Re: Re: Re: [PATCH v2] treewide: const qualify ctl_tables where applicable

Joel Granados
2025-01-27T13:50:00Z

On Wed, Jan 22, 2025 at 01:41:35PM +0100, Ard Biesheuvel wrote:
> On Wed, 22 Jan 2025 at 13:25, Joel Granados <joel.granados@kernel.org> wrote:
> >
> > On Tue, Jan 21, 2025 at 02:40:16PM +0100, Alexander Gordeev wrote:
> > > On Fri, Jan 10, 2025 at 03:16:08PM +0100, Joel Granados wrote:
> > >
> > > Hi Joel,
> > >
> > > > Add the const qualifier to all the ctl_tables in the tree except for
> > > > watchdog_hardlockup_sysctl, memory_allocation_profiling_sysctls,
> > > > loadpin_sysctl_table and the ones calling register_net_sysctl (./net,
> > > > drivers/inifiniband dirs). These are special cases as they use a
> > > > registration function with a non-const qualified ctl_table argument or
> > > > modify the arrays before passing them on to the registration function.
> > > >
> > > > Constifying ctl_table structs will prevent the modification of
> > > > proc_handler function pointers as the arrays would reside in .rodata.
> > > > This is made possible after commit 78eb4ea25cd5 ("sysctl: treewide:
> > > > constify the ctl_table argument of proc_handlers") constified all the
> > > > proc_handlers.
> > >
> > > I could identify at least these occurences in s390 code as well:
> > Hey Alexander
> >
> > Thx for bringing these to my attention. I had completely missed them as
> > the spatch only deals with ctl_tables outside functions.
> >
> > Short answer:
> > These should not be included in the current patch because they are a
> > different pattern from how sysctl tables are usually used. So I will not
> > include them.
> >
> > With that said, I think it might be interesting to look closer at them
> > as they seem to be complicating the proc_handler (I have to look at them
> > closer).
> >
> > I see that they are defining a ctl_table struct within the functions and
> > just using the data (from the incoming ctl_table) to forward things down
> > to proc_do{u,}intvec_* functions. This is very odd and I have only seen
> > it done in order to change the incoming ctl_table (which is not what is
> > being done here).
> >
> > I will take a closer look after the merge window and circle back with
> > more info. Might take me a while as I'm not very familiar with s390
> > code; any additional information on why those are being used inside the
> > functions would be helpfull.
> >
>
> Using const data on the stack is not as useful, because the stack is
> always mapped writable.
>
> Global data structures marked 'const' will be moved into an ELF
> section that is typically mapped read-only in its entirely, and so the
> data cannot be modified by writing to it directly. No such protection
> is possible for the stack, and so the constness there is only enforced
> at compile time.
I completely agree with you. No reason to use const within those
functions. But why define those ctl_tables in function to begin with.
Can't you just use the ones that are defined outside the functions?

Best

--

Joel Granados