linux-kernel - Re: [PATCH v2 3/4] docs: admin-guide: document the kernel.modprobe sysctl

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Date:   Fri, 13 Mar 2020 01:07:27 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     NeilBrown <neilb@...e.de>
Cc:     Eric Biggers <ebiggers@...nel.org>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeff Vander Stoep <jeffv@...gle.com>,
        Jessica Yu <jeyu@...nel.org>,
        Kees Cook <keescook@...omium.org>, NeilBrown <neilb@...e.com>
Subject: Re: [PATCH v2 3/4] docs: admin-guide: document the kernel.modprobe
 sysctl

On Fri, Mar 13, 2020 at 09:04:01AM +1100, NeilBrown wrote:
> On Thu, Mar 12 2020, Eric Biggers wrote:
> 
> > From: Eric Biggers <ebiggers@...gle.com>
> >
> > Document the kernel.modprobe sysctl in the same place that all the other
> > kernel.* sysctls are documented.  Make sure to mention how to use this
> > sysctl to completely disable module autoloading, and how this sysctl
> > relates to CONFIG_STATIC_USERMODEHELPER.
> >
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Cc: Jeff Vander Stoep <jeffv@...gle.com>
> > Cc: Jessica Yu <jeyu@...nel.org>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Luis Chamberlain <mcgrof@...nel.org>
> > Cc: NeilBrown <neilb@...e.com>
> > Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> > ---
> >  Documentation/admin-guide/sysctl/kernel.rst | 25 ++++++++++++++++++++-
> >  1 file changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> > index def074807cee9..454f3402ed321 100644
> > --- a/Documentation/admin-guide/sysctl/kernel.rst
> > +++ b/Documentation/admin-guide/sysctl/kernel.rst
> > @@ -49,7 +49,7 @@ show up in /proc/sys/kernel:
> >  - kexec_load_disabled
> >  - kptr_restrict
> >  - l2cr                        [ PPC only ]
> > -- modprobe                    ==> Documentation/debugging-modules.txt
> > +- modprobe
> >  - modules_disabled
> >  - msg_next_id		      [ sysv ipc ]
> >  - msgmax
> > @@ -444,6 +444,29 @@ l2cr: (PPC only)
> >  This flag controls the L2 cache of G3 processor boards. If
> >  0, the cache is disabled. Enabled if nonzero.
> >  
> > +modprobe:
> > +=========
> > +
> > +The path to the usermode helper for autoloading kernel modules, by
> > +default "/sbin/modprobe".  This binary is executed when the kernel
> > +requests a module.  For example, if userspace passes an unknown
> > +filesystem type "foo" to mount(), then the kernel will automatically
> > +request the module "fs-foo.ko" by executing this usermode helper.
> 
> I don't think it is right to add the ".ko" there.  The string "fs-foo"
> is what is passed to the named executable, and it make well end up
> loading "bar.ko", depending what aliases are set up.
> I would probably write  '... request the module named 'fs-foo" by executing..'

And that is just because filesystems, in this case a mount call, will
use the fs- prefix for aliases. This is tribal knowledge in the context
above, and so someone not familiar with this won't easily grasp this.

Is there an easier autoloading example other than filesystems we can use that
doesn't require you to explain the aliasing thing?

What is module autoloading? Where is this documented ? If that
can be slightly clarified this would be even easier to understand as
well.

  Luis

> (The "name" for a module can come from the file that stores it, and
> alias inside it, or configuration in modprobe.d).
> 
> Thanks,
> NeilBrown
> 
> 
> > +This usermode helper should insert the needed module into the kernel.
> > +
> > +This sysctl only affects module autoloading.  It has no effect on the
> > +ability to explicitly insert modules.
> > +
> > +If this sysctl is set to the empty string, then module autoloading is
> > +completely disabled.  The kernel will not try to execute a usermode
> > +helper at all, nor will it call the kernel_module_request LSM hook.
> > +
> > +If CONFIG_STATIC_USERMODEHELPER=y is set in the kernel configuration,
> > +then the configured static usermode helper overrides this sysctl,
> > +except that the empty string is still accepted to completely disable
> > +module autoloading as described above.
> > +
> > +Also see Documentation/debugging-modules.txt.
> >  
> >  modules_disabled:
> >  =================
> > -- 
> > 2.25.1



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