Message-ID: <20210414070025.GB6048@kadam>
Date:   Wed, 14 Apr 2021 10:00:25 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Julia Lawall <julia.lawall@...ia.fr>,
        outreachy-kernel@...glegroups.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [Outreachy kernel] [PATCH] :staging: rtl8723bs: Remove useless
 led_blink_hdl()

On Wed, Apr 14, 2021 at 08:33:48AM +0200, Fabio M. De Francesco wrote:
> On Wednesday, April 14, 2021 7:21:50 AM CEST Dan Carpenter wrote:
> > On Tue, Apr 13, 2021 at 10:08:32PM +0200, Fabio M. De Francesco wrote:
> > > On Tuesday, April 13, 2021 9:48:44 PM CEST Matthew Wilcox wrote:
> > > > On Tue, Apr 13, 2021 at 09:45:03PM +0200, Fabio M. De Francesco 
> wrote:
> > > > > 1) The driver doesn't call that function from anywhere else than
> > > > > the
> > > > > macro. 2) You have explained that the macro add its symbol to a
> > > > > slot
> > > > > in an array that would shift all the subsequent elements down if
> > > > > that
> > > > > macro is not used exactly in the line where it is.
> > > > > 3) Dan Carpenter said that that array is full of null functions (or
> > > > > empty slots?).
> > > > > 
> > > > > Unless that function is called anonymously dereferencing its
> > > > > address
> > > > > from the position it occupies in the array, I'm not able to see
> > > > > what
> > > > > else means can any caller use.
> > > > > 
> > > > > I know I have much less experience than you with C: what can go
> > > > > wrong?
> > > > 
> > > > Here's where the driver calls that function:
> > > > 
> > > > $ git grep wlancmds drivers/staging/rtl8723bs/
> > > > drivers/staging/rtl8723bs/core/rtw_cmd.c:static struct cmd_hdl
> > > > wlancmds[] = { drivers/staging/rtl8723bs/core/rtw_cmd.c:            
> > > >   if
> > > > (pcmd->cmdcode < ARRAY_SIZE(wlancmds)) {
> > > > drivers/staging/rtl8723bs/core/rtw_cmd.c:                      
> > > > cmd_hdl
> > > > = wlancmds[pcmd->cmdcode].h2cfuns;
> > > 
> > > OK, I had imagined an anonymous call from its location in the array (as
> > > I wrote in the last phrase of my message). However, I thought that it
> > > could have been an improbable possibility, not a real one.
> > > 
> > > Linux uses a lot of interesting ideas that newcomers like me should
> > > learn. Things here are trickier than they appear at first sight.
> > 
> > One trick would be to build the Smatch cross function database.
> > 
> > https://www.spinics.net/lists/smatch/msg00568.html 
> > 
> > Then you could do:
> > 
> > $ ~/path/to/smatch_data/db/smdb.py led_blink_hdl
> > file | caller | function | type | parameter | key | value |
> > drivers/staging/rtl8723bs/core/rtw_cmd.c |       rtw_cmd_thread |
> > rtw_cmd_thread ptr cmd_hdl |           INTERNAL | -1 |                 |
> > uchar(*)(struct adapter*, uchar*)
> > drivers/staging/rtl8188eu/core/rtw_cmd.c |       rtw_cmd_thread |
> > rtw_cmd_thread ptr cmd_hdl |           INTERNAL | -1 |                 |
> > uchar(*)(struct adapter*, uchar*)
> > drivers/staging/rtl8188eu/core/rtw_cmd.c |       rtw_cmd_thread |
> > rtw_cmd_thread ptr cmd_hdl |           BUF_SIZE |  1 |            pbuf |
> > 1,4,6,8,12,14,16,19-20,23-24,48,740,884,892,900,960
> > 
> > 
> > Which says that led_blink_hdl() is called as a function pointer called
> > "cmd_hdl" from rtw_cmd_thread().
> > 
> > Hm...  It says it can be called from either rtw_cmd_thread() function
> > (the rtl8723bs or rtl8188eu version) which is not ideal.  But also
> > basically harmless so whatever...
> > 
> > regards,
> > dan carpenter
> >
> Nice tool, thanks. I'll surely use it when it is needed to find out which  
> callers use a function pointer.
> 
> However I cannot see how it can help in this context. That function *does* 
> something, even if I cannot understand why someone needs a function to test 
> the initialization of a pointer. Furthermore it is actually called by 
> rtw_cmd_thread() (as you found out by using smatch) that expect one of the 
> two possible values that led_blink_hdl() returns.
> 
> That said, what trick could I use for the purpose of getting rid of that 
> function? At this point I'm not sure it could be made.

If you look at how this is called:

drivers/staging/rtl8723bs/core/rtw_cmd.c
   449                  memcpy(pcmdbuf, pcmd->parmbuf, pcmd->cmdsz);
   450  
   451                  if (pcmd->cmdcode < ARRAY_SIZE(wlancmds)) {
   452                          cmd_hdl = wlancmds[pcmd->cmdcode].h2cfuns;
   453  
   454                          if (cmd_hdl) {
   455                                  ret = cmd_hdl(pcmd->padapter, pcmdbuf);
                                                                      ^^^^^^^

   456                                  pcmd->res = ret;
   457                          }
   458  
   459                          pcmdpriv->cmd_seq++;
   460                  } else {
   461                          pcmd->res = H2C_PARAMETERS_ERROR;
   462                  }
   463  
   464                  cmd_hdl = NULL;

The led_blink_hdl() function returns success if "pcmdbuf" is non-NULL
and fail if it's NULL.  "pcmdbuf" is never supposed to be NULL.  (The
"supposed" caveat is because there may be a race in rtw_sdio_if1_init()
which briefly allows a NULL "pcmdbuf", but that is an unrelated bug).

Anyway, there is no point to the led_blink_hdl() function.  Likely
they intended it to do something but never got around to implementing
it.  Just delete it.

regards,
dan carpenter

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