From: Eric Biggers <[email protected]>
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 <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jeff Vander Stoep <[email protected]>
Cc: Jessica Yu <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: NeilBrown <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
---
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.
+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
On Thu, Mar 12 2020, Eric Biggers wrote:
> From: Eric Biggers <[email protected]>
>
> 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 <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Greg Kroah-Hartman <[email protected]>
> Cc: Jeff Vander Stoep <[email protected]>
> Cc: Jessica Yu <[email protected]>
> Cc: Kees Cook <[email protected]>
> Cc: Luis Chamberlain <[email protected]>
> Cc: NeilBrown <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>
> ---
> 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..'
(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
On Fri, Mar 13, 2020 at 09:04:01AM +1100, NeilBrown wrote:
> On Thu, Mar 12 2020, Eric Biggers wrote:
>
> > From: Eric Biggers <[email protected]>
> >
> > 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 <[email protected]>
> > Cc: Andrew Morton <[email protected]>
> > Cc: Greg Kroah-Hartman <[email protected]>
> > Cc: Jeff Vander Stoep <[email protected]>
> > Cc: Jessica Yu <[email protected]>
> > Cc: Kees Cook <[email protected]>
> > Cc: Luis Chamberlain <[email protected]>
> > Cc: NeilBrown <[email protected]>
> > Signed-off-by: Eric Biggers <[email protected]>
> > ---
> > 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
On Fri, Mar 13, 2020 at 01:07:27AM +0000, Luis Chamberlain wrote:
> > > +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.
>
I think we're getting too down into the weeds here. The purpose of this patch
is just to document the modprobe sysctl, not to to give a full explanation of
how module autoloading works including modaliases and everything. And this
sysctl isn't needed to enable module autoloading; it's enabled by default.
Most users already use module autoloading without ever touching this sysctl.
Let's just write instead:
For example, if userspace passes an unknown filesystem type to mount(),
then the kernel will automatically request the corresponding filesystem
module by executing this usermode helper. This usermode helper should
insert the needed module into the kernel.
If someone wants to write a new documentation file that fully explains kernel
modules (I don't see any yet), they should should certainly do so. It's more
than I set out to do, though. IMO, just documenting this sysctl is already a
nice improvement by itself.
- Eric
On Fri, Mar 13, 2020 at 12:05:29PM -0700, Eric Biggers wrote:
> Let's just write instead:
>
> For example, if userspace passes an unknown filesystem type to mount(),
> then the kernel will automatically request the corresponding filesystem
> module by executing this usermode helper. This usermode helper should
> insert the needed module into the kernel.
Works with me.
Luis