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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 4 Nov 2022 14:56:40 -0700
Message-ID: <CAEf4BzY+qP1wwVddjg7_rypcUAW8iPRzSa=1O6aFG5dSLX+1Gg@mail.gmail.com>
Subject: Re: [PATCH 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access
 for 32-bit architecture
To:     Yang Jihong <yangjihong1@huawei.com>
Cc:     ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
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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)

either way, please make sure to add tests specifically for this case
in test_verifier


>                 info->reg_type = PTR_TO_SOCK_COMMON_OR_NULL;
>                 break;
>         case offsetof(struct __sk_buff, tstamp_type):
> --
> 2.30.GIT
>