liblp: Add a helper to differentiate empty from non-empty images.

Also, finally explain the difference between these somewhere.

Bug: 140204341
Test: lpdump
Change-Id: I128780b01a28e893afac65f3aa4f4a8d36032d63

fs_mgr/liblp/images.cpp

@@ -17,6 +17,7 @@
 #include "images.h"
 
 #include <limits.h>
+#include <sys/stat.h>
 
 #include <android-base/file.h>
 
@@ -27,12 +28,45 @@
 namespace android {
 namespace fs_mgr {
 
+using android::base::borrowed_fd;
 using android::base::unique_fd;
 
 #if defined(_WIN32)
 static const int O_NOFOLLOW = 0;
 #endif
 
+static bool IsEmptySuperImage(borrowed_fd fd) {
+    struct stat s;
+    if (fstat(fd.get(), &s) < 0) {
+        PERROR << __PRETTY_FUNCTION__ << " fstat failed";
+        return false;
+    }
+    if (s.st_size < LP_METADATA_GEOMETRY_SIZE) {
+        return false;
+    }
+
+    // Rewind back to the start, read the geometry struct.
+    LpMetadataGeometry geometry = {};
+    if (SeekFile64(fd.get(), 0, SEEK_SET) < 0) {
+        PERROR << __PRETTY_FUNCTION__ << " lseek failed";
+        return false;
+    }
+    if (!android::base::ReadFully(fd, &geometry, sizeof(geometry))) {
+        PERROR << __PRETTY_FUNCTION__ << " read failed";
+        return false;
+    }
+    return geometry.magic == LP_METADATA_GEOMETRY_MAGIC;
+}
+
+bool IsEmptySuperImage(const std::string& file) {
+    unique_fd fd = GetControlFileOrOpen(file, O_RDONLY | O_CLOEXEC);
+    if (fd < 0) {
+        PERROR << __PRETTY_FUNCTION__ << " open failed";
+        return false;
+    }
+    return IsEmptySuperImage(fd);
+}
+
 std::unique_ptr<LpMetadata> ReadFromImageFile(int fd) {
     std::unique_ptr<uint8_t[]> buffer = std::make_unique<uint8_t[]>(LP_METADATA_GEOMETRY_SIZE);
     if (SeekFile64(fd, 0, SEEK_SET) < 0) {

fs_mgr/liblp/include/liblp/liblp.h

@@ -70,8 +70,15 @@ bool UpdatePartitionTable(const std::string& super_partition, const LpMetadata&
                           uint32_t slot_number);
 std::unique_ptr<LpMetadata> ReadMetadata(const std::string& super_partition, uint32_t slot_number);
 
+// Returns whether an image is an "empty" image or not. An empty image contains
+// only metadata. Unlike a flashed block device, there are no reserved bytes or
+// backup sections, and only one slot is stored (even if multiple slots are
+// supported). It is a format specifically for storing only metadata.
+bool IsEmptySuperImage(const std::string& file);
+
 // Read/Write logical partition metadata to an image file, for diagnostics or
-// flashing.
+// flashing. If no partition images are specified, the file will be in the
+// empty format.
 bool WriteToImageFile(const std::string& file, const LpMetadata& metadata, uint32_t block_size,
                       const std::map<std::string, std::string>& images, bool sparsify);
 bool WriteToImageFile(const std::string& file, const LpMetadata& metadata);

fs_mgr/liblp/utility.cpp

@@ -205,9 +205,9 @@ bool SetBlockReadonly(int fd, bool readonly) {
 #endif
 }
 
-base::unique_fd GetControlFileOrOpen(const char* path, int flags) {
+base::unique_fd GetControlFileOrOpen(std::string_view path, int flags) {
 #if defined(__ANDROID__)
-    int fd = android_get_control_file(path);
+    int fd = android_get_control_file(path.data());
     if (fd >= 0) {
         int newfd = TEMP_FAILURE_RETRY(dup(fd));
         if (newfd >= 0) {
@@ -216,7 +216,7 @@ base::unique_fd GetControlFileOrOpen(const char* path, int flags) {
         PERROR << "Cannot dup fd for already controlled file: " << path << ", reopening...";
     }
 #endif
-    return base::unique_fd(open(path, flags));
+    return base::unique_fd(open(path.data(), flags));
 }
 
 bool UpdateMetadataForInPlaceSnapshot(LpMetadata* metadata, uint32_t source_slot_number,

fs_mgr/liblp/utility.h

@@ -21,6 +21,9 @@
 #include <stdint.h>
 #include <sys/types.h>
 
+#include <string>
+#include <string_view>
+
 #include <android-base/logging.h>
 #include <android-base/unique_fd.h>
 
@@ -94,7 +97,7 @@ bool UpdatePartitionName(LpMetadataPartition* partition, const std::string& name
 // Call BLKROSET ioctl on fd so that fd is readonly / read-writable.
 bool SetBlockReadonly(int fd, bool readonly);
 
-::android::base::unique_fd GetControlFileOrOpen(const char* path, int flags);
+::android::base::unique_fd GetControlFileOrOpen(std::string_view path, int flags);
 
 // For Virtual A/B updates, modify |metadata| so that it can be written to |target_slot_number|.
 bool UpdateMetadataForInPlaceSnapshot(LpMetadata* metadata, uint32_t source_slot_number,