linux-kernel - Re: [Outreachy kernel] [PATCH] :staging: rtl8723bs: Remove useless led_blink

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Date:   Wed, 14 Apr 2021 08:21:50 +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 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