On 6/11/18 7:42 PM, Jakub Kicinski wrote:
On Mon, 11 Jun 2018 19:01:08 -0700, Yonghong Song wrote: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(-) 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) {Looking at this again, why change the comparison from less than to greater than 0?
Totally my fault. I altered the condition to test p_err output since I cannot trigger it normally, but forgot to change it back. Will send yet another revision :-(
+ p_err("perf_query_support: cannot open directory \"/\" (%s)", + strerror(errno)); goto out; }
