logd: add getEventTag id= command

This is the precursor for "Plan B" recovery when access to
/dev/event-log-tags is blocked to untrusted zones.  Also
deals with mitigating issues with long-lived mappings that
do not update /dev/event-log-tags when dynamically changed.

Test: gTest logd-unit-test --gtest_filter=logd.getEventTag_42
Bug: 31456426
Bug: 35326290
Change-Id: I3db2e73763603727a369da3952c5ab4cf709f901

logd/CommandListener.cpp

@@ -301,6 +301,7 @@ int CommandListener::GetEventTagCmd::runCommand(SocketClient *cli,
 
     const char *name = NULL;
     const char *format = NULL;
+    const char *id = NULL;
     for (int i = 1; i < argc; ++i) {
         static const char _name[] = "name=";
         if (!strncmp(argv[i], _name, strlen(_name))) {
@@ -313,6 +314,21 @@ int CommandListener::GetEventTagCmd::runCommand(SocketClient *cli,
             format = argv[i] + strlen(_format);
             continue;
         }
+
+        static const char _id[] = "id=";
+        if (!strncmp(argv[i], _id, strlen(_id))) {
+            id = argv[i] + strlen(_id);
+            continue;
+        }
+    }
+
+    if (id) {
+        if (format || name) {
+            cli->sendMsg("can not mix id= with either format= or name=");
+            return 0;
+        }
+        cli->sendMsg(package_string(mBuf.formatEntry(atoi(id), uid)).c_str());
+        return 0;
     }
 
     cli->sendMsg(package_string(mBuf.formatGetEventTag(uid,

logd/LogBuffer.h

@@ -140,6 +140,9 @@ public:
                                   const char *name, const char *format) {
         return tags.formatGetEventTag(uid, name, format);
     }
+    std::string formatEntry(uint32_t tag, uid_t uid) {
+        return tags.formatEntry(tag, uid);
+    }
     const char *tagToName(uint32_t tag) { return tags.tagToName(tag); }
 
     // helper must be protected directly or implicitly by lock()/unlock()

logd/LogTags.cpp

@@ -836,6 +836,11 @@ std::string LogTags::formatEntry_locked(uint32_t tag, uid_t uid,
     return ret;
 }
 
+std::string LogTags::formatEntry(uint32_t tag, uid_t uid) {
+    android::RWLock::AutoRLock readLock(rwlock);
+    return formatEntry_locked(tag, uid);
+}
+
 std::string LogTags::formatGetEventTag(uid_t uid,
                                        const char* name, const char* format) {
     bool all = name && (name[0] == '*') && !name[1];

logd/LogTags.h

@@ -106,6 +106,7 @@ public:
     // reverse lookup from tag
     const char* tagToName(uint32_t tag) const;
     const char* tagToFormat(uint32_t tag) const;
+    std::string formatEntry(uint32_t tag, uid_t uid);
     // find associated tag
     uint32_t nameToTag(const char* name) const;
 

logd/tests/logd_test.cpp

@@ -836,6 +836,23 @@ TEST(logd, getEventTag_list) {
 #endif
 }
 
+TEST(logd, getEventTag_42) {
+#ifdef __ANDROID__
+    char buffer[256];
+    memset(buffer, 0, sizeof(buffer));
+    snprintf(buffer, sizeof(buffer), "getEventTag id=42");
+    send_to_control(buffer, sizeof(buffer));
+    buffer[sizeof(buffer) - 1] = '\0';
+    char *cp;
+    long ret = strtol(buffer, &cp, 10);
+    EXPECT_GT(ret, 16);
+    EXPECT_TRUE(strstr(buffer, "\t(to life the universe etc|3)") != NULL);
+    EXPECT_TRUE(strstr(buffer, "answer") != NULL);
+#else
+    GTEST_LOG_(INFO) << "This test does nothing.\n";
+#endif
+}
+
 TEST(logd, getEventTag_newentry) {
 #ifdef __ANDROID__
     char buffer[256];