netdev - Re: [PATCH 2/4] bpf: Remove size check for sk in bpf_skb_is_valid

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Date:   Fri, 4 Nov 2022 14:56:40 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yang Jihong <yangjihong1@...wei.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, illusionist.neo@...il.com,
        linux@...linux.org.uk, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, mykolal@...com,
        shuah@...nel.org, benjamin.tissoires@...hat.com, memxor@...il.com,
        delyank@...com, asavkov@...hat.com, colin.i.king@...il.com,
        bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        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 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)

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
>