base: kernel logging get pre-opened /dev/kmsg from environment

If the environment has an instance of /dev/kmsg pre-opened, pick up.
This happens when an init script has 'file w /dev/kmsg' in the
daemon's recipe.

libcutils android_get_control_file operations are open-coded to
prevent a dependency on libcutils itself.  Also check that /dev/kmsg
file descriptor is valid and open'd for write only.

Test: check to make sure init() kernel logging still works
Bug: 77541952
Bug: 77661605
Change-Id: I007553acd594ef0815d23f32f8aa2867518570ba

1 files changed, 19 insertions, 1 deletions

diff --git a/base/logging.cpp b/base/logging.cpp
index 30d7f8d57..a33da2211 100644
--- a/base/logging.cpp
+++ b/base/logging.cpp
@@ -54,6 +54,7 @@
 #endif
 
 #include <android-base/macros.h>
+#include <android-base/parseint.h>
 #include <android-base/strings.h>
 #include <android-base/threads.h>
 
@@ -82,6 +83,23 @@ const char* getprogname() {
   return progname;
 }
 #endif
+
+#if defined(__linux__)
+int OpenKmsg() {
+#if defined(__ANDROID__)
+  // pick up 'file w /dev/kmsg' environment from daemon's init rc file
+  const auto val = getenv("ANDROID_FILE__dev_kmsg");
+  if (val != nullptr) {
+    int fd;
+    if (android::base::ParseInt(val, &fd, 0)) {
+      auto flags = fcntl(fd, F_GETFL);
+      if ((flags != -1) && ((flags & O_ACCMODE) == O_WRONLY)) return fd;
+    }
+  }
+#endif
+  return TEMP_FAILURE_RETRY(open("/dev/kmsg", O_WRONLY | O_CLOEXEC));
+}
+#endif
 } // namespace
 
 namespace android {
@@ -150,7 +168,7 @@ void KernelLogger(android::base::LogId, android::base::LogSeverity severity,
   static_assert(arraysize(kLogSeverityToKernelLogLevel) == android::base::FATAL + 1,
                 "Mismatch in size of kLogSeverityToKernelLogLevel and values in LogSeverity");
 
-  static int klog_fd = TEMP_FAILURE_RETRY(open("/dev/kmsg", O_WRONLY | O_CLOEXEC));
+  static int klog_fd = OpenKmsg();
   if (klog_fd == -1) return;
 
   int level = kLogSeverityToKernelLogLevel[severity];