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

[Yonghong Song]

On 6/11/18 6:49 PM, Jakub Kicinski wrote:

On Mon, 11 Jun 2018 18:15:16 -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 
Signed-off-by: Yonghong Song 
---
  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",


nit: no \n at the end of p_err() format, because it breaks JSON :(


Thanks for letting me know. Will send v2 to fix this!




+ strerror(errno));
goto out;
}
  

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

[Jakub Kicinski]

On Mon, 11 Jun 2018 18:15:16 -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 
> Signed-off-by: Yonghong Song 
> ---
>  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",

nit: no \n at the end of p_err() format, because it breaks JSON :(

> +   strerror(errno));
>   goto out;
>   }
>