netdev - Re: [PATCH bpf-next] selftests/bpf: Fix stat probe in d

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Date:   Wed, 16 Sep 2020 19:25:02 +0200
From:   KP Singh <kpsingh@...omium.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix stat probe in d_path test

On Wed, Sep 16, 2020 at 1:24 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Some kernels builds might inline vfs_getattr call within fstat
> syscall code path, so fentry/vfs_getattr trampoline is not called.
>
> Alexei suggested [1] we should use security_inode_getattr instead,
> because it's less likely to get inlined.
>
> Adding security_inode_getattr to the d_path allowed list and
> switching the stat trampoline to security_inode_getattr.
>
> Adding flags that indicate trampolines were called and failing
> the test if any of them got missed, so it's easier to identify
> the issue next time.
>
> [1] https://lore.kernel.org/bpf/CAADnVQJ0FchoPqNWm+dEppyij-MOvvEG_trEfyrHdabtcEuZGg@mail.gmail.com/
> Fixes: e4d1af4b16f8 ("selftests/bpf: Add test for d_path helper")
> Signed-off-by: Jiri Olsa <jolsa@...hat.com>

Acked-by: KP Singh <kpsingh@...gle.com>

> ---
>  kernel/trace/bpf_trace.c                        | 1 +
>  tools/testing/selftests/bpf/prog_tests/d_path.c | 6 ++++++
>  tools/testing/selftests/bpf/progs/test_d_path.c | 9 ++++++++-
>  3 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b2a5380eb187..1001c053ebb3 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1122,6 +1122,7 @@ BTF_ID(func, vfs_truncate)
>  BTF_ID(func, vfs_fallocate)
>  BTF_ID(func, dentry_open)
>  BTF_ID(func, vfs_getattr)
> +BTF_ID(func, security_inode_getattr)
>  BTF_ID(func, filp_close)
>  BTF_SET_END(btf_allowlist_d_path)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/d_path.c b/tools/testing/selftests/bpf/prog_tests/d_path.c
> index fc12e0d445ff..f507f1a6fa3a 100644
> --- a/tools/testing/selftests/bpf/prog_tests/d_path.c
> +++ b/tools/testing/selftests/bpf/prog_tests/d_path.c
> @@ -120,6 +120,12 @@ void test_d_path(void)
>         if (err < 0)
>                 goto cleanup;
>
> +       if (CHECK(!bss->called_stat || !bss->called_close,
> +                 "check",
> +                 "failed to call trampolines called_stat %d, bss->called_close %d\n",
> +                  bss->called_stat, bss->called_close))

optional:

maybe it's better to add two separate checks with specific error messages?

"stat", "trampoline for security_inode_getattr was not called\n"
"close", "trampoline for filp_close was not called\n"

I think this would make the output more readable.

- KP

> +               goto cleanup;
> +
>         for (int i = 0; i < MAX_FILES; i++) {
>                 CHECK(strncmp(src.paths[i], bss->paths_stat[i], MAX_PATH_LEN),
>                       "check",

[...]

>         if (pid != my_pid)
>                 return 0;
>
> --
> 2.26.2
>