linux-kernel - Re: [PATCH v2 4/4] ax.25: Remove the now superfluous sentinel elements from ctl

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Date: Fri, 5 Apr 2024 15:26:58 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <j.granados@...sung.com>
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Subject: Re: [PATCH v2 4/4] ax.25: Remove the now superfluous sentinel elements from ctl_table array

From: Joel Granados <j.granados@...sung.com>
Date: Fri, 5 Apr 2024 09:15:31 +0200
> On Thu, Mar 28, 2024 at 12:49:34PM -0700, Kuniyuki Iwashima wrote:
> > From: Joel Granados via B4 Relay <devnull+j.granados.samsung.com@...nel.org>
> > Date: Thu, 28 Mar 2024 16:40:05 +0100
> > > This commit comes at the tail end of a greater effort to remove the
> > > empty elements at the end of the ctl_table arrays (sentinels) which will
> > > reduce the overall build time size of the kernel and run time memory
> > > bloat by ~64 bytes per sentinel (further information Link :
> > > https://lore.kernel.org/all/ZO5Yx5JFogGi%2FcBo@bombadil.infradead.org/)
> > > 
> > > When we remove the sentinel from ax25_param_table a buffer overflow
> > > shows its ugly head. The sentinel's data element used to be changed when
> > > CONFIG_AX25_DAMA_SLAVE was not defined.
> > 
> > I think it's better to define the relation explicitly between the
> > enum and sysctl table by BUILD_BUG_ON() in ax25_register_dev_sysctl()
> > 
> >   BUILD_BUG_ON(AX25_MAX_VALUES != ARRAY_SIZE(ax25_param_table));
> > 
> > and guard AX25_VALUES_DS_TIMEOUT with #ifdef CONFIG_AX25_DAMA_SLAVE
> > as done for other enum.
> 
> When I remove AX25_VALUES_DS_TIMEOUT from the un-guarded build it
> complains in net/ax25/ax25_ds_timer.c (ax25_ds_set_timer). Here is the
> report https://lore.kernel.org/oe-kbuild-all/202404040301.qzKmVQGB-lkp@intel.com/.
> 
> How best to address this? Should we just guard the whole function and do
> nothing when not set? like this:

It seems fine to me.

ax25_ds_timeout() checks !ax25_dev->dama.slave_timeout, but it's
initialised by kzalloc() during dev setup, so it will be a noop.


> 
> ```
> void ax25_ds_set_timer(ax25_dev *ax25_dev)
> {
> #ifdef COFNIG_AX25_DAMA_SLAVE
>         if (ax25_dev == NULL)        ···/* paranoia */
>                 return;
> 
>         ax25_dev->dama.slave_timeout =
>                 msecs_to_jiffies(ax25_dev->values[AX25_VALUES_DS_TIMEOUT]) / 10;
>         mod_timer(&ax25_dev->dama.slave_timer, jiffies + HZ);
> #else
>         return;
> #endif
> }
> 
> ```
> 
> I'm not too familiar with this, so pointing me to the "correct" way to
> handle this would be helpfull.

Also, you will need to guard another use of AX25_VALUES_DS_TIMEOUT in
ax25_dev_device_up().

Thanks!