Re: [PATCH bpf-next v12 2/2] selftests/bpf: test for bpf_get_file_path() from tracepoint

[Yonghong Song <yhs@fb.com>]

On 12/14/19 8:01 PM, Wenbo Zhang wrote:
> trace fstat events by tracepoint syscalls/sys_enter_newfstat, and handle
> events only produced by test_file_get_path, which call fstat on several
> different types of files to test bpf_get_file_path's feature.
> 
> v4->v5: addressed Andrii's feedback
> - pass NULL for opts as bpf_object__open_file's PARAM2, as not really
> using any
> - modify patch subject to keep up with test code
> - as this test is single-threaded, so use getpid instead of SYS_gettid
> - remove unnecessary parens around check which after if (i < 3)
> - in kern use bpf_get_current_pid_tgid() >> 32 to fit getpid() in
> userspace part
> - with the patch adding helper as one patch series
> 
> v3->v4: addressed Andrii's feedback
> - use a set of fd instead of fds array
> - use global variables instead of maps (in v3, I mistakenly thought that
> the bpf maps are global variables.)
> - remove uncessary global variable path_info_index
> - remove fd compare as the fstat's order is fixed
> 
> v2->v3: addressed Andrii's feedback
> - use global data instead of perf_buffer to simplified code
> 
> v1->v2: addressed Daniel's feedback
> - rename bpf_fd2path to bpf_get_file_path to be consistent with other
> helper's names
> 
> Signed-off-by: Wenbo Zhang <ethercflow@gmail.com>
> ---
>   .../selftests/bpf/prog_tests/get_file_path.c  | 171 ++++++++++++++++++
>   .../selftests/bpf/progs/test_get_file_path.c  |  43 +++++
>   2 files changed, 214 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/prog_tests/get_file_path.c
>   create mode 100644 tools/testing/selftests/bpf/progs/test_get_file_path.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/get_file_path.c b/tools/testing/selftests/bpf/prog_tests/get_file_path.c
> new file mode 100644
> index 000000000000..7ec11e43e0fc
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/get_file_path.c
> @@ -0,0 +1,171 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#define _GNU_SOURCE
> +#include <test_progs.h>
> +#include <sys/stat.h>
> +#include <linux/sched.h>
> +#include <sys/syscall.h>
> +
> +#define MAX_PATH_LEN		128
> +#define MAX_FDS			7
> +#define MAX_EVENT_NUM		16
> +
> +static struct file_path_test_data {
> +	pid_t pid;
> +	__u32 cnt;
> +	__u32 fds[MAX_EVENT_NUM];
> +	char paths[MAX_EVENT_NUM][MAX_PATH_LEN];
> +} src, dst;
> +
> +static inline int set_pathname(int fd)

In non-bpf .c file, typically we do not add 'inline' attribute.
It is up to compiler to decide whether it should be inlined.

> +{
> +	char buf[MAX_PATH_LEN];
> +
> +	snprintf(buf, MAX_PATH_LEN, "/proc/%d/fd/%d", src.pid, fd);
> +	src.fds[src.cnt] = fd;
> +	return readlink(buf, src.paths[src.cnt++], MAX_PATH_LEN);
> +}
> +
[...]
> diff --git a/tools/testing/selftests/bpf/progs/test_get_file_path.c b/tools/testing/selftests/bpf/progs/test_get_file_path.c
> new file mode 100644
> index 000000000000..eae663c1262a
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_get_file_path.c
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/bpf.h>
> +#include <linux/ptrace.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include "bpf_helpers.h"
> +#include "bpf_tracing.h"
> +
> +#define MAX_PATH_LEN		128
> +#define MAX_EVENT_NUM		16
> +
> +static struct file_path_test_data {
> +	pid_t pid;
> +	__u32 cnt;
> +	__u32 fds[MAX_EVENT_NUM];
> +	char paths[MAX_EVENT_NUM][MAX_PATH_LEN];
> +} data;
> +
> +struct sys_enter_newfstat_args {
> +	unsigned long long pad1;
> +	unsigned long long pad2;
> +	unsigned int fd;
> +};

The BTF generated vmlinux.h has the following structure,
struct trace_entry {
         short unsigned int type;
         unsigned char flags;
         unsigned char preempt_count;
         int pid;
};
struct trace_event_raw_sys_enter {
         struct trace_entry ent;
         long int id;
         long unsigned int args[6];
         char __data[0];
};

The third parameter type should be long, otherwise,
it may have issue on big endian machines?