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

[Jakub Kicinski <jakub.kicinski@netronome.com>]

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 <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) {

Looking at this again, why change the comparison from less than to
greater than 0?

> +		p_err("perf_query_support: cannot open directory \"/\" (%s)",
> +		      strerror(errno));
>  		goto out;
>  	}
>