Force init to load fstab from file in 2SI

Patching DTBs is proven to be difficult and problematic as there are
tons of different formats out there. Adding support for all the formats
in magiskboot has been quite an headache in the past year, and it still
definitely does not cover all possible cases of them out there.

There is another issue: fake dt fstabs. Some super old devices do not
have device trees in their boot images, so some custom ROM developers
had came up with a "genius" solution: hardcode fstab entries directly
in the kernel source code and create fake device tree nodes even if
Android 10+ init can graciously take fstab files instead (-_-) 。。。

And there is YET another issue: DTBs are not always in boot images!
Google is crazy enough to litter DTBs all over the place, it is like
they cannot make up their minds (duh). This means the dt fstabs can be
either concatnated after the kernel (1), in the DTB partition (2), in
the DTBO partition (3), in the recovery_dtbo section in boot images (4),
or in the dtb section in boot images (5). FIVE f**king places, how can
anyone keep up with that!

With Android 10+ that uses 2 stage inits, it is crutual for Magisk to
be able to modify fstab mount points in order to let the original init
mount partitions for us, but NOT switch root and continue booting. For
devices using dt for early mount fstab, we used to patch the DTB at
install time with magiskboot. However these changes are permanent and
cannot be restored back at reinstallation.

With this commit, Magisk will read dt fstabs and write them to ramdisk
at boot time. And in that case, the init binary will also be patched
to force it to NEVER use fstabs in device-tree. By doing so, we can
unify ramdisk based 2SI fstab patching as basically we are just patching
fstab files. This also means we can manipulate fstab whatever Magisk
needs in the future without the need to going through the headache that
is patching DTBs at installation.

native/jni/init/init.hpp

@@ -4,7 +4,6 @@
 #include <unistd.h>
 #include <stdlib.h>
 #include <vector>
-#include <map>
 
 #include <logging.hpp>
 
@@ -44,6 +43,7 @@ struct fstab_entry {
 	fstab_entry() = default;
 	fstab_entry(const fstab_entry &o) = delete;
 	fstab_entry(fstab_entry &&o) = default;
+	void to_file(FILE *fp);
 };
 
 #define INIT_SOCKET "MAGISKINIT"
@@ -79,7 +79,7 @@ public:
 	cmd(cmd), argv(argv), mount_list{"/sys", "/proc"} {}
 	virtual ~BaseInit() = default;
 	virtual void start() = 0;
-	void read_dt_fstab(std::map<std::string_view, fstab_entry> &fstab);
+	void read_dt_fstab(std::vector<fstab_entry> &fstab);
 	void dt_early_mount();
 };
 
@@ -118,6 +118,7 @@ public:
 class FirstStageInit : public BaseInit {
 private:
 	void prepare();
+
 public:
 	FirstStageInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {
 		LOGD("%s\n", __FUNCTION__);

native/jni/init/mount.cpp

@@ -104,16 +104,26 @@ if (access(#val, F_OK) == 0) {\
 	entry.val = rtrim(full_read(#val)); \
 }
 
-void BaseInit::read_dt_fstab(map<string_view, fstab_entry> &fstab) {
+void BaseInit::read_dt_fstab(vector<fstab_entry> &fstab) {
 	if (access(cmd->dt_dir, F_OK) != 0)
 		return;
+
+	char cwd[128];
+	getcwd(cwd, sizeof(cwd));
 	chdir(cmd->dt_dir);
-	run_finally cr([]{ chdir("/"); });
+	run_finally cd([&]{ chdir(cwd); });
 
 	if (access("fstab", F_OK) != 0)
 		return;
 	chdir("fstab");
 
+	// Make sure dt fstab is enabled
+	if (access("status", F_OK) == 0) {
+		auto status = rtrim(full_read("status"));
+		if (status != "okay" && status != "ok")
+			return;
+	}
+
 	auto dir = xopen_dir(".");
 	for (dirent *dp; (dp = xreaddir(dir.get()));) {
 		if (dp->d_type != DT_DIR)
@@ -138,14 +148,14 @@ void BaseInit::read_dt_fstab(map<string_view, fstab_entry> &fstab) {
 		read_info(mnt_flags);
 		read_info(fsmgr_flags);
 
-		fstab.emplace(entry.mnt_point, std::move(entry));
+		fstab.emplace_back(std::move(entry));
 	}
 }
 
 void BaseInit::dt_early_mount() {
-	map<string_view, fstab_entry> fstab;
+	vector<fstab_entry> fstab;
 	read_dt_fstab(fstab);
-	for (const auto &[_, entry] : fstab) {
+	for (const auto &entry : fstab) {
 		if (is_lnk(entry.mnt_point.data()))
 			continue;
 		// Derive partname from dev

native/jni/init/twostage.cpp

@@ -7,42 +7,15 @@
 
 using namespace std;
 
-static void patch_fstab(const char *fstab) {
-	string patched = fstab + ".p"s;
-	FILE *fp = xfopen(patched.data(), "we");
-	file_readline(fstab, [=](string_view l) -> bool {
-		if (l[0] == '#' || l.length() == 1)
-			return true;
-		char *line = (char *) l.data();
-		int src0, src1, mnt0, mnt1, type0, type1, opt0, opt1, flag0, flag1;
-		sscanf(line, "%n%*s%n %n%*s%n %n%*s%n %n%*s%n %n%*s%n",
-			   &src0, &src1, &mnt0, &mnt1, &type0, &type1, &opt0, &opt1, &flag0, &flag1);
-		const char *src, *mnt, *type, *opt, *flag;
-		src = &line[src0];
-		line[src1] = '\0';
-		mnt = &line[mnt0];
-		line[mnt1] = '\0';
-		type = &line[type0];
-		line[type1] = '\0';
-		opt = &line[opt0];
-		line[opt1] = '\0';
-		flag = &line[flag0];
-		line[flag1] = '\0';
-
-		// Redirect system to system_root
-		if (mnt == "/system"sv)
-			mnt = "/system_root";
-
-		fprintf(fp, "%s %s %s %s %s\n", src, mnt, type, opt, flag);
-		return true;
-	});
-	fclose(fp);
-
-	// Replace old fstab
-	clone_attr(fstab, patched.data());
-	rename(patched.data(), fstab);
+void fstab_entry::to_file(FILE *fp) {
+	fprintf(fp, "%s %s %s %s %s\n", dev.data(), mnt_point.data(),
+			type.data(), mnt_flags.data(), fsmgr_flags.data());
 }
 
+#define set_info(val) \
+line[val##1] = '\0'; \
+entry.val = &line[val##0];
+
 #define FSR "/first_stage_ramdisk"
 
 void FirstStageInit::prepare() {
@@ -62,14 +35,100 @@ void FirstStageInit::prepare() {
 		rename("/.backup/init", "/init");
 	}
 
-	// Patch fstab
-	auto dir = xopen_dir(".");
-	for (dirent *de; (de = xreaddir(dir.get()));) {
-		if (strstr(de->d_name, "fstab")) {
-			patch_fstab(de->d_name);
+	// Try to load fstab from dt
+	vector<fstab_entry> fstab;
+	read_dt_fstab(fstab);
+
+	char fstab_file[128];
+	fstab_file[0] = '\0';
+
+	// Find existing fstab file
+	for (const char *hw : { cmd->hardware, cmd->hardware_plat }) {
+		if (hw[0] == '\0')
+			continue;
+		sprintf(fstab_file, "fstab.%s", hw);
+		if (access(fstab_file, F_OK) != 0) {
+			fstab_file[0] = '\0';
+			continue;
+		} else {
+			LOGD("Found fstab file: %s\n", fstab_file);
 			break;
 		}
 	}
+
+	if (fstab.empty()) {
+		// fstab has to be somewhere in ramdisk
+		if (fstab_file[0] == '\0') {
+			LOGE("Cannot find fstab file in ramdisk!\n");
+			return;
+		}
+
+		// Parse and load fstab file
+		file_readline(fstab_file, [&](string_view l) -> bool {
+			if (l[0] == '#' || l.length() == 1)
+				return true;
+			char *line = (char *) l.data();
+
+			int dev0, dev1, mnt_point0, mnt_point1, type0, type1,
+					mnt_flags0, mnt_flags1, fsmgr_flags0, fsmgr_flags1;
+
+			sscanf(line, "%n%*s%n %n%*s%n %n%*s%n %n%*s%n %n%*s%n",
+				   &dev0, &dev1, &mnt_point0, &mnt_point1, &type0, &type1,
+				   &mnt_flags0, &mnt_flags1, &fsmgr_flags0, &fsmgr_flags1);
+
+			fstab_entry entry;
+
+			set_info(dev);
+			set_info(mnt_point);
+			set_info(type);
+			set_info(mnt_flags);
+			set_info(fsmgr_flags);
+
+			fstab.emplace_back(std::move(entry));
+			return true;
+		});
+	} else {
+		// All dt fstab entries should be first_stage_mount
+		for (auto &entry : fstab) {
+			if (!str_contains(entry.fsmgr_flags, "first_stage_mount")) {
+				if (!entry.fsmgr_flags.empty())
+					entry.fsmgr_flags += ',';
+				entry.fsmgr_flags += "first_stage_mount";
+			}
+		}
+
+		if (fstab_file[0] == '\0') {
+			const char *hw = cmd->hardware[0] ? cmd->hardware :
+					(cmd->hardware_plat[0] ? cmd->hardware_plat : nullptr);
+			if (hw == nullptr) {
+				LOGE("Cannot determine hardware name!\n");
+				return;
+			}
+			sprintf(fstab_file, "fstab.%s", hw);
+		}
+
+		// Patch init to force ignore dt fstab
+		uint8_t *addr;
+		size_t sz;
+		mmap_rw("/init", addr, sz);
+		raw_data_patch(addr, sz, {
+			make_pair("android,fstab", "xxx")  /* Force IsDtFstabCompatible() to return false */
+		});
+		munmap(addr, sz);
+	}
+
+	{
+		LOGD("Write fstab file: %s\n", fstab_file);
+		auto fp = xopen_file(fstab_file, "we");
+		for (auto &entry : fstab) {
+			// Redirect system mnt_point so init won't switch root in first stage init
+			if (entry.mnt_point == "/system")
+				entry.mnt_point = "/system_root";
+			entry.to_file(fp.get());
+		}
+	}
+	chmod(fstab_file, 0644);
+
 	chdir("/");
 }
 
@@ -82,20 +141,16 @@ void SARFirstStageInit::prepare() {
 	xmount("tmpfs", "/dev", "tmpfs", 0, "mode=755");
 
 	// Patch init binary
-	raw_data init;
 	int src = xopen("/init", O_RDONLY);
-	fd_full_read(src, init.buf, init.sz);
-	for (uint8_t *p = init.buf, *eof = init.buf + init.sz; p < eof; ++p) {
-		if (memcmp(p, INIT_PATH, sizeof(INIT_PATH)) == 0) {
-			LOGD("Patch init [" INIT_PATH "] -> [" REDIR_PATH "]\n");
-			memcpy(p, REDIR_PATH, sizeof(REDIR_PATH));
-			break;
-		}
-	}
 	int dest = xopen("/dev/init", O_CREAT | O_WRONLY, 0);
-	write(dest, init.buf, init.sz);
-	fclone_attr(src, dest);
-	close(dest);
+	{
+		raw_data init;
+		fd_full_read(src, init.buf, init.sz);
+		raw_data_patch(init.buf, init.sz, { make_pair(INIT_PATH, REDIR_PATH) });
+		write(dest, init.buf, init.sz);
+		fclone_attr(src, dest);
+		close(dest);
+	}
 
 	// Replace redirect init with magiskinit
 	dest = xopen("/dev/magiskinit", O_CREAT | O_WRONLY, 0);