Re: [PATCH v10 05/15] perf evlist: implement control command handling functions

[Alexey Budankov]

On 13.07.2020 6:20, Namhyung Kim wrote:
> On Wed, Jul 8, 2020 at 4:50 PM Alexey Budankov
> <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
>>
>>
>> Implement functions of initialization, finalization and processing
>> of control command messages coming from control file descriptors.
>> Allocate control file descriptor as descriptor at struct pollfd
>> object of evsel_list for atomic poll() operation.
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
>> ---
> [SNIP]
>> +static int evlist__ctlfd_recv(struct evlist *evlist, enum evlist_ctl_cmd *cmd,
>> +                             char *cmd_data, size_t data_size)
>> +{
>> +       int err;
>> +       char c;
>> +       size_t bytes_read = 0;
>> +
>> +       memset(cmd_data, 0, data_size--);
> 
> I overlooked the '--' at the end and thought there might be
> buffer overflow..  Care to add a comment?

If it wants to be more explicit then it could possibly be implemented
like this:

memset(cmd_data, 0, data_size);
data_size--;

> 
>> +
>> +       do {
>> +               err = read(evlist->ctl_fd.fd, &c, 1);
> 
> Maybe I missed earlier discussion, but do we really want
> this 1 byte read in a loop?

That was the explicit request to support commands of variable
length so it implements it like that.

Alexei

> 
> Thanks
> Namhyung
> 
> 
>> +               if (err > 0) {
>> +                       if (c == '\n' || c == '\0')
>> +                               break;
>> +                       cmd_data[bytes_read++] = c;
>> +                       if (bytes_read == data_size)
>> +                               break;
>> +               } else {
>> +                       if (err == -1)
>> +                               pr_err("Failed to read from ctlfd %d: %m\n", evlist->ctl_fd.fd);
>> +                       break;
>> +               }
>> +       } while (1);
>> +
>> +       pr_debug("Message from ctl_fd: \"%s%s\"\n", cmd_data,
>> +                bytes_read == data_size ? "" : c == '\n' ? "\\n" : "\\0");
>> +
>> +       if (err > 0) {
>> +               if (!strncmp(cmd_data, EVLIST_CTL_CMD_ENABLE_TAG,
>> +                            (sizeof(EVLIST_CTL_CMD_ENABLE_TAG)-1))) {
>> +                       *cmd = EVLIST_CTL_CMD_ENABLE;
>> +               } else if (!strncmp(cmd_data, EVLIST_CTL_CMD_DISABLE_TAG,
>> +                                   (sizeof(EVLIST_CTL_CMD_DISABLE_TAG)-1))) {
>> +                       *cmd = EVLIST_CTL_CMD_DISABLE;
>> +               }
>> +       }
>> +
>> +       return err;
>> +}