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Logical Resizable Android Partitions support

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The way how logical partition, or "Logical Resizable Android Partitions"
as they say in AOSP source code, is setup makes it impossible to early
mount the partitions from the shared super partition with just
a few lines of code; in fact, AOSP has a whole "fs_mgr" folder which
consist of multiple complex libraries, with 15K lines of code just
to deal with the device mapper shenanigans.

In order to keep the already overly complicated MagiskInit more
managable, I chose NOT to go the route of including fs_mgr directly
into MagiskInit. Luckily, starting from Android Q, Google decided to
split init startup into 3 stages, with the first stage doing _only_
early mount. This is great news, because we can simply let the stock
init do its own thing for us, and we intercept the bootup sequence.

So the workflow can be visualized roughly below:

Magisk First Stage --> First Stage Mount --> Magisk Second Stage --+
   (MagiskInit)         (Original Init)         (MagiskInit)       +
                                                                   +
                                                                   +
     ...Rest of the boot... <-- Second Stage <-- Selinux Setup  <--+
      (__________________ Original Init ____________________)

The catch here is that after doing all the first stage mounting, /init
will pivot /system as root directory (/), leaving us impossible to
regain control after we hand it over. So the solution here is to patch
fstab in /first_stage_ramdisk on-the-fly to redirect /system to
/system_root, making the original init do all the hard work for
us and mount required early mount partitions, but skips the step of
switching root directory. It will also conveniently hand over execution
back to MagiskInit, which we will reuse the routine for patching
root directory in normal system-as-root situations.
This commit is contained in:
topjohnwu
2019-06-29 00:47:29 -07:00
parent a48c4f9e05
commit f1112fdf37
6 changed files with 172 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files

66
native/jni/init/rootdir.cpp
View File

@@ -34,7 +34,7 @@ static void patch_socket_name(const char *path) {
}
static void patch_init_rc(FILE *rc) {
file_readline("/init.rc", [&](string_view line) -> bool {
file_readline("/init.rc", [=](string_view line) -> bool {
// Do not start vaultkeeper
if (str_contains(line, "start vaultkeeper")) {
LOGD("Remove vaultkeeper\n");
@@ -197,7 +197,7 @@ static void sbin_overlay(const raw_data &self, const raw_data &config) {
#define PATCHPOLICY "/sbin/.se"
#define LIBSELINUX "/system/" LIBNAME "/libselinux.so"
void SARInit::patch_rootdir() {
void SARCommon::patch_rootdir() {
sbin_overlay(self, config);
// Mount system_root mirror
@@ -300,6 +300,68 @@ void SARInit::patch_rootdir() {
xmount(ROOTOVERLAY "/init.rc", "/init.rc", nullptr, MS_BIND, nullptr);
}
#define FSR "/first_stage_ramdisk"
void FirstStageInit::patch_fstab() {
// Find fstab
DIR *dir = xopendir(FSR);
if (!dir)
return;
dirent *de;
string fstab(FSR "/");
while ((de = readdir(dir))) {
if (strstr(de->d_name, "fstab")) {
fstab += de->d_name;
break;
}
}
closedir(dir);
if (fstab.length() == sizeof(FSR))
return;
// Patch fstab
string patched = fstab + ".p";
FILE *fp = xfopen(patched.data(), "we");
file_readline(fstab.data(), [=](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.data(), patched.data());
rename(patched.data(), fstab.data());
// Move stuffs for next stage
xmkdir(FSR "/system", 0755);
xmkdir(FSR "/system/bin", 0755);
rename("/init", FSR "/system/bin/init");
xmkdir(FSR "/.backup", 0);
rename("/.backup/.magisk", FSR "/.backup/.magisk");
}
#ifdef MAGISK_DEBUG
static FILE *kmsg;
static int vprintk(const char *fmt, va_list ap) {