ueventd: duplicate /dev/ashmem

We want ashmem to only be used via libcutils API, with long-term goal
being deprecation of ashmem with memfd. To do that we route libcutils to
a new source of ashmem fds. We then phase out uses of /dev/ashmem that
doesn't go through libcutils using SELinux.

In Q, we introduced ashmemd as the source of ashmem fds to libcutils.
However, having a separate process and, consequently, binder hops to
handle /dev/ashmem results in performance/memory overhead.

To address the overhead, replace ashmemd with a duplicate of
/dev/ashmem. Name it /dev/ashmem<boot_id>, where boot_id is a random
number generated on each boot. This way we make sure that developers
don't accidentally depend on /dev/ashmem<boot_id>, as that name can't be
hardcoded.

Bug: 139855428
Test: writing "add"/"remove" to /sys/class/misc/ashmem/uevent correctly
adds/removes /dev/ashmem and /dev/ashmem/boot_id
Change-Id: I36d23116048bfcd99903ba46cc133161835a2cfa

init/devices.cpp

@@ -441,6 +441,23 @@ void DeviceHandler::HandleDevice(const std::string& action, const std::string& d
     }
 }
 
+void DeviceHandler::HandleAshmemUevent(const Uevent& uevent) {
+    if (uevent.device_name == "ashmem") {
+        static const std::string boot_id_path = "/proc/sys/kernel/random/boot_id";
+        std::string boot_id;
+        if (!ReadFileToString(boot_id_path, &boot_id)) {
+            PLOG(ERROR) << "Cannot duplicate ashmem device node. Failed to read " << boot_id_path;
+            return;
+        };
+        boot_id = Trim(boot_id);
+
+        Uevent dup_ashmem_uevent = uevent;
+        dup_ashmem_uevent.device_name += boot_id;
+        dup_ashmem_uevent.path += boot_id;
+        HandleUevent(dup_ashmem_uevent);
+    }
+}
+
 void DeviceHandler::HandleUevent(const Uevent& uevent) {
     if (uevent.action == "add" || uevent.action == "change" || uevent.action == "online") {
         FixupSysPermissions(uevent.path, uevent.subsystem);
@@ -485,6 +502,10 @@ void DeviceHandler::HandleUevent(const Uevent& uevent) {
     mkdir_recursive(Dirname(devpath), 0755);
 
     HandleDevice(uevent.action, devpath, block, uevent.major, uevent.minor, links);
+
+    // Duplicate /dev/ashmem device and name it /dev/ashmem<boot_id>.
+    // TODO(b/111903542): remove once all users of /dev/ashmem are migrated to libcutils API.
+    HandleAshmemUevent(uevent);
 }
 
 void DeviceHandler::ColdbootDone() {

init/devices.h

@@ -130,6 +130,7 @@ class DeviceHandler : public UeventHandler {
     void HandleDevice(const std::string& action, const std::string& devpath, bool block, int major,
                       int minor, const std::vector<std::string>& links) const;
     void FixupSysPermissions(const std::string& upath, const std::string& subsystem) const;
+    void HandleAshmemUevent(const Uevent& uevent);
 
     std::vector<Permissions> dev_permissions_;
     std::vector<SysfsPermissions> sysfs_permissions_;

rootdir/ueventd.rc

@@ -33,7 +33,7 @@ subsystem sound
 /dev/urandom              0666   root       root
 # Make HW RNG readable by group system to let EntropyMixer read it.
 /dev/hw_random            0440   root       system
-/dev/ashmem               0666   root       root
+/dev/ashmem*              0666   root       root
 /dev/binder               0666   root       root
 /dev/hwbinder             0666   root       root
 /dev/vndbinder            0666   root       root