From b479666e3c9c96c93a53c334c9cd881106b3bda2 Mon Sep 17 00:00:00 2001
From: "A. Cody Schuffelen" <schuffelen@google.com>
Date: Tue, 23 Apr 2024 18:59:00 -0700
Subject: [PATCH] ueventd: Add `devname sys_name` to `subsystem`, pulling
 device names from sysfs

This change introduces a new mode to `subsystem.devname` in `ueventd.rc`
configuration files, which sets the file name to the contents of
`/sys/DEVNAME/name`.

The objective of this change is to help Cuttlefish distinguish between
console devices, which are only different in uevents by initialization
order. Cuttlefish currently relies on `/dev/hvc##` devices which are
created for non-multiport virtio-console devices.

https://cs.android.com/android/platform/superproject/main/+/main:device/google/cuttlefish/shared/config/ueventd.rc;l=18;drc=5204f119d859d3ae5f1a2ee1c6a05ee68d6a28ed

On Cuttlefish we're considering moving to multiport virtio-console
devices ( https://fedoraproject.org/wiki/Features/VirtioSerial ). It
would be possible to rely on device order here as well, but using names
to distinguish devices makes it possible to drop unused devices in the
future, rather than reserving indexes indefinitely.

Multiport virtio-console devices create uevents with DEVNAME=vport#p#
and DEVPATH=.../vport#p#, only exposing the name in a sysfs file.

Bug: 336663898
Test: Attach multiport console, run with `-DLOG_UEVENTS=1`
Test: Introduce ueventd policy using `devname sys_name`
Change-Id: I59632b556db4a47883eab97e90c0e6ca81a9c650
---
 init/README.ueventd.md  |  5 +++--
 init/devices.h          | 17 +++++++++++++----
 init/ueventd_parser.cpp |  4 ++++
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/init/README.ueventd.md b/init/README.ueventd.md
index 3c7107a4d..7d00195eb 100644
--- a/init/README.ueventd.md
+++ b/init/README.ueventd.md
@@ -59,9 +59,10 @@ device in. The section takes the below format of
 
 `subsystem_name` is used to match uevent `SUBSYSTEM` value
 
-`devname` takes one of two options
+`devname` takes one of three options
   1. `uevent_devname` specifies that the name of the node will be the uevent `DEVNAME`
-  2. `uevent_devpath` specified that the name of the node will be basename uevent `DEVPATH`
+  2. `uevent_devpath` specifies that the name of the node will be basename uevent `DEVPATH`
+  3. `sys_name` specifies that the name of the node will be the contents of `/sys/DEVPATH/name`
 
 `dirname` is an optional parameter that specifies a directory within `/dev` where the node will be
 created.
diff --git a/init/devices.h b/init/devices.h
index f9f4d7989..44ce2a920 100644
--- a/init/devices.h
+++ b/init/devices.h
@@ -82,6 +82,7 @@ class Subsystem {
     enum DevnameSource {
         DEVNAME_UEVENT_DEVNAME,
         DEVNAME_UEVENT_DEVPATH,
+        DEVNAME_SYS_NAME,
     };
 
     Subsystem() {}
@@ -92,10 +93,18 @@ class Subsystem {
     // Returns the full path for a uevent of a device that is a member of this subsystem,
     // according to the rules parsed from ueventd.rc
     std::string ParseDevPath(const Uevent& uevent) const {
-        std::string devname = devname_source_ == DEVNAME_UEVENT_DEVNAME
-                                      ? uevent.device_name
-                                      : android::base::Basename(uevent.path);
-
+        std::string devname;
+        if (devname_source_ == DEVNAME_UEVENT_DEVNAME) {
+            devname = uevent.device_name;
+        } else if (devname_source_ == DEVNAME_UEVENT_DEVPATH) {
+            devname = android::base::Basename(uevent.path);
+        } else if (devname_source_ == DEVNAME_SYS_NAME) {
+            if (android::base::ReadFileToString("/sys/" + uevent.path + "/name", &devname)) {
+                devname.pop_back();  // Remove terminating newline
+            } else {
+                devname = uevent.device_name;
+            }
+        }
         return dir_name_ + "/" + devname;
     }
 
diff --git a/init/ueventd_parser.cpp b/init/ueventd_parser.cpp
index d34672e24..4395d8838 100644
--- a/init/ueventd_parser.cpp
+++ b/init/ueventd_parser.cpp
@@ -218,6 +218,10 @@ Result<void> SubsystemParser::ParseDevName(std::vector<std::string>&& args) {
         subsystem_.devname_source_ = Subsystem::DEVNAME_UEVENT_DEVPATH;
         return {};
     }
+    if (args[1] == "sys_name") {
+        subsystem_.devname_source_ = Subsystem::DEVNAME_SYS_NAME;
+        return {};
+    }
 
     return Error() << "invalid devname '" << args[1] << "'";
 }