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

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.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@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jeff Vander Stoep <jeffv@google.com>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Eric Biggers <ebiggers@google.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.
+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