Check file uri sharing owner rather than prohibiting outright

Fixes #5381
// FREEBIE

jni/Android.mk

@@ -53,6 +53,14 @@ libwebrtc_spl \
 libwebrtc_vad \
 libcrypto_static
 
-
 include $(BUILD_SHARED_LIBRARY)
 
+include $(CLEAR_VARS)
+
+LOCAL_MODULE     := native-utils
+LOCAL_C_INCLUDES := $(JNI_DIR)/utils/
+LOCAL_CFLAGS     += -Wall
+
+LOCAL_SRC_FILES := $(JNI_DIR)/utils/org_thoughtcrime_securesms_util_FileUtils.cpp
+
+include $(BUILD_SHARED_LIBRARY)

jni/utils/org_thoughtcrime_securesms_util_FileUtils.cpp

@@ -0,0 +1,31 @@
+#include "org_thoughtcrime_securesms_util_FileUtils.h"
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+jint JNICALL Java_org_thoughtcrime_securesms_util_FileUtils_getFileDescriptorOwner
+  (JNIEnv *env, jclass clazz, jobject fileDescriptor)
+{
+  jclass fdClass = env->GetObjectClass(fileDescriptor);
+
+  if (fdClass == NULL) {
+    return -1;
+  }
+
+  jfieldID fdFieldId = env->GetFieldID(fdClass, "descriptor", "I");
+
+  if (fdFieldId == NULL) {
+    return -1;
+  }
+
+  int fd = env->GetIntField(fileDescriptor, fdFieldId);
+
+  struct stat stat_struct;
+
+  if (fstat(fd, &stat_struct) != 0) {
+    return -1;
+  }
+
+  return stat_struct.st_uid;
+}

jni/utils/org_thoughtcrime_securesms_util_FileUtils.h

@@ -0,0 +1,21 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class org_thoughtcrime_securesms_util_FileUtils */
+
+#ifndef _Included_org_thoughtcrime_securesms_util_FileUtils
+#define _Included_org_thoughtcrime_securesms_util_FileUtils
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     org_thoughtcrime_securesms_util_FileUtils
+ * Method:    getFileDescriptorOwner
+ * Signature: (Ljava/io/FileDescriptor;)I
+ */
+JNIEXPORT jint JNICALL Java_org_thoughtcrime_securesms_util_FileUtils_getFileDescriptorOwner
+  (JNIEnv *, jclass, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif