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modules: add CONFIG_MODPROBE_PATH

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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
made the initramfs unpacking asynchronous - 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.

For a relatively slow ppc board I'm working on, the two patches combined
lead to 0.2s faster boot - but more importantly, the fact that the
initramfs unpacking proceeds completely in the background while devices
get probed means I get to handle the gpio watchdog in time without getting
reset.

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

Link: https://lkml.kernel.org/r/20210313212528.2956377-3-linux@rasmusvillemoes.dk
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Jessica Yu <jeyu@kernel.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Rasmus Villemoes 2021-05-06 18:05:45 -07:00 committed by Linus Torvalds
parent e7cb072eb9
commit 17652f4240
2 changed files with 13 additions and 1 deletions
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12
init/Kconfig
View File

@ -2299,6 +2299,18 @@ 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" if EXPERT
depends on !COMPILE_TEST

2
kernel/kmod.c
View File

@ -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)
{