[PATCH bpf v2] tools/bpftool: fix a bug in bpftool perf

[Yonghong Song <yhs@fb.com>]

Commit b04df400c302 ("tools/bpftool: add perf subcommand")
introduced bpftool subcommand perf to query bpf program
kuprobe and tracepoint attachments.

The perf subcommand will first test whether bpf subcommand
BPF_TASK_FD_QUERY is supported in kernel or not. It does it
by opening a file with argv[0] and feeds the file descriptor
and current task pid to the kernel for querying.

Such an approach won't work if the argv[0] cannot be opened
successfully in the current directory. This is especially
true when bpftool is accessible through PATH env variable.
The error below reflects the open failure for file argv[0]
at home directory.

  [yhs@localhost ~]$ which bpftool
  /usr/local/sbin/bpftool
  [yhs@localhost ~]$ bpftool perf
  Error: perf_query_support: No such file or directory

To fix the issue, let us open root directory ("/")
which exists in every linux system. With the fix, the
error message will correctly reflect the permission issue.

  [yhs@localhost ~]$ which bpftool
  /usr/local/sbin/bpftool
  [yhs@localhost ~]$ bpftool perf
  Error: perf_query_support: Operation not permitted
  HINT: non root or kernel doesn't support TASK_FD_QUERY

Fixes: b04df400c302 ("tools/bpftool: add perf subcommand")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 tools/bpf/bpftool/perf.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Changelogs:
 v1 -> v2:
   . remove '\n' in the p_err format string in order to
     have valid json output.

diff --git a/tools/bpf/bpftool/perf.c b/tools/bpf/bpftool/perf.c
index ac6b1a12c9b7..239715aa6fb9 100644
--- a/tools/bpf/bpftool/perf.c
+++ b/tools/bpf/bpftool/perf.c
@@ -29,9 +29,10 @@ static bool has_perf_query_support(void)
 	if (perf_query_supported)
 		goto out;
 
-	fd = open(bin_name, O_RDONLY);
-	if (fd < 0) {
-		p_err("perf_query_support: %s", strerror(errno));
+	fd = open("/", O_RDONLY);
+	if (fd > 0) {
+		p_err("perf_query_support: cannot open directory \"/\" (%s)",
+		      strerror(errno));
 		goto out;
 	}
 
-- 
2.14.3