netdev - Re: [PATCH v2 bpf-next 3/3] libbpf: btf typed dump does not need to allocate dump data

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Date:   Fri, 16 Jul 2021 17:32:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Bill Wendling <morbo@...gle.com>,
        Shuah Khan <shuah@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 3/3] libbpf: btf typed dump does not need to
 allocate dump data

On Fri, Jul 16, 2021 at 3:47 PM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> By using the stack for this small structure, we avoid the need
> for freeing memory in error paths.
>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> ---
>  tools/lib/bpf/btf_dump.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> index e5fbfb8..bd8e005 100644
> --- a/tools/lib/bpf/btf_dump.c
> +++ b/tools/lib/bpf/btf_dump.c
> @@ -2240,6 +2240,7 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
>                              const void *data, size_t data_sz,
>                              const struct btf_dump_type_data_opts *opts)
>  {
> +       struct btf_dump_data typed_dump = {};
>         const struct btf_type *t;
>         int ret;
>
> @@ -2250,7 +2251,7 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
>         if (!t)
>                 return libbpf_err(-ENOENT);
>
> -       d->typed_dump = calloc(1, sizeof(struct btf_dump_data));
> +       d->typed_dump = &typed_dump;
>         if (!d->typed_dump)
>                 return libbpf_err(-ENOMEM);

can't happen, removed, please pay attention to the surrounding code

>
> @@ -2269,7 +2270,5 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
>
>         ret = btf_dump_dump_type_data(d, NULL, t, id, data, 0, 0);
>
> -       free(d->typed_dump);
> -

added resetting to NULL here, so that we don't have an accidental
dangling pointers

>         return libbpf_err(ret);
>  }

> --
> 1.8.3.1
>