Commit 73df93c5 authored Jun 11, 2018 by Yonghong Song Committed by Daniel Borkmann Jun 13, 2018

tools/bpftool: fix a bug in bpftool perf



Commit b04df400 ("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: b04df400 ("tools/bpftool: add perf subcommand")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

parent 5d902372

tools/bpf/bpftool/perf.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -29,9 +29,10 @@ static bool has_perf_query_support(void)
		if (perf_query_supported)
		goto out;

		fd = open(bin_name, O_RDONLY);
		fd = open("/", O_RDONLY);
		if (fd < 0) {
		p_err("perf_query_support: %s", strerror(errno));
		p_err("perf_query_support: cannot open directory \"/\" (%s)",
		strerror(errno));
		goto out;
		}

Admin message