133c2d6c3ded@linux.dev>
Date: Mon, 7 Nov 2022 18:28:51 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Yang Jihong <yangjihong1@...wei.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Shubham Bansal <illusionist.neo@...il.com>,
Russell King <linux@...linux.org.uk>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
Benjamin Tissoires <benjamin.tissoires@...hat.com>,
Kumar Kartikeya Dwivedi <memxor@...il.com>,
Delyan Kratunov <delyank@...com>,
Artem Savkov <asavkov@...hat.com>, colin.i.king@...il.com,
bpf <bpf@...r.kernel.org>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
LKML <linux-kernel@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 2/4] bpf: Remove size check for sk in
bpf_skb_is_valid_access for 32-bit architecture
On 11/7/22 5:07 PM, Andrii Nakryiko wrote:
> On Fri, Nov 4, 2022 at 4:32 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>>
>> On Fri, Nov 4, 2022 at 2:56 PM Andrii Nakryiko
>> <andrii.nakryiko@...il.com> wrote:
>>>
>>> On Thu, Nov 3, 2022 at 1:36 AM Yang Jihong <yangjihong1@...wei.com> wrote:
>>>>
>>>> The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
>>>> This is because bpf_object__relocate modifies the instruction to change memory
>>>> size to 4 bytes, as shown in the following messages:
>>>>
>>>> libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
>>>> libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
>>>> libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
>>>>
>>>> As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
>>>> unnecessary checks need to be deleted.
>>>>
>>>> Signed-off-by: Yang Jihong <yangjihong1@...wei.com>
>>>> ---
>>>> net/core/filter.c | 2 --
>>>> 1 file changed, 2 deletions(-)
>>>>
>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>> index bb0136e7a8e4..eab7ce89740c 100644
>>>> --- a/net/core/filter.c
>>>> +++ b/net/core/filter.c
>>>> @@ -8269,8 +8269,6 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
>>>> return false;
>>>> break;
>>>> case offsetof(struct __sk_buff, sk):
>>>> - if (type == BPF_WRITE || size != sizeof(__u64))
>>>> - return false;
>>>
>>> this probably should be specific to host architecture bitness? I'd
>>> imagine that size = 4 should be invalid on 64-bit arches (reading half
>>> of the pointer is bad)
>>
>> Not quite.
>> In __sk_buff the field 'sk' is defined as:
>> __bpf_md_ptr(struct bpf_sock *, sk);
>> so it's always 64-bit load when bpf prog reads it.
>> In this case CO_RE shouldn't have been applied to uapi struct __sk_buff.
>
> Ok, hold on. __bpf_md_ptr just creates a 8-byte sized and aligned
> union. It doesn't change the pointer itself in any way:
>
> union {
> struct bpf_sock* sk;
> __u64 :64;
> };
>
>
> It's a 64-bit pointer only because any pointer in the BPF target is
> 64-bit. But on 32-bit architectures such struct bpf_sock *sk pointer
> will *actually* be 4-byte pointer (and __u64 :64 will just make
> compiler add 4 bytes of padding after it, effectively), and BPF
> verifier will actually generate LDX instruction of BPF_W size (4 byte
> load):
>
> case offsetof(struct __sk_buff, sk):
> *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct sk_buff, sk),
> si->dst_reg, si->src_reg,
> offsetof(struct sk_buff, sk));
> break;
>
>
> BPF_FIELD_SIZEOF(struct sk_buff, sk) is 4 for 32-bit kernels.
>
> So while you are correct that it will be 8-byte load from the BPF
> side, allowing 4-byte load for such pointers should also be correct.
> It's our choice, there is no fundamental limitation why this shouldn't
> be the case.
>
> Note also that we do this transformation when fentry/fexit/raw_tp_btf
> programs traverse pointers in kernel structures. There pretending like
> pointer to an 8-byte value is actually invalid. So libbpf adjusts such
> loads to 4-byte loads for CO-RE-relocatable types, which makes it all
> work transparently on 32-bit architectures. Context accesses deviate
> from that, as they came earlier and we didn't have CO-RE at that time.
>
> So what you are saying is that __sk_buff shouldn't be
> CO-RE-relocatable, and yes, that would be good. But I think that's
> orthogonal in this case.
This issue should be from
commit c1ff181ffabc ("selftests/bpf: Extend kfunc selftests") which replaced the
uapi's bpf.h with vmlinux.h. One option to unblock this for now is to separate
those tests that read __sk_buff->sk to its own prog.c and use the uapi's bpf.h
instead of vmlinux.h.
It would be nice if the bpf-tc program can take 'struct sk_buff *skb' instead of
'struct __sk_buff *skb' but it will be a separate topic.
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