[PATCH/RFC 2/2] modules: add CONFIG_MODPROBE_PATH

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

Allow the developer to specifiy the initial value of the
modprobe_path[] string. This can be used to set it to the empty string
initially, thus effectively disabling request_module() during early
boot until userspace writes a new value via the
/proc/sys/kernel/modprobe interface. [1]

When building a custom kernel (often for an embedded target), it's
normal to build everything into the kernel that is needed for booting,
and indeed the initramfs often contains no modules at all, so every
such request_module() done before userspace init has mounted the real
rootfs is a waste of time.

This is particularly useful when combined with the previous patch,
which allowed the initramfs to be unpacked asynchronously - for that
to work, it had to make any usermodehelper call wait for the unpacking
to finish before attempting to invoke the userspace helper. By
eliminating all such (known-to-be-futile) calls of usermodehelper, the
initramfs unpacking and the {device,late}_initcalls can proceed in
parallel for much longer.

[1] __request_module() already has an early -ENOENT return when
modprobe_path is the empty string.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 init/Kconfig  | 12 ++++++++++++
 kernel/kmod.c |  2 +-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index ba8bd5256980..e3f61e15445d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2272,6 +2272,18 @@ config MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS
 
 	  If unsure, say N.
 
+config MODPROBE_PATH
+	string "Path to modprobe binary"
+	default "/sbin/modprobe"
+	help
+	  When kernel code requests a module, it does so by calling
+	  the "modprobe" userspace utility. This option allows you to
+	  set the path where that binary is found. This can be changed
+	  at runtime via the sysctl file
+	  /proc/sys/kernel/modprobe. Setting this to the empty string
+	  removes the kernel's ability to request modules (but
+	  userspace can still load modules explicitly).
+
 config TRIM_UNUSED_KSYMS
 	bool "Trim unused exported kernel symbols"
 	depends on BROKEN
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 3cd075ce2a1e..b717134ebe17 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -58,7 +58,7 @@ static DECLARE_WAIT_QUEUE_HEAD(kmod_wq);
 /*
 	modprobe_path is set via /proc/sys.
 */
-char modprobe_path[KMOD_PATH_LEN] = "/sbin/modprobe";
+char modprobe_path[KMOD_PATH_LEN] = CONFIG_MODPROBE_PATH;
 
 static void free_modprobe_argv(struct subprocess_info *info)
 {
-- 
2.29.2