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init: Bind mount /mnt/installer early for scoped storage.

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Scoped storage has some unique requirements that are hard to implement
with the two mount namespaces, because the daemon that does the mounting
(vold) lives in a different namespace than the processes using those
mounts.

In particular, /mnt/installer is a special bind mount that should
receive mount events under /mnt/user, but at the same time only
only propagate mount events under /mnt/installer to /mnt/installer in
the other namespace. More details in the code.

Bug: 134706060
Test: /mnt/installer shows up and is setup correctly.
Change-Id: I6dab5ace5a345d9d684a9f1ae94c833fc294d49e
This commit is contained in:
Martijn Coenen 2020-01-10 15:42:15 +01:00
parent 7c7d1c600e
commit c70c0665fc
1 changed files with 58 additions and 0 deletions
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58
init/mount_namespace.cpp
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@ -35,6 +35,19 @@ namespace android {
namespace init {
namespace {
static bool BindMount(const std::string& source, const std::string& mount_point,
bool recursive = false) {
unsigned long mountflags = MS_BIND;
if (recursive) {
mountflags |= MS_REC;
}
if (mount(source.c_str(), mount_point.c_str(), nullptr, mountflags, nullptr) == -1) {
PLOG(ERROR) << "Failed to bind mount " << source;
return false;
}
return true;
}
static bool MakeShared(const std::string& mount_point, bool recursive = false) {
unsigned long mountflags = MS_SHARED;
if (recursive) {
@ -47,6 +60,18 @@ static bool MakeShared(const std::string& mount_point, bool recursive = false) {
return true;
}
static bool MakeSlave(const std::string& mount_point, bool recursive = false) {
unsigned long mountflags = MS_SLAVE;
if (recursive) {
mountflags |= MS_REC;
}
if (mount(nullptr, mount_point.c_str(), nullptr, mountflags, nullptr) == -1) {
PLOG(ERROR) << "Failed to change propagation type to slave";
return false;
}
return true;
}
static bool MakePrivate(const std::string& mount_point, bool recursive = false) {
unsigned long mountflags = MS_PRIVATE;
if (recursive) {
@ -191,6 +216,39 @@ bool SetupMountNamespaces() {
// namespace
if (!(MakePrivate("/linkerconfig"))) return false;
// The two mount namespaces present challenges for scoped storage, because
// vold, which is responsible for most of the mounting, lives in the
// bootstrap mount namespace, whereas most other daemons and all apps live
// in the default namespace. Scoped storage has a need for a
// /mnt/installer view that is a slave bind mount of /mnt/user - in other
// words, all mounts under /mnt/user should automatically show up under
// /mnt/installer. However, additional mounts done under /mnt/installer
// should not propagate back to /mnt/user. In a single mount namespace
// this is easy to achieve, by simply marking the /mnt/installer a slave
// bind mount. Unfortunately, if /mnt/installer is only created and
// bind mounted after the two namespaces are created below, we end up
// with the following situation:
// /mnt/user and /mnt/installer share the same peer group in both the
// bootstrap and default namespaces. Marking /mnt/installer slave in either
// namespace means that it won't propagate events to the /mnt/installer in
// the other namespace, which is still something we require - vold is the
// one doing the mounting under /mnt/installer, and those mounts should
// show up in the default namespace as well.
//
// The simplest solution is to do the bind mount before the two namespaces
// are created: the effect is that in both namespaces, /mnt/installer is a
// slave to the /mnt/user mount, and at the same time /mnt/installer in the
// bootstrap namespace shares a peer group with /mnt/installer in the
// default namespace.
if (!mkdir_recursive("/mnt/user", 0755)) return false;
if (!mkdir_recursive("/mnt/installer", 0755)) return false;
if (!(BindMount("/mnt/user", "/mnt/installer", true))) return false;
// First, make /mnt/installer a slave bind mount
if (!(MakeSlave("/mnt/installer"))) return false;
// Then, make it shared again - effectively creating a new peer group, that
// will be inherited by new mount namespaces.
if (!(MakeShared("/mnt/installer"))) return false;
bootstrap_ns_fd.reset(OpenMountNamespace());
bootstrap_ns_id = GetMountNamespaceId();