Re: [PATCH v9 bpf-next 13/14] selftests/bpf: Add test for d_path helper

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Sat, Aug 1, 2020 at 10:05 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding test for d_path helper which is pretty much
> copied from Wenbo Zhang's test for bpf_get_fd_path,
> which never made it in.
>
> The test is doing fstat/close on several fd types,
> and verifies we got the d_path helper working on
> kernel probes for vfs_getattr/filp_close functions.
>
> Original-patch-by: Wenbo Zhang <ethercflow@gmail.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---

Nothing wrong with this BPF implementation, but seem one suggestion below.

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

>  .../testing/selftests/bpf/prog_tests/d_path.c | 147 ++++++++++++++++++
>  .../testing/selftests/bpf/progs/test_d_path.c |  64 ++++++++
>  2 files changed, 211 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/d_path.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_d_path.c
>

[...]

> diff --git a/tools/testing/selftests/bpf/progs/test_d_path.c b/tools/testing/selftests/bpf/progs/test_d_path.c
> new file mode 100644
> index 000000000000..9d342d7a1de6
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_d_path.c
> @@ -0,0 +1,64 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +#define MAX_PATH_LEN           128
> +#define MAX_FILES              7
> +
> +pid_t my_pid = 0;
> +__u32 cnt_stat = 0;
> +__u32 cnt_close = 0;
> +char paths_stat[MAX_FILES][MAX_PATH_LEN] = {};
> +char paths_close[MAX_FILES][MAX_PATH_LEN] = {};
> +int rets_stat[MAX_FILES] = {};
> +int rets_close[MAX_FILES] = {};
> +
> +SEC("fentry/vfs_getattr")
> +int BPF_PROG(prog_stat, struct path *path, struct kstat *stat,
> +            __u32 request_mask, unsigned int query_flags)
> +{
> +       pid_t pid = bpf_get_current_pid_tgid() >> 32;
> +       int ret;
> +
> +       if (pid != my_pid)
> +               return 0;
> +
> +       if (cnt_stat >= MAX_FILES)
> +               return 0;
> +       ret = bpf_d_path(path, paths_stat[cnt_stat], MAX_PATH_LEN);
> +
> +       /* We need to recheck cnt_stat for verifier. */
> +       if (cnt_stat >= MAX_FILES)
> +               return 0;
> +       rets_stat[cnt_stat] = ret;
> +
> +       cnt_stat++;
> +       return 0;
> +}
> +
> +SEC("fentry/filp_close")
> +int BPF_PROG(prog_close, struct file *file, void *id)
> +{
> +       pid_t pid = bpf_get_current_pid_tgid() >> 32;
> +       int ret;
> +
> +       if (pid != my_pid)
> +               return 0;
> +
> +       if (cnt_close >= MAX_FILES)
> +               return 0;
> +       ret = bpf_d_path(&file->f_path,
> +                        paths_close[cnt_close], MAX_PATH_LEN);
> +
> +       /* We need to recheck cnt_stat for verifier. */

you need to do it because you are re-reading a global variable; if you
stored cnt_close in a local variable, then did >= MAX_FILES check
once, you probably could have avoided this duplication. Same for
another instance above.

> +       if (cnt_close >= MAX_FILES)
> +               return 0;
> +       rets_close[cnt_close] = ret;
> +
> +       cnt_close++;
> +       return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> --
> 2.25.4
>