Add a helpful error message if GetUnreachableMemory fails

The primary reason for GetUnreachableMemory failing is selinux
blocking system apps from ptracing themselves.  Add a helpful
error message suggesting setenforce 0.

Bug: 27945735
Test: dumpsys meminfo --unreachable com.android.systemui
Change-Id: Ibfa9507860be36dffd514f82027f7a2f8a72ac48

1 files changed, 4 insertions, 1 deletions

diff --git a/libmemunreachable/MemUnreachable.cpp b/libmemunreachable/MemUnreachable.cpp
index e7c0beb56..1c8474428 100644
--- a/libmemunreachable/MemUnreachable.cpp
+++ b/libmemunreachable/MemUnreachable.cpp
@@ -502,7 +502,10 @@ std::string UnreachableMemoryInfo::ToString(bool log_contents) const {
 std::string GetUnreachableMemoryString(bool log_contents, size_t limit) {
   UnreachableMemoryInfo info;
   if (!GetUnreachableMemory(info, limit)) {
-    return "Failed to get unreachable memory\n";
+    return "Failed to get unreachable memory\n"
+           "If you are trying to get unreachable memory from a system app\n"
+           "(like com.android.systemui), disable selinux first using\n"
+           "setenforce 0\n";
   }
 
   return info.ToString(log_contents);