Re: [PATCH 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture

[Martin KaFai Lau <martin.lau@linux.dev>]

On 11/7/22 5:07 PM, Andrii Nakryiko wrote:
> On Fri, Nov 4, 2022 at 4:32 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
>>
>> On Fri, Nov 4, 2022 at 2:56 PM Andrii Nakryiko
>> <andrii.nakryiko@gmail.com> wrote:
>>>
>>> On Thu, Nov 3, 2022 at 1:36 AM Yang Jihong <yangjihong1@huawei.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@huawei.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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