website: document the replacement of do_faccessat for some non-GKI (#206)

Signed-off-by: Ookiineko <chiisaineko@protonmail.com>

website/docs/guide/how-to-integrate-for-non-gki.md

@@ -169,4 +169,31 @@ index 068fdbcc9e26..5348b7bb9db2 100644
  		goto out;
 ```
 
+For kernels eariler than 4.17, if you cannot find `do_faccessat`, just go to the definition of the `faccessat` syscall and place the call there:
+
+```diff
+diff --git a/fs/open.c b/fs/open.c
+index 2ff887661237..e758d7db7663 100644
+--- a/fs/open.c
++++ b/fs/open.c
+@@ -355,6 +355,9 @@ SYSCALL_DEFINE4(fallocate, int, fd, int, mode, loff_t, offset, loff_t, len)
+ 	return error;
+ }
+ 
++extern int ksu_handle_faccessat(int *dfd, const char __user **filename_user, int *mode,
++			        int *flags);
++
+ /*
+  * access() needs to use the real uid/gid, not the effective uid/gid.
+  * We do this by temporarily clearing all FS-related capabilities and
+@@ -370,6 +373,8 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
+ 	int res;
+ 	unsigned int lookup_flags = LOOKUP_FOLLOW;
+ 
++	ksu_handle_faccessat(&dfd, &filename, &mode, NULL);
++
+ 	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
+ 		return -EINVAL;
+```
+
 Finally, build your kernel again, KernelSU should works well.