netdev - Re: [PATCH bpf] tools: bpftool: fix a bitfield pretty print issue

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Message-ID: <20181129004210.e2l54yt3s6silvlu@ast-mbp>
Date:   Wed, 28 Nov 2018 16:42:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     ast@...com, daniel@...earbox.net, netdev@...r.kernel.org,
        kafai@...com, kernel-team@...com
Subject: Re: [PATCH bpf] tools: bpftool: fix a bitfield pretty print issue

On Wed, Nov 28, 2018 at 09:38:23AM -0800, Yonghong Song wrote:
> Commit b12d6ec09730 ("bpf: btf: add btf print functionality")
> added btf pretty print functionality to bpftool.
> There is a problem though in printing a bitfield whose type
> has modifiers.
> 
> For example, for a type like
>   typedef int ___int;
>   struct tmp_t {
>           int a:3;
>           ___int b:3;
>   };
> Suppose we have a map
>   struct bpf_map_def SEC("maps") tmpmap = {
>           .type = BPF_MAP_TYPE_HASH,
>           .key_size = sizeof(__u32),
>           .value_size = sizeof(struct tmp_t),
>           .max_entries = 1,
>   };
> and the hash table is populated with one element with
> key 0 and value (.a = 1 and .b = 2).
> 
> In BTF, the struct member "b" will have a type "typedef" which
> points to an int type. The current implementation does not
> pass the bit offset during transition from typedef to int type,
> hence incorrectly print the value as
>   $ bpftool m d id 79
>   [{
>           "key": 0,
>           "value": {
>               "a": 0x1,
>               "b": 0x1
>           }
>       }
>   ]
> 
> This patch fixed the issue by carrying bit_offset along the type
> chain during bit_field print. The correct result can be printed as
>   $ bpftool m d id 76
>   [{
>           "key": 0,
>           "value": {
>               "a": 0x1,
>               "b": 0x2
>           }
>       }
>   ]
> 
> The kernel pretty print is implemented correctly and does not
> have this issue.
> 
> Fixes: b12d6ec09730 ("bpf: btf: add btf print functionality")
> Signed-off-by: Yonghong Song <yhs@...com>

Applied to bpf tree. Thanks