Trim trailing '\0's inserted by libselinux.

libselinux has an off-by-one that causes it to pass the trailing '\0' to
the kernel as if it's part of the security context, and the kernel
dutifully hands it back, since it's an uninterpreted byte array as far
as the kernel is concerned. libselinux accidentally hides this bug by
treating it as a C string and calling strdup(), but debuggerd doesn't
because it reads the file into a std::string.

We could switch to libselinux's getcon()/getpidcon(), but (a) libselinux
is awful (see above) and (b) not currently accessible to apexes (and it
doesn't seem like a great idea to make it accessible). So just manually
drop the last byte from the context we read ourselves, if it happens to
be a '\0'.

Bug: android/ndk#1993
Test: treehugger
Change-Id: I8e7605ac5e618007a8da635cb6f45b0778dc167c

debuggerd/crash_dump.cpp

@@ -510,6 +510,10 @@ int main(int argc, char** argv) {
       if (!android::base::ReadFdToString(attr_fd, &info.selinux_label)) {
         PLOG(WARNING) << "failed to read selinux label";
       }
+      // https://github.com/android/ndk/issues/1993
+      if (!info.selinux_label.empty() && info.selinux_label.back() == '\0') {
+        info.selinux_label.pop_back();
+      }
 
       if (!ptrace_interrupt(thread, &info.signo)) {
         PLOG(WARNING) << "failed to ptrace interrupt thread " << thread;

debuggerd/libdebuggerd/tombstone.cpp

@@ -71,6 +71,10 @@ void engrave_tombstone_ucontext(int tombstone_fd, int proto_fd, uint64_t abort_m
 
   std::string selinux_label;
   android::base::ReadFileToString("/proc/self/attr/current", &selinux_label);
+  // https://github.com/android/ndk/issues/1993
+  if (!selinux_label.empty() && selinux_label.back() == '\0') {
+    selinux_label.pop_back();
+  }
 
   std::map<pid_t, ThreadInfo> threads;
   threads[target_tid] = ThreadInfo {