netdev - Re: [PATCH bpf-next] selftests/bpf: define string const as global for test_sysctl

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Message-ID: <CAEf4Bza3=W5GaVuRFbvQvCKD3WQ9PVnuYJGPdF21zcaztJFBFA@mail.gmail.com>
Date:   Thu, 10 Sep 2020 14:56:27 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next] selftests/bpf: define string const as global for test_sysctl_prog.c

On Thu, Sep 10, 2020 at 1:30 PM Yonghong Song <yhs@...com> wrote:
>
> When tweaking llvm optimizations, I found that selftest build failed
> with the following error:
>   libbpf: elf: skipping unrecognized data section(6) .rodata.str1.1
>   libbpf: prog 'sysctl_tcp_mem': bad map relo against '.L__const.is_tcp_mem.tcp_mem_name'
>           in section '.rodata.str1.1'
>   Error: failed to open BPF object file: Relocation failed
>   make: *** [/work/net-next/tools/testing/selftests/bpf/test_sysctl_prog.skel.h] Error 255
>   make: *** Deleting file `/work/net-next/tools/testing/selftests/bpf/test_sysctl_prog.skel.h'
>
> The local string constant "tcp_mem_name" is put into '.rodata.str1.1' section
> which libbpf cannot handle. Using untweaked upstream llvm, "tcp_mem_name"
> is completely inlined after loop unrolling.
>
> Commit 7fb5eefd7639 ("selftests/bpf: Fix test_sysctl_loop{1, 2}
> failure due to clang change") solved a similar problem by defining
> the string const as a global. Let us do the same here
> for test_sysctl_prog.c so it can weather future potential llvm changes.
>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---

Acked-by: Andrii Nakryiko <andriin@...com>

>  tools/testing/selftests/bpf/progs/test_sysctl_prog.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/test_sysctl_prog.c b/tools/testing/selftests/bpf/progs/test_sysctl_prog.c
> index 50525235380e..5489823c83fc 100644
> --- a/tools/testing/selftests/bpf/progs/test_sysctl_prog.c
> +++ b/tools/testing/selftests/bpf/progs/test_sysctl_prog.c
> @@ -19,11 +19,11 @@
>  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
>  #endif
>
> +const char tcp_mem_name[] = "net/ipv4/tcp_mem";

nit: I'd prefer keeping an empty line between variables and functions

>  static __always_inline int is_tcp_mem(struct bpf_sysctl *ctx)
>  {
> -       char tcp_mem_name[] = "net/ipv4/tcp_mem";
>         unsigned char i;
> -       char name[64];
> +       char name[sizeof(tcp_mem_name)];
>         int ret;
>
>         memset(name, 0, sizeof(name));
> --
> 2.24.1
>