libprocessgroup: Add a function to remove only empty process groups

When system_server and zygote crash or get killed, all apps also get
killed but their process groups are left empty. Provide a function to
remove all empty process groups so that init can purge them when this
even happens.

Bug: 228160715
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Ife38ca021e80cd38106f218ae13183e8c2631bf0
Merged-In: Ife38ca021e80cd38106f218ae13183e8c2631bf0

2 files changed, 29 insertions, 5 deletions

diff --git a/libprocessgroup/include/processgroup/processgroup.h b/libprocessgroup/include/processgroup/processgroup.h
index c5badc941..39b9f3fc0 100644
--- a/libprocessgroup/include/processgroup/processgroup.h
+++ b/libprocessgroup/include/processgroup/processgroup.h
@@ -67,6 +67,7 @@ bool setProcessGroupSoftLimit(uid_t uid, int initialPid, int64_t softLimitInByte
 bool setProcessGroupLimit(uid_t uid, int initialPid, int64_t limitInBytes);
 
 void removeAllProcessGroups(void);
+void removeAllEmptyProcessGroups(void);
 
 // Provides the path for an attribute in a specific process group
 // Returns false in case of error, true in case of success
diff --git a/libprocessgroup/processgroup.cpp b/libprocessgroup/processgroup.cpp
index e3a80e97b..267e62c67 100644
--- a/libprocessgroup/processgroup.cpp
+++ b/libprocessgroup/processgroup.cpp
@@ -200,7 +200,7 @@ static int RemoveProcessGroup(const char* cgroup, uid_t uid, int pid, unsigned i
     return ret;
 }
 
-static bool RemoveUidProcessGroups(const std::string& uid_path) {
+static bool RemoveUidProcessGroups(const std::string& uid_path, bool empty_only) {
     std::unique_ptr<DIR, decltype(&closedir)> uid(opendir(uid_path.c_str()), closedir);
     bool empty = true;
     if (uid != NULL) {
@@ -215,6 +215,21 @@ static bool RemoveUidProcessGroups(const std::string& uid_path) {
             }
 
             auto path = StringPrintf("%s/%s", uid_path.c_str(), dir->d_name);
+            if (empty_only) {
+                struct stat st;
+                auto procs_file = StringPrintf("%s/%s", path.c_str(),
+                                               PROCESSGROUP_CGROUP_PROCS_FILE);
+                if (stat(procs_file.c_str(), &st) == -1) {
+                    PLOG(ERROR) << "Failed to get stats for " << procs_file;
+                    continue;
+                }
+                if (st.st_size > 0) {
+                    // skip non-empty groups
+                    LOG(VERBOSE) << "Skipping non-empty group " << path;
+                    empty = false;
+                    continue;
+                }
+            }
             LOG(VERBOSE) << "Removing " << path;
             if (rmdir(path.c_str()) == -1) {
                 if (errno != EBUSY) {
@@ -227,9 +242,7 @@ static bool RemoveUidProcessGroups(const std::string& uid_path) {
     return empty;
 }
 
-void removeAllProcessGroups() {
-    LOG(VERBOSE) << "removeAllProcessGroups()";
-
+void removeAllProcessGroupsInternal(bool empty_only) {
     std::vector<std::string> cgroups;
     std::string path, memcg_apps_path;
 
@@ -256,7 +269,7 @@ void removeAllProcessGroups() {
                 }
 
                 auto path = StringPrintf("%s/%s", cgroup_root_path.c_str(), dir->d_name);
-                if (!RemoveUidProcessGroups(path)) {
+                if (!RemoveUidProcessGroups(path, empty_only)) {
                     LOG(VERBOSE) << "Skip removing " << path;
                     continue;
                 }
@@ -269,6 +282,16 @@ void removeAllProcessGroups() {
     }
 }
 
+void removeAllProcessGroups() {
+    LOG(VERBOSE) << "removeAllProcessGroups()";
+    removeAllProcessGroupsInternal(false);
+}
+
+void removeAllEmptyProcessGroups() {
+    LOG(VERBOSE) << "removeAllEmptyProcessGroups()";
+    removeAllProcessGroupsInternal(true);
+}
+
 /**
  * Process groups are primarily created by the Zygote, meaning that uid/pid groups are created by
  * the user root. Ownership for the newly created cgroup and all of its files must thus be