Commit b04df400c3 ("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: b04df400c3 ("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>
73df93c57c