Suggest non-gki kernel users to backport path_umount (#1464)

Most kernel builders have to touch their kernel source code anyway, why
not also tell them to backport path_umount so even non-gki users can
benefit from this?

I know this might be a bit controversial as this will raise the barrier
of entry, but the benefits are just so high.

Idea was from OnlyTomInSecond on KernelSU group chat way back, and it
has been on the discussions for some time

references:
https://t.me/KernelSU_group/27237/176515
https://t.me/KernelSU_group/3249/184908

tiann/KernelSU#955 (comment)

OnlyTomInSecond/android_kernel_xiaomi_sdm845@03d233d
tiann/KernelSU#1060

https://elixir.bootlin.com/linux/v5.9.1/source/fs/namespace.c#L1728
https://elixir.bootlin.com/linux/v5.10.9/source/fs/namespace.c#L1730
https://elixir.bootlin.com/linux/v6.5/source/fs/namespace.c#L1887
tiann/KernelSU#1464 (comment)

Kernel side change examples
5.4
natsumerinchan/kernel_oneplus_sm8350@961d978
4.19
https://github.com/backslashxx/android_karnol_ximi_fog/commit/164917f56d0e75ab51bb9f1bdf489acac7a6d3db
4.14
crdroidandroid/android_kernel_xiaomi_sm6150@c07c70a
4.9
backslashxx/msm8953-kernel@195f075
4.4
https://github.com/riarumoda/android_kernel_samsung_a9y18qlte/commit/21ea33fe41ce079ec1d663c0bd2201bc00a8084a
tiann/KernelSU#1464 (comment)
ofcourse having someone on 3.18 confirm this will be nice.

**PROS**: umount modules for everyone
**CONS**: barrier of entry +1

---------

Co-authored-by: Christoph Hellwig <[email protected]>
Co-authored-by: Fede2782 <[email protected]>
Co-authored-by: Tom <[email protected]>

Author: 4 people

drivers/kernelsu/Makefile

@@ -56,5 +56,12 @@ $(info -- KernelSU Manager signature hash: $(KSU_EXPECTED_HASH))
 ccflags-y += -DEXPECTED_SIZE=$(KSU_EXPECTED_SIZE)
 ccflags-y += -DEXPECTED_HASH=\"$(KSU_EXPECTED_HASH)\"
 
+ifeq ($(shell grep -q "int path_umount" $(srctree)/fs/namespace.c; echo $$?),0)
+ccflags-y += -DKSU_UMOUNT
+else
+$(info -- Did you know you can backport path_umount to fs/namespace.c from 5.9?)
+$(info -- Read: https://kernelsu.org/guide/how-to-integrate-for-non-gki.html#path_umount)
+endif
+
 ccflags-y += -Wno-implicit-function-declaration -Wno-strict-prototypes -Wno-int-conversion -Wno-gcc-compat
 ccflags-y += -Wno-declaration-after-statement -Wno-unused-function

drivers/kernelsu/core_hook.c

@@ -522,7 +522,7 @@ static bool should_umount(struct path *path)
 
 static void ksu_umount_mnt(struct path *path, int flags)
 {
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 9, 0)
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 9, 0) || KSU_UMOUNT
 	int err = path_umount(path, flags);
 	if (err) {
 		pr_info("umount %s failed: %d\n", path->dentry->d_iname, err);