From e3f63994737ad062d5dbd5c9700c71f735a7874d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E6=AE=8B=E9=A1=B5?=
 <31466456+canyie@users.noreply.github.com>
Date: Fri, 29 Oct 2021 09:00:43 +0800
Subject: [PATCH] Don't use xwrite() when patching legacy rootfs init

Fix topjohnwu#4810
> [    2.927463]  [1:           init:    1] magiskinit: Replace [/system/etc/selinux/plat_sepolicy.cil] -> [xxx]
[    2.936801]  [1:           init:    1] magiskinit: write failed with 14: Bad address

Since topjohnwu#4596, magisk fails to patch `/init`, xwrite() fails with EFAULT, break the original `/init` file and make the device unbootable. Reverting this commit for legacy rootfs devices fixes the problem. I think this is a Samsung kernel magic since currently I can't reproduce this on other devices or find something special in the log currently we have.
---
 native/jni/init/rootdir.cpp | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/native/jni/init/rootdir.cpp b/native/jni/init/rootdir.cpp
index a93ae7d96..436748f0f 100644
--- a/native/jni/init/rootdir.cpp
+++ b/native/jni/init/rootdir.cpp
@@ -343,11 +343,16 @@ void RootFSBase::patch_rootfs() {
     }
 
     if (patch_sepolicy("/sepolicy")) {
-        auto init = mmap_data::ro(access("/system/bin/init",F_OK) == 0 ? "/system/bin/init" : "/init");
-        init.patch({ make_pair(SPLIT_PLAT_CIL, "xxx") });
-        int dest = xopen("/init", O_TRUNC | O_WRONLY | O_CLOEXEC, 0);
-        xwrite(dest, init.buf, init.sz);
-        close(dest);
+        if (access("/system/bin/init", F_OK) == 0) {
+            auto init = mmap_data::ro("/system/bin/init");
+            init.patch({ make_pair(SPLIT_PLAT_CIL, "xxx") });
+            int dest = xopen("/init", O_TRUNC | O_WRONLY | O_CLOEXEC, 0);
+            xwrite(dest, init.buf, init.sz);
+            close(dest);
+        } else {
+            auto init = mmap_data::rw("/init");
+            init.patch({ make_pair(SPLIT_PLAT_CIL, "xxx") });
+        }
     }
 
     // Handle overlays