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Subject: Re: [PATCH BlueZ v2] src/gatt-client: always check properties in
 WriteValue
To:     Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Cc:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
References: <20200427150153.17055-1-david@lechnology.com>
 <CABBYNZKhVUqLortqL60CNtu0MZBdtW+qqdtn6zfacOVKDezYxQ@mail.gmail.com>
From:   David Lechner <david@lechnology.com>
Message-ID: <35f37936-e737-ac35-c538-9447a00d1cbc@lechnology.com>
Date:   Tue, 28 Apr 2020 09:39:48 -0500
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On 4/27/20 11:28 AM, Luiz Augusto von Dentz wrote:
> Hi David,
> 
> On Mon, Apr 27, 2020 at 8:07 AM David Lechner <david@lechnology.com> wrote:
>>
>> This modifies the GATT client characteristic WriteValue D-Bus method to
>> always check that the characteristic supports the requested type of
>> write by checking for the corresponding property before attempting to
>> write.
>>
>> Before this change, if the "type" option was used and it was set to
>> "reliable" or "request", then BlueZ would attempt the write even if the
>> characteristic does not support that write type. On the other hand, if
>> "type" was set to "command" or was not specified, the method would
>> return a org.bluez.Error.NotSupported error without attempting to write.
>>
>> After this change, the WriteValue method will consistently return
>> org.bluez.Error.NotSupported if the corresponding property flag is not
>> set for all types of writes.
>> ---
>>
>> v2 changes:
>> - remove extra check of test variable not NULL
>> - fix one line over 80 chars
>>
>>   src/gatt-client.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/src/gatt-client.c b/src/gatt-client.c
>> index a9bfc2802..f80742fbb 100644
>> --- a/src/gatt-client.c
>> +++ b/src/gatt-client.c
>> @@ -1016,8 +1016,8 @@ static DBusMessage *characteristic_write_value(DBusConnection *conn,
>>           *     - If value is larger than MTU - 3: long-write
>>           *   * "write-without-response" property set -> write command.
>>           */
>> -       if ((!type && (chrc->ext_props & BT_GATT_CHRC_EXT_PROP_RELIABLE_WRITE))
>> -                       || (type && !strcasecmp(type, "reliable"))) {
>> +       if ((!type || !strcasecmp(type, "reliable")) && chrc->ext_props &
>> +                               BT_GATT_CHRC_EXT_PROP_RELIABLE_WRITE) {
>>                  supported = true;
>>                  chrc->write_op = start_long_write(msg, chrc->value_handle, gatt,
>>                                                  true, value, value_len, offset,
>> @@ -1026,8 +1026,8 @@ static DBusMessage *characteristic_write_value(DBusConnection *conn,
>>                          return NULL;
>>          }
>>
>> -       if ((!type && chrc->props & BT_GATT_CHRC_PROP_WRITE) ||
>> -                       (type && !strcasecmp(type, "request"))) {
>> +       if ((!type || !strcasecmp(type, "request")) && chrc->props &
>> +                                               BT_GATT_CHRC_PROP_WRITE) {
>>                  uint16_t mtu;
>>
>>                  supported = true;
>> --
>> 2.17.1
> 
> As far I can remember this is on purpose so the application can
> overwrite the type if it needs to disabling the checks on the client
> side, though the kernel can still return not supported, so if we want
> to change this it will now not be possible to overwrite it anymore.
> 

If this is the case, does it make sense to make the following change
instead so that write without response can also be forced?

diff --git a/src/gatt-client.c b/src/gatt-client.c
index f80742fbb..3153f571f 100644
--- a/src/gatt-client.c
+++ b/src/gatt-client.c
@@ -1050,8 +1050,8 @@ static DBusMessage *characteristic_write_value(DBusConnection *conn,
                         return NULL;
         }
  
-       if ((type && strcasecmp(type, "command")) || offset ||
-                       !(chrc->props & BT_GATT_CHRC_PROP_WRITE_WITHOUT_RESP))
+       if ((type && strcasecmp(type, "command")) || offset || (!type &&
+                       !(chrc->props & BT_GATT_CHRC_PROP_WRITE_WITHOUT_RESP)))
                 goto fail;
  
         supported = true;