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 <a...@kernel.org>
Signed-off-by: Yonghong Song <y...@fb.com>
---
tools/bpf/bpftool/perf.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/tools/bpf/bpftool/perf.c b/tools/bpf/bpftool/perf.c
index ac6b1a12c9b7..f1e4c9b270e2 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)\n",
+ strerror(errno));
goto out;
}
--
2.14.3
